Thin-gauge high-toughness Q355-grade hot-rolled structural steel plate and manufacturing method thereof

Abstract

The invention discloses a thin-gauge high-toughness Q355-grade hot-rolled structural steel plate and a manufacturing method thereof, and belongs to the technical field of steel production. The steel plate is prepared from, by mass, 0.14%-0.17% of C, 0.2%-0.4% of Si, 0.9%-1.3% of Mn, 0.010%-0.025% of Ti, 0.020%-0.040% of Al, smaller than or equal to 0.006% of N, smaller than or equal to 0.005% of S and the balance Fe and inevitable impurities, and Ti / Ngt is contained. And 3.42). The thickness of the steel plate is 6-30 mm, the typical microstructure is ferrite and pearlite (the ferrite grain grade is larger than or equal to 9 grade), the Charpy impact value at the temperature of-40 DEG C is larger than or equal to 50 J, and no crack exists after 180-degree three-point bending (the diameter of a pressure head is equal to the thickness of the steel plate). The manufacturing method comprises the steps of vacuum smelting, casting blank heating, controlled rolling and controlled cooling, large-size harmful precipitated phases are avoided, crystal grains are refined, the structure uniformity is improved, balance of high performance and low cost is achieved by precisely controlling the component proportion and technological parameters, and the high-performance low-cost steel is suitable for structural parts such as engineering machinery, bridges and buildings.

Description

Technical Field

[0001] This invention belongs to the field of steel production technology, and specifically relates to a thin-gauge high-toughness Q355 grade structural steel plate and its manufacturing method. It is applicable to the manufacturing of structural components with high strength, high and low temperature toughness and good weldability in fields such as engineering machinery, bridges, buildings and machinery manufacturing. Background Technology

[0002] Q355 grade structural steel, with its excellent strength, toughness, and weldability, is widely used in engineering machinery, bridges, buildings, and other structures. In recent years, with the rapid development of China's steel industry, overcapacity has become a prominent issue, and homogeneous competition has intensified, especially for low-alloy Q355 series medium and heavy plates, which have the largest application volume. To reduce costs and improve market competitiveness, some steel mills have even completely removed microalloying elements such as Nb, V, and Ti, relying entirely on "water-based alloying"—increasing the intensity of controlled rolling and cooling—to compensate for insufficient strength. This easily leads to excessive differences in the surface and central microstructure of the steel plate, resulting in a significant decrease in toughness, typically only meeting the minimum requirements of national standards.

[0003] Patent CN116694993A discloses "A Low-Cost Q355D Steel Plate and Its Production Method," which significantly reduces the Mn content to below 0.8% through a low-manganese, micro-niobium composition design and the addition of trace amounts of niobium alloy. This replaces a large amount of inexpensive alloy with a small amount of precious alloy, thus lowering the overall cost. This process adds the precious alloying element Nb to basic steel in the 6-30mm specification range, greatly increasing Nb consumption.

[0004] Patent CN110129652A discloses "A Low-Manganese Microalloyed Q355 Structural Steel and Its Preparation Process," which replaces Mn with Ti, adding 0.04~0.050% Ti to reduce the Mn content to below 1.0%. Through a reasonable controlled rolling and cooling process, a Q355 steel plate meeting the standards is obtained. However, the excessive addition of Ti in this process easily causes large-sized TiN to precipitate from the liquid phase during the solidification process of the cast billet, significantly reducing the low-temperature toughness of the steel plate.

[0005] Grain size and microstructure uniformity are two key factors affecting the low-temperature toughness of steel plates. Existing technologies refine grains and improve low-temperature toughness by microalloying with Nb, increasing rolling intensity, and lowering the final cooling temperature. However, these methods often result in high alloy costs, low rolling efficiency, and poor plate shape due to low final cooling temperatures. Furthermore, the microstructure inhomogeneity caused by banded pearlite is particularly pronounced in steel plates with a diameter ≤15mm, leading to poor low-temperature impact toughness in thin-gauge steel plates. Existing technologies often employ low-carbon content (less than 0.1%) compositions to weaken banded microstructure and improve low-temperature impact toughness, but this increases costs. This invention, addressing the need for high toughness and low cost, provides a micro-Ti alloy-designed, inherently fine-grained steel and its manufacturing process, resulting in thin-gauge Q355 grade hot-rolled structural steel plates that meet the -40℃ low-temperature impact toughness requirements. Summary of the Invention

[0006] The technical problem to be solved by the present invention is to provide a thin-gauge high-toughness Q355 grade structural steel plate with reasonable composition design, simple process and controllable cost, and its manufacturing method, so as to achieve a balance between high performance and low cost.

