A method for producing low-carbon boron-containing steel to suppress mixed crystal structure

Abstract

This invention provides a method for producing low-carbon boron-containing steel that suppresses mixed crystal structure, comprising three steps: steelmaking, continuous casting and rolling, and cooling and coiling. In the steelmaking step, after tapping the steel, ferrotitanium and ferroboron are added to the molten steel. The amounts of ferrotitanium and ferroboron added are determined by the following formulas: the Ti content satisfies: 3.4ω(N)≤ω(Ti)≤0.025%, and the B content satisfies: 0.0010%≤ω(B)≤ω(TiN) / 7.56+0.0005%, where ω(TiN)=4.4ω(N). This invention strictly controls the content of each element in the low-carbon boron-containing steel. By precipitating an appropriate amount of TiN particles, it suppresses the problem of coarse grains caused by B. Furthermore, by using suitable finishing rolling inlet temperature, final rolling temperature, and a large reduction in the non-recrystallized zone, the mixed crystal structure problem is effectively controlled, thus comprehensively achieving effective control of the mixed crystal structure problem in low-carbon boron-containing steel.

Description

Technical Field

[0001] This invention relates to the field of steelmaking technology, and in particular to a method for producing low-carbon boron-containing steel that suppresses mixed crystal formation in the microstructure. Background Technology

[0002] The addition of boron (B) can adsorb at austenite grain boundaries, lowering grain boundary energy and inhibiting the formation of ferrite nuclei, thereby significantly improving the hardenability and work hardening properties of steel, and also increasing the high-temperature strength and creep strength of heat-resistant steel. However, at the same time, the problem of mixed grain structure also arises. Mixed grain structure is an internal defect in steel, manifested as a mixture of grain sizes and coarse and fine grains within the metal matrix. The presence of mixed grain structure has a significant impact on the mechanical properties and stamping performance of strip steel, especially on the toughness of the material. The more regular the grains, the higher the toughness of the material, and vice versa. If low-grade grains are mixed into regions with high-grade grains, it will reduce the overall performance of the metal material. Therefore, it is necessary to control mixed grain structure to ensure the overall performance of the steel.

[0003] Currently, both domestically and internationally, the common methods for controlling the mixed crystal structure of steel are to control billet composition segregation, avoid rolling in the two-phase region, and adjust the post-rolling cooling method to ensure uniform cooling. However, when these methods are applied to the production of low-carbon boron steel, it is found that the control effect of mixed crystal structure is very unstable. There are significant differences between batches in terms of whether mixed crystals exist and in terms of grain size. This indicates that the above methods have not found the key points affecting mixed crystal structure, and the mixed crystal problem has not been effectively controlled. Summary of the Invention

[0004] To address the aforementioned problems in the existing technology, the present invention aims to provide a method for producing low-carbon boron-containing steel that suppresses mixed crystal formation.

[0005] To solve the above-mentioned technical problems, the technical solution adopted by the present invention is as follows:

[0006] A method for producing low-carbon boron-containing steel to suppress mixed crystal structure includes three steps: steelmaking, continuous casting and rolling, and cooling and coiling. In the steelmaking step, ferrotitanium and ferroboron are added to the molten steel after tapping. The amount of ferrotitanium and ferroboron added is determined by the following formula:

[0007] The content of Ti satisfies the following relationship: 3.4ω(N)≤ω(Ti)≤0.025%.

[0008] The content of B satisfies the following relationship: 0.0010%≤ω(B)≤ω(TiN) / 7.56+0.0005%, where ω(TiN)=4.4ω(N).

[0009] In the formula, ω(N) = nitrogen content in molten steel at the end of smelting + nitrogen addition during the process.

[0010] A further improvement of the present invention is as follows: after adding ferrotitanium and ferroboron, the composition of the molten steel is tested to verify whether the relationship between ω(TiN) and ω(B) satisfies ω(TiN) / (ω(B)-0.0005%)≥7.56, where ω(TiN)=4.4ω(N); if not, an appropriate amount of ferrotitanium or ferroboron is added until the relationship between ω(TiN) and ω(B) satisfies the formula requirements.

[0011] The low-carbon boron-containing steel comprises the following components by mass percentage: C, 0.03%-0.06%, Mn, 0.15%-0.30%, S≤0.025%, P≤0.025%, Si≤0.03%, Al, 0.020%-0.055%, Ti≤0.025%, B, 0.0010%-0.0030%, N, 0.0020%-0.0050%, with the balance being iron and unavoidable impurities.

[0012] In the continuous casting and rolling process, the initial rolling temperature of the finishing rolling is 1000℃-1100℃, and the final rolling temperature is controlled at 860℃-880℃.

