Method for determining solid solution temperature required for heating high-titanium microalloy high-strength structural steel slab

Abstract

This invention discloses a method for determining the solution treatment temperature required for heating high-titanium microalloyed high-strength structural steel slabs, relating to the field of hot rolling technology. The method includes the following steps: S1, calculating the solution treatment temperature of niobium-titanium carbide; S2, normalizing the solution treatment temperature calculated in step S1 to obtain a normalization temperature T1; S3, using T1 as the center temperature and with a gradient of 10℃, setting several adjustment temperatures to heat and solution treat the high-titanium microalloyed high-strength structural steel slabs, followed by rolling them into hot-rolled coils. The microalloying elements in the high-titanium microalloyed high-strength structural steel include Ti. This invention establishes a method for determining the solution treatment temperature required for heating high-titanium microalloyed high-strength structural steel slabs. This method determines the most reasonable solution treatment temperature required for hot rolling of high-titanium microalloyed high-strength structural steel slabs, which is beneficial for fully utilizing the strengthening effect of microalloying elements, achieving economical and efficient production while ensuring that the product strength meets requirements.

Description

Technical Field

[0001] This invention belongs to the field of hot rolling technology, specifically a method for determining the solution treatment temperature required for heating high-titanium microalloyed high-strength structural steel slabs. Background Technology

[0002] Qualified molten steel is continuously cast into billets using a continuous casting machine. These billets are then heated and rolled into finished products, which is the basic production method for hot-rolled steel products in related technologies. However, during the cooling process of the continuously cast slab, the slow cooling rate causes microalloyed carbonitrides to precipitate in a near-equilibrium state. Generally, the size of the microalloyed carbonitrides precipitated in undeformed austenite is typically 50nm to 100nm. These microalloyed carbonitrides do not produce a significant strengthening effect. Therefore, these coarse precipitates of 50nm to 100nm need to be fully dissolved during the heating process before hot rolling of the slab to produce a strengthening effect during subsequent rolling and cooling. Thus, the solution treatment temperature of the slab is the most important factor affecting the strength of hot-rolled microalloyed high-strength structural steel.

[0003] In related technologies, the solubility product formula is used to calculate the solid solution temperature required in equilibrium for precipitates formed by microalloying elements (such as Ti and Nb) with carbon and nitrogen elements. However, the solubility product formulas derived from these technologies for the same microalloying elements and carbon and nitrogen elements are not the same, as follows:

[0004] For example, the formula for calculating the solid solubility product of TiC in austenite:

[0005] lg{[C]*[Ti]}=5.33-10475 / T…………………Phase Analysis Method (1);

[0006] lg{[C]*[Ti]}=2.75-7000 / T……………………Phase Analysis Method (2);

[0007] For example, the formula for calculating the solid solubility product of NbC in austenite:

[0008] lg{[C]*[Nb]}=2.90-7500 / T………………Hardness method (3);

[0009] lg{[C]*[Nb]}=3.04-7290 / T………………Phase Analysis Method (4);

[0010] lg{[C]*[Nb]}=3.70-9100 / T…………………Carbon analysis method (5);

[0011] lg{[C]*[Nb]}=3.42-7900 / T…………………Phase Analysis Method (6);

[0012] lg{[C]*[Nb]}=4.37-9290 / T…………………Carbon analysis method (7);

[0013] For example, the formula for calculating the solid solubility product of TiN in austenite:

[0014] lg{[Ti]*[N]}=3.94-15190 / T…………………Phase Analysis Method (8);

[0015] lg{[Ti]*[N]}=0.32-8000 / T…………………Phase Analysis Method (9).

[0016] As can be seen from the above calculation formulas, the theoretical method for calculating the solution temperature required for complete solid solution of microalloying element precipitates is not perfect. For composite precipitates such as Ti(NC) and Nb(NC), there is currently no universally accepted formula for calculating the equilibrium solubility product. Furthermore, the solution temperature required for slab heating in actual production is non-equilibrium, hence the difference between the two. Excessively high solution temperatures waste energy and adversely affect the toughness of the product, while excessively low temperatures fail to fully utilize the strengthening effect of microalloying elements, resulting in insufficient strength and failing to meet standards.