[0007] The technical solution adopted by the present invention to solve the above problems is as follows: a thin-gauge, high-toughness Q355 grade structural steel plate, wherein the composition of the steel plate by mass percentage includes C: 0.14%~0.17%, Si: 0.2%~0.4%, Mn: 0.9%~1.3%, Ti: 0.010%~0.025%, Al: 0.020~0.040%, N: ≤0.006%, S≤0.005%, and the balance being Fe and unavoidable impurities, wherein the Ti / N ratio is >3.42.

[0008] This invention achieves grain refinement, microstructure homogenization, and performance improvement through the synergistic control of various elements. The design basis for each element is as follows: Carbon (C): 0.14%~0.17% : Carbon is a core strengthening element. An appropriate amount of C ensures the strength of the steel plate, but too high a concentration (>0.17%) will worsen weldability, increase water-cooling sensitivity, and lead to uneven microstructure. Too low a concentration (<0.14%) will fail to meet the strength requirements of Q355 grade, requiring additional alloying and increasing costs. Therefore, a medium carbon range of 0.14%~0.17% is chosen to balance strength and weldability.

[0009] Si: 0.2%~0.4%: As a deoxidizing element, Si can effectively remove oxygen from molten steel and enhance the strength of ferrite through solid solution strengthening. When the Si content is below 0.2%, the deoxidation effect is insufficient, and when it is above 0.4%, it will increase the brittleness of the steel plate and reduce the impact toughness. Therefore, this range is limited.

[0010] Mn: 0.9%~1.3%: Mn is a common solid solution strengthening element and the substitutional solid solution element that has the most drastic effect on Ar3 temperature. Because Mn has a large segregation coefficient and a low diffusion coefficient, dendritic microsegregation will lead to a decrease in Ar3, and through its influence on the carbon activity coefficient, it will cause carbon segregation, ultimately exacerbating the formation of inhomogeneous banded pearlite structure. Simultaneously, the decrease in Mn content increases the driving force for the transformation of austenite to ferrite during post-rolling cooling, which is beneficial for grain refinement. Furthermore, studies have shown that when the Mn content is above 1.3%, the degree of Mn microsegregation increases significantly; therefore, the upper limit of Mn is specified as 1.3%. To ensure the strength of the steel plate, the lower limit of Mn is designed to be 0.9%. The optimal Mn content is 1.0%~1.3%.

[0011] Ti: 0.010%~0.025%: Ti's deoxidizing ability is second only to aluminum, and it has a strong ability to bond with nitrogen. The formation of TiN reduces the precipitation of AlN at grain boundaries, preventing grain boundary embrittlement and avoiding hot cracking of the cast billet. Simultaneously, TiN can precipitate during solidification or within austenite. TiN particles distributed on austenite grain boundaries strongly hinder the migration of austenite grains, thereby refining the initial austenite grains. However, when Ti and N exceed their solid solubility product, large-sized square TiN particles easily precipitate in the liquid state, deteriorating impact performance. Therefore, the upper limit for Ti is specified as 0.025%, and the upper limit for N is 0.006%. Ti can also combine with S to form Ti4C2S2, and their large size also affects the toughness of the steel. To ensure the formation of a sufficient amount of TiN, the lower limit for Ti is specified as 0.01%, while the S content is less than 0.005%, resolving the contradiction between low cost and high quality.

[0012] Al: 0.020~0.040%: Al is a strong deoxidizing element that can form Al2O3 inclusions and promote the precipitation of TiN. When the Al content is below 0.020%, deoxidation is insufficient, and when it is above 0.040%, coarse Al2O3 inclusions will be formed, affecting toughness. Therefore, this range is limited.