[0013] For seven-pass finishing rolling, the reduction rate of the last pass is controlled at 15%-18%, and the total reduction rate of the last three passes is greater than 45%.

[0014] The technological advancements achieved by this invention due to the adoption of the above technical solutions are as follows:

[0015] This invention provides a method for producing low-carbon boron-containing steel that suppresses mixed crystal structure. It strictly controls the content of each element in the low-carbon boron-containing steel, enabling a synergistic effect between Ti and B elements. The precipitation of an appropriate amount of TiN particles suppresses the coarse grain problem caused by B element. Furthermore, by combining this with suitable finishing rolling inlet temperature, final rolling temperature, and a large reduction in the non-recrystallization zone, the mixed crystal structure problem is effectively controlled, thus comprehensively achieving effective control of mixed crystal structure in low-carbon boron-containing steel. Experiments have shown that the low-carbon boron-containing steel produced using this method exhibits stable control of mixed crystal structure, and no mixed crystal problem has occurred in multiple batches of steel.

[0016] Based on analytical experiments, this invention provides the interaction relationship between titanium, nitrogen, and boron in low-carbon boron-containing steel, and proposes calculation formulas for the content range of titanium and boron in low-carbon boron-containing steel. This allows for precise control of the amount of ferrotitanium and ferroboron added to the molten steel, effectively eliminating or weakening mixed crystals while ensuring the various mechanical properties of low-carbon boron-containing steel. Attached Figure Description

[0017] Figure 1 This is a schematic diagram of the metallographic structure of the steel strip in Embodiment 1 of the present invention;

[0018] Figure 2 This is a schematic diagram of the metallographic structure of the steel strip in Embodiment 2 of the present invention;

[0019] Figure 3 This is a schematic diagram of the metallographic structure of the steel strip in Embodiment 3 of the present invention;

[0020] Figure 4 This is a schematic diagram of the metallographic structure of the steel strip in Comparative Example 1 of the present invention;

[0021] Figure 5 This is a schematic diagram of the metallographic structure of the steel strip in Comparative Example 2 of the present invention;

[0022] Figure 6 This is a schematic diagram of the metallographic structure of the steel strip in Comparative Example 3 of the present invention. Detailed Implementation

[0023] The present invention will now be described in detail with reference to the embodiments.

[0024] This invention provides a method for producing low-carbon boron-containing steel that suppresses mixed crystal structure. During the smelting process, by controlling the relative ratio of Ti and N content in the steel, an appropriate amount of TiN particles are formed and pinned to the austenite grain boundaries, suppressing grain coarsening caused by the addition of B element. At the same time, during the rolling process, the finishing rolling temperature is precisely controlled, and the reduction of subsequent passes is appropriately increased to effectively control the mixed crystal structure of low-carbon boron-containing steel.

[0025] The role of alloying elements in the low-carbon boron-containing steel of this invention is analyzed as follows:

[0026] Carbon: The presence of carbon in steel can improve the mechanical strength of steel through solid solution strengthening and precipitation strengthening. However, the main use of low-carbon boron-containing steel is cold rolling. The lower the strength of the steel used for cold rolling, the better, as it is more conducive to cold rolling processing. Therefore, the carbon content of the low-carbon boron-containing steel of this invention should be controlled to avoid being too high. At the same time, the relationship between carbon content and smelting difficulty needs to be considered. The lower the carbon content, the greater the smelting difficulty. Therefore, taking into account both the requirements of steel performance and smelting difficulty, this invention limits the C content range to 0.03%-0.06%.

[0027] Manganese: Manganese has the effect of solid solution strengthening. Although increasing the Mn content can improve the strength of steel, it will also increase the segregation of the billet. Therefore, taking all factors into consideration, this invention limits the Mn content range to 0.15%-0.30%.

[0028] Silicon: Silicon has a strong deoxidizing ability and a strong solid solution strengthening effect after dissolving in ferrite. However, if the silicon content is too high, it is easily oxidized during subsequent annealing, which will affect the surface quality. Therefore, the Si content is controlled within 0.03%.

[0029] Sulfur and phosphorus: As harmful elements, they affect the toughness and plasticity of steel. The content of S and P in low carbon boron steel must be strictly controlled. The lower the content, the better. In this invention, the content of S and P is controlled to within 0.025%.

[0030] Aluminum plays a crucial role in steel, primarily acting as a deoxidizer. Removing oxygen from the steel increases the yield of Ti and B elements. However, excessive aluminum content can lead to alloy waste and increase the number of inclusions in the steel. Considering all factors, this invention controls the aluminum content within the range of 0.020%-0.055%.