[0017] Therefore, the present invention provides a method for determining the solution treatment temperature required for heating high-titanium microalloyed high-strength structural steel slabs, in order to solve the problems mentioned in the background art. Summary of the Invention

[0018] The purpose of this invention is to provide a method for determining the solution treatment temperature required for heating high-titanium microalloyed high-strength structural steel slabs, so as to solve at least one aspect of the problems and defects mentioned in the background art.

[0019] Specifically, this invention provides a method for determining the solution treatment temperature required for heating high-titanium microalloyed high-strength structural steel slabs, comprising the following steps:

[0020] S1. Calculate the solution solution temperature of niobium titanium carbide;

[0021] The calculation method described in step S1 includes the following formula:

[0022] lg{[Ti+Nb]×[C]}=2.75-7000 / T;

[0023] In the formula: Ti represents the mass percentage content of Ti in the high-titanium microalloyed high-strength structural steel;

[0024] Nb represents the mass percentage content of Nb in the high-titanium microalloyed high-strength structural steel.

[0025] C represents the mass percentage content of C in the high-titanium microalloyed high-strength structural steel;

[0026] T represents the absolute solution temperature, and the unit is K;

[0027] Converting T to Celsius temperature yields the Celsius solid solution temperature of niobium titanium carbide;

[0028] S2. The Celsius solution temperature of niobium titanium carbide calculated in step S1 is normalized to obtain the normalization temperature T1.

[0029] The normalization process involves resetting the units digit and decimal part of the Celsius solution temperature in step S1 to zero.

[0030] If the units digit and the decimal part of the Celsius solution temperature mentioned in step S1 are both zero, then the normalization treatment temperature is the same as the solution temperature in step S1.

[0031] If the units or decimal part of the Celsius solution temperature mentioned in step S1 is non-zero, then the tens digit of the normalization temperature is increased by one, and the units and decimal parts are reduced to zero.

[0032] S3. Using T1 temperature as the center temperature and ±10℃ as the gradient, several adjustment solution temperatures are set to heat and solution treat the high-titanium microalloy high-strength structural steel slab. After heating and solution treatment, rough rolling and fine rolling are performed to obtain hot-rolled steel coils.

[0033] The heating and solution treatment time is 30 min to 90 min;

[0034] The solution treatment temperature at which the difference between the tensile strength at the middle position and the tensile strength at a position 7m from the tail end of the hot-rolled high-titanium microalloy high-strength structural steel coil is 5MPa to 15MPa is the solution treatment temperature required for heating the high-titanium microalloy steel slab.

[0035] The microalloying elements in the high-titanium microalloyed high-strength structural steel include Ti.

[0036] One of the technical solutions determined by the method of the present invention has at least the following beneficial effects:

[0037] In the study of the hot rolling process of high-Ti microalloyed steel (microalloying elements are Ti and other elements), this invention found that for high-Ti microalloyed high-strength structural steel with a given composition, since the mechanical property sampling point of the hot-rolled coil of high-Ti microalloyed high-strength structural steel (i.e., the hot-rolled steel coil in step S3) is located 7m from the end of the hot-rolled steel coil during production inspection, in order to ensure that the longitudinal temperature uniformity of the slab and the CT temperature uniformity meet the requirements, the slab heating solution temperature needs to make the tensile strength of the middle part of the hot-rolled coil (1 / 2 length of the steel coil) 5MPa to 15MPa greater than the tensile strength of the sampling point at the end, in order to ensure that the mechanical properties of the whole coil are qualified.

[0038] The following is an example of a regularization process:

[0039] If the calculation result in step S1 is >1250℃ but <1260℃, it is normalized to 1260℃. If the calculation result is strictly equal to 1260℃, it is still 1260℃.

[0040] For example, 1253.714℃ is regularized to 1260℃, and 1250.011℃ is regularized to 1260℃.

[0041] An example of the calculation method in the formula described in this invention is as follows:

[0042] lg{[Ti+Nb]×[C]}=2.75-7000 / T;

[0043] When the mass percentage of Ti is 0.12%, the mass percentage of Nb is 0.05%, and the mass percentage of C is 0.05%, then [Ti + Nb] = 0.12% * 100 + 0.05% * 100 = 0.17; that is, [Ti + Nb] is the product of the sum of the mass percentages of Ti and Nb and 100; [C] = 0.05% * 100 = 0.05; that is, [C] is the product of the mass percentage of C and 100.