[0013] N: ≤0.006%: N is a harmful element. Excessive N will combine with Ti to form large-sized TiN, and when it exists alone, it will cause aging embrittlement of the steel plate. This invention controls N to ≤0.006% and ensures that all N combines with Ti to form fine TiN by using a Ti / N ratio of >3.42, thus avoiding the harm of free N.

[0014] S: ≤0.005%: S combines with Ti to form TiS or Ti4C2S2. These inclusions are relatively large and will reduce the toughness of the steel plate. Therefore, S should be strictly controlled to ≤0.005%, preferably ≤0.002%.

[0015] Another objective of this application is to provide a method for manufacturing thin-gauge, high-toughness Q355 grade structural steel plates, comprising the following process steps: vacuum smelting – billet heating – controlled rolling – controlled cooling. The manufacturing method specifically includes the following steps: Step 1: Vacuum smelting 1) Hot metal KR pretreatment: Mechanical stirring desulfurization is adopted to control the sulfur content of hot metal to below 0.03%, laying the foundation for subsequent refining to reduce sulfur content; 2) Converter smelting and LF refining: Converter smelting removes impurities such as P and C, and LF furnace performs deoxidation and alloying. The composition is adjusted by feeding aluminum wire and ferrotitanium to ensure that the S content in the molten steel is less than 0.005%. Argon gas is blown throughout the converter and ladle bottom blowing process, and nitrogen gas is not blown to avoid nitrogen increase, ensuring that N≤0.006%.

[0016] 3) Continuous casting: Low superheat casting (superheat 15-30℃) is adopted to reduce component segregation; the continuous casting billet specifications are 150mm thick and ≤3100mm wide, which is compatible with existing hot rolling production lines.

[0017] Step 2: Heating the billet The furnace temperature should not be lower than 500℃, preferably 550~650℃ (high-temperature hot delivery) to avoid cracking caused by excessive temperature difference in the billet; the heating temperature is 1080~1150℃, which is more than 50℃ lower than the conventional process (1150~1200℃), which reduces energy consumption and avoids coarsening of austenite grains; the holding time is 2-6 hours to ensure that the billet composition is homogeneous and that elements such as Ti and N are fully dissolved.

[0018] Step 3: Controlled rolling 1) Rough rolling: High temperature and large reduction process is adopted, with an opening temperature of 1020~1120℃ and a single-pass reduction rate of ≥10%. The large reduction breaks down the coarse austenite grains of the billet, creating conditions for subsequent fine rolling to refine the grains. For steel plates with a thickness of ≥15mm, a waiting process is set after rough rolling. The waiting thickness is more than 1.8 times the thickness of the finished product to ensure uniform temperature during fine rolling. 2) Finish rolling: The final rolling temperature is 750~800℃. This temperature range is in the non-recrystallized austenite region. The austenite grains are refined through finish rolling deformation. At the same time, the final rolling temperature is avoided from being too high (greater than 800℃) which would cause grain coarsening, or too low (<750℃) which would cause excessive rolling force and poor plate shape.

[0019] Step 4: Controlling Cooling 1) Cooling method: Laminar flow accelerated cooling is adopted, which has good cooling uniformity and avoids uneven tissue caused by localized excessively rapid cooling; 2) Cooling rate and final cooling temperature: Cooling rate 3~15℃ / s, final cooling temperature 620~680℃; when the steel plate thickness is 6~15mm, the cooling rate is 3~8℃ / s, and when the steel plate thickness is 15mm~30mm, the cooling rate is 8~15℃ / s. This quickly inhibits austenite grain growth and ensures that the austenite transforms into ferrite + pearlite. The final cooling temperature is higher than 600℃ to avoid the formation of hard and brittle structures such as bainite and improve toughness.

[0020] This invention significantly reduces the banded structure caused by elemental segregation by reducing the content of easily segregating manganese to below 1.3%, thereby improving the uniformity of the structure and solving the problem of producing thin-gauge, high-toughness low-alloy structural steel in medium-carbon systems. It achieves a balance between high performance and low cost. At the same time, it rationally controls the content of Ti, N, and S to avoid the precipitation of large-sized TiN in the liquid state, which would deteriorate the low-temperature toughness. It also utilizes the solid precipitation of TiN to refine the initial austenite grains and further obtains a ferrite structure with finer grain size through controlled cooling technology.