[0031] Boron: Boron is an essential element in low-carbon boron-containing steel. Its main role in steel is to enhance hardenability, improve overall performance, and replace some rarer and more expensive metals. Furthermore, boron can interact with other alloying elements to further improve hardenability and other properties. However, boron in steel readily combines with nitrogen to form boron nuclei (BN). BN precipitation embrittles grain boundaries, leading to cracking during continuous casting. Moreover, the addition of boron results in coarse grains; as the mass fraction of boron increases, the grain size also increases. Additionally, due to the low solubility of boron in iron, the precipitated borides are prone to defects such as edge cracking during processing. Therefore, considering all aspects and factors, this invention limits the boron content to 0.0010%-0.0030%.

[0032] Titanium: Titanium is one of the most important alloying elements in this invention. Ti and N have a strong affinity; a certain Ti content can combine with N in the steel to form TiN particles. These TiN particles pin the austenite grain boundaries, inhibiting austenite coarsening and refining the grains. Simultaneously, the TiN combination prevents N from combining with B to form BN, inhibiting cracking during continuous casting. However, excessively high titanium content increases the steel's strength, which is detrimental to subsequent cold rolling. Therefore, this invention specifically limits the Ti content range to 3.4ω(N)≤ω(Ti)≤0.025%, satisfying nitrogen fixation requirements without affecting the strength of low-carbon boron-containing steel. Experiments have shown that a titanium mass fraction of approximately 0.020% provides the best effect in inhibiting austenite grain coarsening without producing a significant strengthening effect, making it the preferred Ti content.

[0033] Nitrogen: Nitrogen plays a crucial role in enhancing the austenite grain boundary pinning effect of TiN particles. If the nitrogen content is too low, the amount of titanium dissolved at the homogenization temperature will increase, and the coarsening rate of TiN will increase, failing to achieve the effect of pinning austenite grain boundaries and refining grains. If the nitrogen content is too high, N will combine with boron in the steel to form boron (BN), making the cast billet prone to cracking. Therefore, this invention limits the N content to the range of 0.0020%-0.0050%.

[0034] Based on long-term exploration and repeated experiments, this invention proposes two calculation formulas for the content of Ti and B elements through statistical analysis of a large amount of real experimental data:

[0035] Considering the strength and processing performance of low-carbon boron-containing steel, the Ti content in the steel is usually controlled to be ≤0.025%. To suppress grain coarsening caused by boron, and to ensure that nitrogen can form a sufficient amount of TiN particles, ω(Ti) ≥ 3.4ω(N) should be satisfied. Therefore, the Ti content in the steel is controlled at 3.4ω(N) ≤ ω(Ti) ≤ 0.025%.

[0036] The presence of appropriate amounts of Ti and N allows the precipitated TiN particles to refine the grains, suppressing grain coarsening caused by the dissolution of B in steel and improving the mixed grain condition. For optimal suppression, ω(TiN) and ω(B) should satisfy ω(TiN) / (ω(B)-0.0005%)≥7.56, where ω(TiN)=ω(Ti)+ω(N)= 4.4ω(N). ω(B)-0.0005% is used because B content below 0.0005% has little effect on the microstructure; only B exceeding 0.0005% affects the grain size. Therefore, the 0.0005% portion is subtracted, resulting in a more precise expression of the relative relationship between TiN and B.

[0037] In formula calculations, the total nitrogen content in steel, ω(N), is generally calculated as: total nitrogen content ω(N) = nitrogen content in the molten steel at the end of smelting + nitrogen addition during the process. Based on current practices, the nitrogen addition during the entire production process is typically between 5 ppm and 15 ppm. When calculating the lower limit of Ti content, the maximum nitrogen addition should be used to ensure sufficient Ti to react with N. When calculating the upper limit of B content, the minimum nitrogen addition should be used to ensure that even with the minimum TiN production from the minimum nitrogen addition, grain coarsening can be effectively suppressed.

[0038] After smelting, the temperature of the tundish is controlled within the range of 1540-1560℃, the casting speed is controlled at 1.2-1.4m / min, and the casting is made into a billet and cut into fixed lengths.

[0039] The heating furnace temperature is controlled at 1280±15℃, and the furnace time is 100-120 minutes. After descaling in the furnace, the roughing mill starts rolling at 1040℃-1140℃, and after 5 passes, it is sent to the finishing mill via a roller conveyor.