[0044] According to some embodiments of the present invention, the middle position of the hot-rolled steel coil is halfway along the length of the hot-rolled steel coil.

[0045] According to some embodiments of the present invention, the microalloying elements in the high-titanium microalloyed steel further include at least one of Nb, V and Mo.

[0046] According to some embodiments of the present invention, the microalloying element in the high-titanium microalloyed steel is Ti.

[0047] According to some embodiments of the present invention, the microalloying elements in the high-titanium microalloyed steel are Ti and Nb.

[0048] According to some embodiments of the present invention, the microalloying elements in the high-titanium microalloyed steel are Ti, Nb and V.

[0049] According to some embodiments of the present invention, the microalloying elements in the high-titanium microalloyed steel are Ti, Nb and Mo.

[0050] According to some embodiments of the present invention, the mass fraction of Ti element in the high-titanium microalloyed steel is 0.06% to 0.2%.

[0051] According to some embodiments of the present invention, the mass fraction of Ti element in the high-titanium microalloyed steel is 0.12% to 0.15%.

[0052] According to some embodiments of the present invention, the mass fraction of Nb in the high-titanium microalloyed steel is 0.01% to 0.08%.

[0053] According to some embodiments of the present invention, the mass fraction of Nb in the high-titanium microalloyed steel is 0.06% to 0.08%.

[0054] According to some embodiments of the present invention, the mass fraction of V in the high-titanium microalloyed steel is ≥0.01%.

[0055] According to some embodiments of the present invention, the mass fraction of V in the high-titanium microalloyed steel is 0.01% to 0.02%.

[0056] According to some embodiments of the present invention, the mass fraction of carbon element in the high-titanium microalloyed steel is 0.03% to 0.12%.

[0057] According to some embodiments of the present invention, the mass fraction of nitrogen in the high-titanium microalloyed steel is ≤0.01%.

[0058] According to some embodiments of the present invention, the mass fraction of nitrogen in the high-titanium microalloyed steel is ≤0.006%.

[0059] According to some embodiments of the present invention, the mass fraction of nitrogen in the high-titanium microalloyed steel is 0.003% to 0.006%.

[0060] According to some embodiments of the present invention, the mass fraction of nitrogen in the high-titanium microalloyed steel is ≤0.005%.

[0061] According to some embodiments of the present invention, the mass fraction of Mn element in the high-titanium microalloyed steel is 0.2% to 2%.

[0062] According to some embodiments of the present invention, the mass fraction of Mn element in the high-titanium microalloyed steel is 1.8% to 2.0%.

[0063] Microalloyed hot-rolled high-strength structural steel can be produced by adding microalloying elements such as Ti, Nb, and V, either alone or in combination, to low-carbon alloy steel with a C mass fraction of 0.03%–0.12% and a Mn mass fraction of 0.2%–2%, followed by controlled rolling and cooling processes.

[0064] High-strength structural steel with added Nb, V and Mo elements can achieve tensile strength levels of over 900 MPa.

[0065] According to some embodiments of the present invention, the high-titanium microalloyed high-strength structural steel comprises the following elements by mass fraction:

[0066] C 0.03%–0.12%, Si 0.1%–0.7%, Mn 0.2%–2.0%, P ≤0.02%, S ≤0.02%, Al 0.02%–0.06%, Ti 0.06%–0.2%, Nb 0.01%–0.08%, V 0.01%–0.2%, N ≤0.01%, and the balance being Fe and unavoidable residual elements and impurities from the smelting process.

[0067] According to some embodiments of the present invention, for hot-rolled coil products where impact toughness is difficult to guarantee, the heating and solution treatment time in step S3 is 30 min to 60 min, so that the strength and toughness of the product meet the requirements.

[0068] According to some embodiments of the present invention, for products whose impact toughness is relatively easy to guarantee, the heating and solution treatment time in step S3 is 30 min to 90 min.

[0069] Controlling the above-mentioned solution treatment time is beneficial for production implementation and avoids the adverse effects on impact toughness that may result from excessively long solution treatment times.

[0070] According to some embodiments of the present invention, for products whose impact toughness is easy to guarantee, the solution treatment time in step S3 is controlled at ≥30 min to facilitate production.