[0021] Compared with the prior art, the present invention has the following beneficial effects: 1. Excellent performance: The steel plate thickness is 6~30mm, the ferrite grain grade is ≥9 (up to 10~11), the Charpy impact value at -40℃ is ≥50J (up to 155J), far exceeding the requirements of GB / T 1591-2018 standard; no cracks are found in 180° three-point bending (indenter diameter = steel plate thickness =), the welding performance is good (carbon equivalent CEV≤0.42%), and it can adapt to low temperature and high stress conditions.

[0022] 2. Controllable cost: No need to add precious alloys such as Nb and V, reasonable Mn usage, no need to modify existing equipment for smelting and rolling processes, and the overall production cost is reduced by 8% to 15% compared with existing microalloying schemes.

[0023] 3. Strong process adaptability: The process path is fully compatible with existing hot rolling production lines, enabling large-scale production with high efficiency (rolling cycle is shortened by 5%~10% compared to conventional processes), good plate shape (flatness ≤3mm / m), and no additional straightening process required.

[0024] 4. Good uniformity of structure: By controlling Mn segregation and refining TiN grains, the steel plate has no obvious banded pearlite structure, the performance fluctuation is small (impact value fluctuation of the same batch ≤ ±30J), and the product qualification rate is ≥99%. Attached Figure Description

[0025] Appendix Figure 1 Example 1-1 shows the typical metallographic structure of a 6mm thick Q355E steel plate.

[0026] Appendix Figure 2Examples 1-2 show typical metallographic structures of 28mm thick Q355E steel plates.

[0027] Appendix Figure 3 Example 2-1 shows the typical metallographic structure of a 15mm thick Q355E steel plate. Detailed Implementation

[0028] The technical solution of the present invention will be described in more detail below with reference to preferred embodiments. However, these embodiments are merely descriptions of preferred implementations of the present invention and should not be construed as limiting the scope of the present invention.

[0029] The core of the specific implementation of this invention lies in the preparation of thin-gauge, high-toughness Q355E grade hot-rolled structural steel plates through precise composition design and optimized production process. The implementation process and effects are described in detail below with reference to specific embodiments, comparative examples and relevant test data.

[0030] I. Core Design Basis (I) Principles of Ingredient Design Based on the core requirements of "low cost + high toughness", the chemical composition of the steel plate was determined as follows (mass percentage): C: 0.14%~0.17%, Si: 0.25%~0.35%, Mn: 1.0%~1.3%, Ti: 0.010%~0.020%, Al: 0.020~0.040%, N: ≤0.005%, S≤0.002%, with the balance being Fe and unavoidable impurities, and the Ti / N ratio strictly controlled to >3.42. This design weakens the banded structure by reducing the content of easily segregated Mn (≤1.3%), rationally controls the content of Ti, N, and S to avoid large-sized precipitates from deteriorating toughness, and utilizes TiN solid precipitation to refine the grains without the need to add expensive alloys such as Nb, thus achieving a balance between cost and performance.

[0031] (II) Process Design Principles The entire process of "vacuum smelting - billet heating - controlled rolling - controlled cooling" is adopted. The core optimization directions are: reducing the heating temperature to reduce grain coarsening, using high temperature and high pressure to improve the uniformity of the microstructure, and precisely controlling the final rolling and final cooling temperatures to ensure the phase transformation refinement effect. Finally, a uniform microstructure of ferrite + pearlite is obtained, and the ferrite grain grade is ≥9.

[0032] II. Specific Implementation Parameters (a) Composition of test materials (Table 1) Three groups of test subjects were selected. Examples 1 and 2 were Q355E steel plates conforming to the design of this invention, and Comparative Example 1 was a Q355 grade steel plate produced by conventional processes. The specific chemical composition is as follows: Table 1 shows the main chemical components (wt%) of typical embodiments and comparative examples of the present invention.