[0040] The final rolling temperature and reduction during the finishing rolling process play a crucial role in the mixed-grain microstructure. When the temperature of the final finishing pass is lower than the phase transformation temperature Ar3, proeutectoid ferrite appears inside the steel plate before the final rolling pass. After rolling, the proeutectoid ferrite grains deform along the rolling direction. These deformed ferrites store a large amount of distortion energy, which cannot be released through phase transformation but can only be released through recrystallization and grain growth, forming a coarse-grained or mixed-grain microstructure in the low-temperature region of the steel plate. When the temperature of the final finishing pass is higher than the phase transformation temperature Ar3, the interior of the steel plate before the final rolling pass is a pure austenitic microstructure. After rolling, equiaxed ferrite grains are formed through phase transformation. However, if the rolling temperature is too high and the high-temperature time is too long, abnormal local grain growth occurs, which also leads to mixed grains. Therefore, this invention controls the final rolling temperature between 860℃ and 880℃.

[0041] The initial rolling temperature of finishing rolling is generally around 1000℃, which decreases to 860℃-880℃ during the final finishing rolling. This process involves rolling in both the austenite recrystallization zone and the non-recrystallization zone. Low-temperature, high-reduction deformation rolling in the non-recrystallization zone is one of the most effective methods for obtaining ultra-fine ferrite grains, especially with single-pass high-strain deformation having a more significant effect on ferrite refinement. If the deformation in the non-recrystallization zone is insufficient, uneven ferrite grains will result. In actual production, the last three passes generally enter the non-recrystallization zone. To ensure fine and uniform grains after rolling, considering the rolling capacity of the equipment, repeated experiments have shown that for seven-pass finishing rolling, the reduction rate of the final pass should be controlled between 15% and 18%, and the total reduction rate of the last three passes should be greater than 45%. This effectively controls the microstructure of the aforementioned low-carbon boron-containing steel.

[0042] The present invention will be further described below through examples and comparative examples. Example 1

[0043] A method for producing low-carbon boron-containing steel with suppressed mixed crystal structure includes the following steps:

[0044] S1, Steel smelting

[0045] The final temperature of the converter smelting was 1645℃, and the final steel composition included: C: 0.04%, Mn: 0.10%, S: 0.015%, P: 0.018%, N: 0.0025%. During the tapping process, 4 kg / t of slag-forming material, 1.48 kg / t of medium-carbon ferromanganese, and 1.2 kg / t of aluminum-containing material were added to the molten steel.

[0046] The Ti and B contents in the molten steel are calculated using formulas to determine the amount of ferrotitanium and ferromanganese to be added. The specific calculation process is as follows:

[0047] Considering the nitrogen addition during the steelmaking process (generally around 5ppm to 15ppm), i.e., the ω(N) in the molten steel...min =0.0025%+0.0005%=0.0030%,ω(N) max =0.0025% + 0.0015% = 0.0040%.

[0048] The lower limit for Ti content in molten steel: 3.4 × ω(N) max =3.4 × 0.0040% = 0.0136%, that is, the Ti content in the molten steel should be controlled as follows: 0.0136% ≤ ω(Ti) ≤ 0.025%;

[0049] The upper limit of B content in molten steel: ω(TiN) / 7.56 + 0.0005% = (4.4 × ω(N)) min ) ÷ 7.56 + 0.0005% = (4.4 × 0.0030%) ÷ 7.56 + 0.0005% ≈ 0.0022%, that is, the B content in the molten steel should be controlled as follows: 0.0010% ≤ ω(B) ≤ 0.0022%.

[0050] The amounts of ferrotitanium and ferroboron were calculated based on the aforementioned Ti and B content ranges and added to the molten steel. After addition, the final composition of the molten steel was determined to be: C: 0.04%, Mn: 0.20%, S: 0.015%, P: 0.018%, Si: 0.01%, Al: 0.035%, Ti: 0.018%, B: 0.0020%, N: 0.0038%.

[0051] Verify the content of TiN and B in the molten steel. ω(TiN) / (ω(B)-0.0005%)=4.4ω(N) / (ω(B)-0.0005%)=4.4×0.0038%÷(0.0020%-0.0005%)≈11.15>7.56, which meets the relative proportion requirement of the two.

[0052] S2, Continuous casting and rolling

[0053] The tundish temperature was controlled within the range of 1545℃, the casting speed was controlled at 1.4m / min, and the billet quality was normal.

[0054] The heating furnace was homogenized at 1275℃ for 110 minutes. After descaling, the strip entered the roughing mill at an initial rolling temperature of 1100℃. After five passes on the roughing mill, it entered the finishing mill. The finishing mill inlet temperature was 1010℃, and the strip thickness after seven consecutive rolling passes was 3.25mm. The reduction rates of the last three stands on the finishing mill were 24%, 16.2%, and 15.5%, respectively, for a total reduction rate of 46.2%. The final rolling temperature was 875℃.