[0071] According to some embodiments of the present invention, the heating and solution treatment time in step S3 is 30 min to 40 min, which is beneficial for the economical and efficient production of hot-rolled products.

[0072] Extending the solution treatment time from the base of 30 minutes will result in a slight increase in the strength of the hot-rolled coil, but the increase is very limited. For example, at a solution temperature of 1270℃, extending the solution treatment time from 30 minutes to 90 minutes will not increase the tensile strength by more than 11 MPa. Therefore, increasing strength by extending the solution treatment time is uneconomical, as it significantly increases energy consumption and reduces production line efficiency. Thus, a solution treatment time of 30-40 minutes is preferred. Excessive solution treatment time may also lead to a decrease in the impact toughness of the hot-rolled coil. For products where impact toughness is difficult to guarantee, the solution treatment time should be controlled at 30-60 minutes; for products where impact toughness is relatively easy to guarantee, the solution treatment time should be controlled at 30-90 minutes; and for products where impact toughness is easily guaranteed, the solution treatment time should be controlled at ≥30 minutes. This facilitates both the control of product mechanical properties and the organization and implementation of production.

[0073] According to some embodiments of the present invention, the length of the hot-rolled steel coil is 70m to 1600m.

[0074] According to some embodiments of the present invention, the calculation method in step S1 further includes using JMatPro software to calculate the solution temperature and the solution temperature curve (the relationship between the microalloy carbonitride content and the slab temperature).

[0075] According to some embodiments of the present invention, the temperature selection range in step S3 is T1-20℃~T1+20℃.

[0076] The temperature selections are: T1-20℃, T1-10℃, T1, T1+10℃, and T1+20℃.

[0077] According to some embodiments of the present invention, if, in the adjustment of the solution treatment temperature of the slab heated within the temperature range of T1±20℃, a solution treatment temperature is not found where the tensile strength of the 1 / 2 length portion of the hot-rolled coil is greater than or equal to the tensile strength of the portion 7m from the tail end, then the solution treatment temperature needs to be adjusted upwards in a 10℃ gradient until the tensile strength of the 1 / 2 length portion of the hot-rolled coil is greater than or equal to the tensile strength of the portion 7m from the tail end.

[0078] According to some embodiments of the present invention, if in the above-mentioned test coil for heating the slab to solidify the temperature within the range of T1±20℃, the tensile strength of the entire hot-rolled coil at half its length is greater than the tensile strength at 7m from the tail end by more than 15 MPa, then the solidification temperature test is continued to be carried out in a 10℃ gradient until a solidification temperature is reached where the tensile strength of the hot-rolled coil at half its length is greater than the tensile strength at 7m from the tail end by 5 MPa to 15 MPa.

[0079] According to some embodiments of the present invention, if during the debugging process, the solution temperature at which the tensile strength of the 1 / 2 length section of the hot-rolled coil is greater than the tensile strength of the section 7m from the tail end by 5MPa to 15MPa, and if the yield strength and tensile strength margin at the section 7m from the tail end is too large, then the alloy composition of Mn, Nb, V, Ti, etc., should be reduced; if the margin is insufficient, then the alloy composition of Ti, Si, Nb, V, Mn, etc., should be increased.

[0080] The location 7m from the end of the steel coil is the routine sampling point for testing the mechanical properties of hot-rolled high-titanium microalloyed high-strength structural steel coils. Attached Figure Description

[0081] To facilitate understanding by those skilled in the art, the present invention will be further described below with reference to the accompanying drawings.

[0082] Figure 1 This is a graph showing the relationship between TiNbV(C, N) content and slab temperature in Example 1 of the present invention.

[0083] Figure 2 This is a graph showing the relationship between TiN content and slab temperature in Example 1 of the present invention.

[0084] Figure 3 This is a graph showing the relationship between Ti4C2S2 content and slab temperature in Example 1 of the present invention.

[0085] Figure 4 This is a graph showing the relationship between tensile strength and solution heating temperature of the slab in Embodiment 1 of the present invention.

[0086] Figure 5 This is a graph showing the relationship between yield strength and slab heating solution temperature in Embodiment 1 of the present invention. Detailed Implementation

[0087] The following will describe the concept and technical effects of the present invention clearly and completely with reference to embodiments, so as to fully understand the purpose, features and effects of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, not all embodiments. Other embodiments obtained by those skilled in the art based on the embodiments of the present invention without creative effort are all within the scope of protection of the present invention.