[0033]

[0034] Note: Examples 1 and 2 both meet the design requirement of Ti / N>3.42. Comparative Example 1 did not add Ti element, and the Mn content exceeded the limit of this invention (>1.3%).

[0035] (ii) Production process parameters (Table 2) All test objects used 150mm thick continuously cast billets, which were fed into the furnace using a high-temperature hot-feeding method. The specific process steps and parameters are as follows: Vacuum smelting: Molten iron is pre-treated by KR for pre-desulfurization (S<0.03%), and after converter smelting + LF deoxidation and alloying, S is controlled to be <0.005%. Argon is blown throughout the bottom blowing of the converter and ladle to avoid nitrogen increase. The width of the continuously cast billet is ≤3100mm.

[0036] Billet heating: furnace temperature 500~600℃, walking beam furnace temperature 1080-1150℃, holding time ≥120min, which is more than 50℃ lower than the conventional heating temperature, reducing austenite grain coarsening.

[0037] Controlled rolling: After high-pressure water descaling, the rough rolling temperature is 1045-1072℃, and a high-temperature and high-reduction process is adopted, with a single-pass reduction rate of ≥10%; the finishing rolling temperature is 850-910℃ (thick specifications need to wait for warming), and the final rolling temperature is ≥750℃.

[0038] Controlled cooling: Laminar flow accelerated cooling is adopted after rolling, with an inlet water temperature of 750-768℃, a final cooling temperature (reddening temperature) of 636-670℃, and a cooling rate of 2-15℃ / s. The cooling rate range is adjusted according to the steel plate specifications.

[0039] Detailed process parameter comparison table: Table 2 shows the main process parameters of typical embodiments and comparative examples of the present invention.

[0040]

[0041] Note: Thin specifications such as 6mm and 8mm do not require preheating and can be directly rolled into the finishing mill; medium and thick specifications such as 28mm and 15mm require preheating to adjust the temperature field and ensure rolling uniformity.

[0042] III. Performance Verification Results (I) Microstructure Analysis The typical metallographic structures of Examples 1-1 (6 mm thick), 1-2 (28 mm thick), and 2-1 (15 mm thick) are all ferrite + pearlite (see Figures 1-3), with almost no banded structure. The ferrite grain grades reach 11, 10, and 11 respectively, all meeting the design requirement of ≥9 grade. In contrast, the ferrite grain grade of Comparative Example 1 is only 8.5 grade, and there is obvious banded pearlite structure.

[0043] Verification of TiN precipitation behavior: In the examples, the ratio of Ti to N was controlled between 3.75 and 5.33. No large-sized square TiN precipitation occurred in the liquid state. TiN was mainly distributed in the form of fine particles at the austenite grain boundaries, which effectively hindered grain boundary migration, refined the initial austenite grains, and laid the foundation for subsequent phase transformation refinement.

[0044] (ii) Mechanical property testing (Table 3) Mechanical properties were tested according to GB / T 1591-2018 standard, and the results are as follows: Table 3 shows the mechanical properties of typical embodiments and comparative examples of the present invention.

[0045]

[0046] Note: The impact test of 6mm and 8mm steel plates used small specimens (5×10×55mm), with the thickness halved; the impact value of the example is the test result of 3 sets of parallel specimens.

[0047] Performance analysis conclusions: Strength meets the standard: The yield strength (378-401MPa) and tensile strength (518-521MPa) of all embodiments meet the requirements of Q355 grade steel standard.

[0048] Excellent low-temperature toughness: The Charpy impact values ​​of the examples at -40℃ were all ≥48J, far exceeding the design requirements (≥50J, and the 6mm thin specification still reached 48J due to the halving of the sample size, which is close to the standard), and they still maintained high impact toughness at -60℃ (average value 33-119J); while the impact value of Comparative Example 1 at -40℃ was only 24J, which only met the minimum standard, and the difference in low-temperature toughness was significant.

[0049] Good bending performance: All embodiments were subjected to a 180° three-point bending test with a bending indenter of the same diameter as the steel plate thickness, and no cracks were found on the surface, indicating that the plasticity met the standard.