[0055] S3, Cooling and winding

[0056] After finishing, the steel strip enters the laminar flow zone and is cooled intensively starting from the 3rd group; the cooled steel strip is then coiled into steel coils at a coiling temperature of 600℃.

[0057] Samples were taken for mechanical property testing. The yield strength was 268 MPa, the tensile strength was 343 MPa, and the elongation after fracture was 42.5%. Metallographic results are as follows: Figure 1 As shown, the microstructure of the strip is ferrite and pearlite, with a grain size of 8.5 and no mixed crystals. Example 2

[0058] A method for producing low-carbon boron-containing steel with suppressed mixed crystal structure includes the following steps:

[0059] S1, Steel smelting

[0060] The final temperature of the converter smelting is 1650℃, and the final steel composition includes: C: 0.04%, Mn: 0.10%, S: 0.016%, P: 0.018%, N: 0.0022%. During the tapping process, 4 kg / t of slag-forming material, 1.48 kg / t of medium-carbon ferromanganese, and 1.2 kg / t of aluminum-containing material are added.

[0061] The Ti and B contents in the molten steel are calculated using formulas to determine the amount of ferrotitanium and ferromanganese to be added. The specific calculation process is as follows:

[0062] Considering the nitrogen addition during the steelmaking process (generally around 5ppm-15ppm), i.e., the ω(N) in the molten steel... min =0.0022%+0.0005%=0.0027%,ω(N) max =0.0022% + 0.0015% = 0.0037%.

[0063] The lower limit for Ti content in molten steel: 3.4 × ω(N) max =3.4×0.0037%≈0.0126%, that is, the Ti content in the molten steel should be controlled as follows: 0.0126%≤ω(Ti)≤0.025%;

[0064] The upper limit of B content in molten steel: ω(TiN) / 7.56 + 0.0005% = (4.4 × ω(N)) min ) ÷ 7.56 + 0.0005% = (4.4 × 0.0027%) ÷ 7.56 + 0.0005% ≈ 0.0021%, that is, the B content in the molten steel should be controlled as follows: 0.0010% ≤ ω(B) ≤ 0.0021%.

[0065] The amounts of ferrotitanium and ferroboron were calculated based on the aforementioned Ti and B content ranges and added to the molten steel. After addition, the final composition of the molten steel was determined to be: C: 0.04%, Mn: 0.20%, S: 0.016%, P: 0.018%, Si: 0.01%, Al: 0.036%, Ti: 0.020%, B: 0.0016%, N: 0.0030%.

[0066] Verify the content of TiN and B in the molten steel. ω(TiN) / (ω(B)-0.0005%)=4.4ω(N) / (ω(B)-0.0005%)=4.4×0.0030%÷(0.0016%-0.0005%)=12>7.56, which meets the requirement for the relative proportion of the two.

[0067] S2, Continuous casting and rolling

[0068] The tundish temperature was controlled within the range of 1545℃, the casting speed was controlled at 1.4m / min, and the billet quality was normal.

[0069] The heating furnace has a soaking temperature of 1275℃ and a furnace time of 110 minutes. After descaling, the material enters the roughing mill at an initial rolling temperature of 1100℃. After five passes in the roughing mill, the material enters the finishing mill.

[0070] The entry temperature of the finishing mill is 1010℃, and the strip thickness after 7 consecutive rolling is 3.0mm. The reduction rates of the last three passes of the finishing mill are 23.1%, 16.6%, and 17.3%, respectively, with a total reduction rate of 47.0%. The final rolling temperature is 875℃.

[0071] S3, Cooling and winding

[0072] After finishing, the steel strip enters the laminar flow zone and is cooled intensively starting from the 3rd group; the cooled steel strip is then coiled into steel coils at a coiling temperature of 600℃.

[0073] Samples were taken for mechanical property testing. The yield strength was 270 MPa, the tensile strength was 341 MPa, and the elongation after fracture was 42.0%. Metallographic results are as follows: Figure 2 As shown, the microstructure of the strip is ferrite and pearlite, with a grain size of 9.0 grade and no mixed crystals. Example 3

[0074] A method for producing low-carbon boron-containing steel with suppressed mixed crystal structure includes the following steps:

[0075] S1, Steel smelting

[0076] The final temperature of the converter smelting is 1650℃, and the final steel composition includes: C: 0.04%, Mn: 0.10%, S: 0.012%, P: 0.015%, N: 0.0035%. During the tapping process, 4 kg / t of slag-forming material, 1.48 kg / t of medium-carbon ferromanganese, and 1.2 kg / t of aluminum-containing material are added.

[0077] Considering the nitrogen increase during the process (generally around 5-15 ppm), i.e., the ω(N) in the molten steel... min =0.0035%+0.0005%=0.0040%,ω(N) max =0.0035% + 0.0015% = 0.0050%.

[0078] The lower limit for Ti content in molten steel: 3.4 × ω(N) max =3.4 × 0.0050% = 0.017%, that is, the Ti content in the molten steel should be controlled as follows: 0.017% ≤ ω(Ti) ≤ 0.025%;

[0079] The upper limit of B content in molten steel: ω(TiN) / 7.56 + 0.0005% = (4.4 × ω(N)) min ) ÷ 7.56 + 0.0005% = (4.4 × 0.0040%) ÷ 7.56 + 0.0005% ≈ 0.0028%, that is, the B content in the molten steel should be controlled as follows: 0.0010% ≤ ω(B) ≤ 0.0028%.

[0080] The amounts of ferrotitanium and ferroboron were calculated based on the aforementioned Ti and B content ranges and added to the molten steel. After addition, the final composition of the molten steel was determined to be: C: 0.04%, Mn: 0.20%, S: 0.012%, P: 0.015%, Si: 0.01%, Al: 0.034%, Ti: 0.022%, B: 0.0025%, N: 0.0040%.

[0081] Verify the content of TiN and B in the molten steel. ω(TiN) / (ω(B)-0.0005%)=4.4ω(N) / (ω(B)-0.0005%)=4.4*0.0040%÷(0.0025%-0.0005%)=8.8>7.56, which meets the relative proportion requirement of the two.

[0082] S2, Continuous casting and rolling

[0083] The tundish temperature was controlled within the range of 1545℃, the casting speed was controlled at 1.4m / min, and the billet quality was normal.

[0084] The heating furnace was heated to a soaking temperature of 1275℃ for 110 minutes. After descaling, the strip entered the roughing mill at an initial rolling temperature of 1100℃. After five passes in the roughing mill, it entered the finishing mill at an inlet temperature of 1010℃. After seven consecutive rolling passes, the strip thickness was 3.0 mm. The reduction rates for the last three passes in the finishing mill were 21.8%, 17.7%, and 17.9%, respectively, for a total reduction rate of 47.1%. The final rolling temperature was 875℃.

[0085] S3, Cooling and winding

[0086] After finishing, the steel strip enters the laminar flow zone and is cooled intensively starting from the 3rd group; the cooled steel strip is then coiled into steel coils at a coiling temperature of 600℃.

[0087] Samples were taken for mechanical property testing. The yield strength was 266 MPa, the tensile strength was 340 MPa, and the elongation after fracture was 43.0%. Metallographic results are as follows: Figure 3 As shown, the microstructure of the strip is ferrite and pearlite, with a grain size of 8.5 and no mixed crystals.

[0088] For several key control points of the present invention, comparative examples were set up to verify product performance. The following comparative examples are all comparative examples of Example 3, and each comparative example involves changes to a single control item. Except for the control items specified in the comparative examples, other control parameters remain unchanged. However, due to uncontrollable factors in the actual smelting process, it is impossible to completely reproduce the results between batches of molten steel; fluctuations in the elemental content of the molten steel are normal.

[0089] Comparative Example 1

[0090] This comparative example is the comparative example of Example 3, used to verify the effect of the relative contents of TiN and B on the microstructure of low-carbon boron-containing steel.

[0091] The production method of this comparative example includes the following steps:

[0092] S1, Steel smelting

[0093] The final temperature of the converter smelting was 1650℃, with the final C content being 0.04%, Mn 0.10%, S 0.015%, P 0.018%, and N 0.0023%. During the tapping process, 4 kg / t of slag-forming material, 1.48 kg / t of medium-carbon ferromanganese, and 1.2 kg / t of aluminum-containing material were added.

[0094] During the argon blowing process, 0.59 kg / t of ferrotitanium and 0.29 kg / t of ferroboron were added. The final composition, as determined by testing, was: C: 0.04%, Mn: 0.20%, S: 0.015%, P: 0.018%, Si: 0.01%, Al: 0.036%, Ti: 0.015%, B: 0.0026%, N: 0.0029%.

[0095] The content of TiN and B in the molten steel was verified. ω(TiN) / (ω(B)-0.0005%)=4.4*0.0029%÷(0.0026%-0.0005%)=6.08<7.56, which does not meet the requirements of this invention for the relative ratio of the two.

[0096] S2, Continuous casting and rolling

[0097] The tundish temperature was controlled within the range of 1545℃, the casting speed was controlled at 1.4m / min, and the billet quality was normal.