[0050] As can be seen from the embodiments, the mechanical properties of the Q355E involved in this invention fully meet the requirements of GB / T 1591-2018. Figures 1-2 - Figure 3The internal microstructures of the Q355E steel plates obtained in Examples 1-1, 1-2 and 2-1 are respectively. The microstructures are typical ferrite and pearlite structures, and there are almost no banded structures.

[0051] The above are merely specific embodiments of the present invention, but the scope of protection of the present invention is not limited thereto. Any variations or substitutions that can be easily conceived by those skilled in the art within the technical scope disclosed in the present invention should be included within the scope of protection of the present invention. Therefore, the scope of protection of the present invention should be determined by the scope of the claims.

Claims

1. A thin-gauge, high-toughness Q355 grade hot-rolled structural steel plate, characterized in that, The chemical composition of the steel plate, by mass percentage, includes: C: 0.14%~0.17%, Si: 0.2%~0.4%, Mn: 0.9%~1.3%, Ti: 0.010%~0.025%, Al: 0.020~0.040%, N: ≤0.006%, S≤0.005%, with the balance being Fe and unavoidable impurities, and Ti / N>3.

42.

2. The thin-gauge, high-toughness Q355 grade hot-rolled structural steel plate according to claim 1, characterized in that, The thickness of the steel plate is 6~30mm; the Charpy impact value of the steel plate at -40℃ is ≥50J, and no cracks appear on the surface when subjected to a 180° three-point bending test with a bending indenter of the same diameter as the steel plate thickness.

3. The thin-gauge, high-toughness Q355 grade hot-rolled structural steel plate according to claim 1, characterized in that, The microstructure of the steel plate is ferrite + pearlite, and the ferrite grain grade is ≥9.

4. The thin-gauge, high-toughness Q355 grade hot-rolled structural steel plate according to claim 1, characterized in that, The preferred range of the chemical composition is: C: 0.15%~0.16%, Si: 0.25%~0.35%, Mn: 1.0%~1.2%, Ti: 0.015%~0.020%, Al: 0.025~0.035%, N: ≤0.005%, S≤0.002%.

5. A method for manufacturing a thin-gauge, high-toughness Q355 grade hot-rolled structural steel plate as described in any one of claims 1 to 4, characterized in that, The process includes the following steps: (1) Vacuum smelting: The molten iron is pre-desulfurized by KR pretreatment, with S≤0.03%. After converter smelting and LF deoxidation and alloying, S≤0.005% is controlled. Argon gas is blown throughout the bottom blowing of the converter and ladle. Continuous casting yields billets with a thickness of 150mm and a width of ≤3100mm. (2) Heating of billet: The billet temperature is ≥500℃ when it enters the furnace, the heating temperature is 1080~1150℃, and the holding time is 2~6h; (3) Controlled rolling: including roughing and finishing rolling. The roughing process adopts a high temperature and large reduction process, with a single-pass reduction rate of ≥10%. The roughing rolling start temperature is 1020~1120℃; the finishing rolling finish temperature is 750~800℃. (4) Controlled cooling: Laminar flow accelerated cooling is adopted, with a cooling rate of 3~15℃ / s and a final cooling temperature of 620~680℃. After cooling, a ferrite + pearlite structure is obtained.

6. The manufacturing method according to claim 5, characterized in that, In step (2), the billet is hot-delivered at high temperature, with a furnace temperature of 500~700℃ and a holding time of ≥120min.

7. The manufacturing method according to claim 5, characterized in that, In step (3), for steel plates with a thickness ≥ 15 mm, a waiting-heating process is set after rough rolling, and the waiting-heating thickness is more than 1.8 times the thickness of the finished product.

8. The manufacturing method according to claim 5, characterized in that, In step (4), when the steel plate thickness is 6~15mm, the cooling rate is 3~8℃ / s, and when the steel plate thickness is 15mm~30mm, the cooling rate is 8~15℃ / s to ensure temperature uniformity.

9. The manufacturing method according to claim 5, characterized in that, In step (1), the continuous casting process adopts low superheat casting, and the superheat is controlled at 15~30℃ to reduce the segregation of the billet composition.

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