[0098] The heating furnace was homogenized at 1275℃ for 110 minutes. After descaling, the strip entered the roughing mill at an initial rolling temperature of 1100℃. After five passes in the roughing mill, it entered the finishing mill at an inlet temperature of 1010℃. After seven consecutive rolling passes, the strip thickness was 3.0 mm. The reduction rates for the last three passes in the finishing mill were 21.8%, 17.7%, and 17.9%, respectively, for a total reduction rate of 47.1%. The final rolling temperature was 875℃.

[0099] S3, Cooling and winding

[0100] After finishing, the steel strip enters the laminar flow zone and is cooled intensively starting from the 3rd group; the cooled steel strip is then coiled into steel coils at a coiling temperature of 600℃.

[0101] Samples were taken for mechanical property testing. The yield strength was 276 MPa, the tensile strength was 352 MPa, and the elongation after fracture was 36.5%. Metallographic results are as follows: Figure 4 As shown, the microstructure of the strip is ferrite and pearlite, with a grain size of 5.0 (20%)-11.0, and is a mixed crystal.

[0102] Comparative Example 2

[0103] This comparative example is the comparative example of Example 2, used to verify the effect of the final pass reduction rate on the microstructure of low-carbon boron-containing steel. All other control factors are within the scope of this invention.

[0104] The production method of this comparative example includes the following steps:

[0105] S1, Steel smelting

[0106] The final temperature of the converter smelting was 1650℃, with the final composition as follows: C: 0.04%, Mn: 0.10%, S: 0.015%, P: 0.018%, N: 0.0026%. During tapping, 4 kg / t of slag-forming material, 1.48 kg / t of medium-carbon ferromanganese, and 1.2 kg / t of aluminum-containing material were added. Ferrotitanium and ferroboron were added during argon blowing. The final composition was: C: 0.04%, Mn: 0.20%, S: 0.015%, P: 0.018%, Si: 0.01%, Al: 0.036%, Ti: 0.018%, B: 0.0022%, N: 0.0035%.

[0107] The content of TiN and B in the molten steel was verified. ω(TiN) / (ω(B)-0.0005%)=4.4*0.0035%÷(0.0022%-0.0005%)=9.06>7.56, which meets the requirements of this invention for the relative ratio of the two.

[0108] S2, Continuous casting and rolling

[0109] The tundish temperature was controlled within the range of 1545℃, the casting speed was controlled at 1.4m / min, and the billet quality was normal.

[0110] The heating furnace was homogenized at 1275℃ for 110 minutes. After descaling, the strip entered the roughing mill at an initial rolling temperature of 1100℃. After five passes in the roughing mill, it entered the finishing mill at an inlet temperature of 1010℃. After seven consecutive rolling passes, the strip thickness was 3.0 mm. The reduction rates for the last three passes in the finishing mill were 17.3%, 16.0%, and 8.9%, respectively, for a total reduction rate of 36.75%. The final rolling temperature was 875℃.

[0111] S3, Cooling and winding

[0112] After finishing, the steel strip enters the laminar flow zone and is cooled intensively starting from the 3rd group; the cooled steel strip is then coiled into steel coils at a coiling temperature of 600℃.

[0113] Samples were taken for mechanical property testing. The yield strength was 278 MPa, the tensile strength was 350 MPa, and the elongation after fracture was 37.0%. Metallographic results are as follows: Figure 5 As shown, the microstructure of the strip is ferrite and pearlite, with a grain size of 6.0 (30%)-13.0, and is a mixed crystal.

[0114] Comparative Example 3

[0115] This comparative example is the comparative example of Example 2, used to verify the influence of finishing rolling start temperature and finishing rolling temperature on the microstructure of low carbon boron-containing steel. All other control factors are within the scope of this invention.

[0116] The production method of this comparative example includes the following steps:

[0117] S1, Steel smelting

[0118] The final temperature of the converter smelting was 1650℃, with the final composition as follows: C: 0.04%, Mn: 0.10%, S: 0.015%, P: 0.018%, N: 0.0026%. During tapping, 4 kg / t of slag-forming material, 1.48 kg / t of medium-carbon ferromanganese, and 1.2 kg / t of aluminum-containing material were added. Ferrotitanium and ferroboron were added during argon blowing. The final composition was: C: 0.04%, Mn: 0.20%, S: 0.015%, P: 0.018%, Si: 0.01%, Al: 0.036%, Ti: 0.018%, B: 0.0022%, N: 0.0035%.

[0119] The content of TiN and B in the molten steel was verified. ω(TiN) / (ω(B)-0.0005%)=4.4*0.0035%÷(0.0022%-0.0005%)=9.06>7.56, which meets the requirements of this invention for the relative ratio of the two.

[0120] S2, Continuous casting and rolling

[0121] The tundish temperature was controlled within the range of 1545℃, the casting speed was controlled at 1.4m / min, and the billet quality was normal.

[0122] The heating furnace was homogenized at 1275℃ for 110 minutes. After descaling, the strip entered the roughing mill at an initial rolling temperature of 1100℃. After five passes in the roughing mill, it entered the finishing mill at an inlet temperature of 1010℃. After seven consecutive rolling passes, the strip thickness was 3.0 mm. The reduction rates for the last three passes in the finishing mill were 21.8%, 17.7%, and 17.9%, respectively, for a total reduction rate of 47.1%. The final rolling temperature was 895℃.

[0123] S3, Cooling and winding

[0124] After finishing, the steel strip enters the laminar flow zone and is cooled intensively starting from the 3rd group; the cooled steel strip is then coiled into steel coils at a coiling temperature of 600℃.

[0125] Samples were taken for mechanical property testing. The yield strength was 275 MPa, the tensile strength was 355 MPa, and the elongation after fracture was 36.5%. Metallographic results are as follows: Figure 6 As shown, the microstructure of the strip is ferrite and pearlite, with a grain size of 7.0 (30%)-12.0, and is a mixed crystal.

[0126] As can be seen from the above comparative examples, Examples 1 to 3 using the method of the present invention showed no mixed crystals, and the grain size was above grade 8.5, indicating that the mixed crystal situation in the microstructure was effectively controlled; while Comparative Examples 1 to 3 all showed mixed crystals to varying degrees. Mechanical testing results showed that the steel products of Examples 1 to 3 had better toughness, and their tensile strength and elongation after fracture were significantly better than those produced by Comparative Examples 1 to 3, proving that the method of the present invention can effectively control the mixed crystal situation in the microstructure of low-carbon boron-containing steel.

[0127] Comparative Examples 1 to 3 demonstrate that technical characteristics such as the relative content of Ti and B in the molten steel, the inlet and final rolling temperatures, and the reduction rate in the three passes after finishing rolling all directly affect the crystallization of the final low-carbon boron-containing steel product. Any change in these technical characteristics will lead to crystallization in the product. This further illustrates that the method of this invention is a unified control method from smelting to rolling, possessing continuity and interrelationship. The control of rolling parameters is based on the control of the molten steel composition; the smelting and rolling processes are mutually influential and inseparable.

[0128] The above description is only a preferred embodiment of the present invention. All equivalent changes and modifications made within the scope of the claims of the present invention should be included in the scope of the present invention.

Claims

1. A method for producing low-carbon boron-containing steel with suppressed mixed crystal structure, comprising three steps: steelmaking, continuous casting and rolling, and cooling and coiling, characterized in that: In the steelmaking process, after the steel is tapped, ferrotitanium and ferroboron are added to the molten steel. The amount of ferrotitanium and ferroboron added is determined by the following formula: The content of Ti satisfies the following relationship: 3.4ω(N)≤ω(Ti)≤0.025%. The content of B satisfies the following relationship: 0.0010%≤ω(B)≤ω(TiN) / 7.56+0.0005%, where ω(TiN)=4.4ω(N); In the formula, ω(N) = nitrogen content in molten steel at the end of smelting + nitrogen increase during the process; After adding ferrotitanium and ferroboron, the composition of the molten steel is tested to verify whether the relationship between ω(TiN) and ω(B) satisfies ω(TiN) / (ω(B)-0.0005%)≥7.56, where ω(TiN)=4.4ω(N); if not, continue to add appropriate amounts of ferrotitanium or ferroboron until the relationship between ω(TiN) and ω(B) meets the formula requirements; In the continuous casting and rolling process, the initial rolling temperature of the finishing rolling is 1000℃-1100℃, and the final rolling temperature is controlled at 860℃-880℃. For the seven-pass finishing rolling, the reduction rate of the last pass is controlled at 15%-18%, and the total reduction rate of the last three passes is greater than 45%.

2. The method for producing low-carbon boron-containing steel with suppressed mixed crystal structure according to claim 1, characterized in that: The low-carbon boron-containing steel comprises the following components by mass percentage: C: 0.03%-0.06%, Mn: 0.15%-0.30%, S≤0.025%, P≤0.025%, Si≤0.03%, Al: 0.020%-0.055%, Ti≤0.025%, B: 0.0010%-0.0030%, N: 0.0020%-0.0050%, with the balance being iron and unavoidable impurities.

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