[0088] In the description of this invention, the terms "one embodiment," "some embodiments," "illustrative embodiment," "example," "specific example," or "some examples," etc., refer to specific features, structures, materials, or characteristics described in connection with that embodiment or example, which are included in at least one embodiment or example of the invention. In this specification, the illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments or examples.

[0089] Unless otherwise specified in the embodiments, the conditions shall be performed under conventional conditions or conditions recommended by the manufacturer. Instruments used without a specified manufacturer are all commercially available standard products. The testing methods for tensile strength and yield strength in the embodiments of this invention conform to GB / T 2975-2018 (Sampling Location and Specimen Preparation for Mechanical Properties Testing of Steel and Steel Products) and GB / T 228.1-2021 (Metallic Materials, Tensile Testing—Part 1: Test Method at Room Temperature). The testing methods for impact energy in the embodiments of this invention conform to GB / T 2975-2018 (Sampling Location and Specimen Preparation for Mechanical Properties Testing of Steel and Steel Products) and GB / T229-2020 (Charpy Pendulum Impact Test Method for Metallic Materials).

[0090] The roughing process in this embodiment of the invention is as follows: inlet thickness 230mm, inlet temperature 1100℃, 7 rolling passes, outlet thickness 48mm, outlet temperature 1090℃.

[0091] The finishing rolling process in this embodiment of the invention is as follows: inlet thickness 48mm, inlet temperature 990℃, 7 rolling passes, outlet thickness 7mm, outlet temperature 850℃, and coiling temperature 630℃.

[0092] The high-titanium microalloyed steel used in the embodiments of the present invention is composed of the following elements by mass fraction:

[0093] The composition consists of 0.075% C, 0.15% Si, 1.85% Mn, 0.014% P, 0.001% S, 0.035% Al, 0.13% Ti, 0.06% Nb, 0.018% V, 0.004% N, and the balance being Fe and unavoidable residual and impurity elements from the smelting process.

[0094] In this embodiment of the invention, JMatPro was used to calculate the solution temperature, and the calculation results are as follows:

[0095] Temperature at which liquid phase begins to appear (5% liquid phase by mass): 1477℃;

[0096] Liquidus temperature (99.99% liquid phase by mass): 1520℃;

[0097] Ti4C2S2 solution temperature: starts at 764℃, complete solution at 1409℃;

[0098] M(C,N), according to the embodiment of the present invention, the composition of the steel is understood as TiNbV(C,N) at the solution-complete (complete solution) temperature of 1257.4℃.

[0099] TiNbV(C, N) is the most significant precipitate, with the highest content (0.135%) at a slab temperature of 1187.63℃. It is the most important precipitate with a strengthening effect. Above 1187.63℃, the content gradually decreases with increasing slab temperature, reaching complete solid solution above 1257.4℃, with a solid solution content of 0.135%. (See details...) Figure 1 .

[0100] When the slab temperature is 1257.43℃, the MN content (in this embodiment of the invention, the steel grade is mainly considered to be TiN) is the highest, reaching 0.066%; when the slab temperature is 1300℃, the content is 0.053%, as detailed in [link to relevant documentation]. Figure 2 That is, the solubility at a slab temperature of 1300℃ is 0.066% - 0.053% = 0.013%, which is only 1 / 10 of the solid solubility of TiNbV(C,N). Therefore, its effect on strength is significantly weaker than that of TiNbV(C,N).

[0101] The highest content of the precipitate Ti4C2S2 is only 0.00695%, and it only begins to slowly dissolve at about 900℃. At 1300℃, the content is 0.0062%. See details... Figure 3 That is, the solid solution content at 1300℃ is 0.00695% - 0.00619% = 0.00076%, which is only 1 / 178 of TiNbV(C,N), and its effect on strength can be ignored.

[0102] Precipitation strengthening is the most important strengthening mechanism for low-carbon microalloyed steels. Strengthening through the precipitation of microalloyed carbonitrides with a volume fraction of 0.1%–0.2% and an average size of 2 nm–5 nm can provide a strength increase of 200 MPa–400 MPa. Given the same precipitate size, the strength increase is proportional to the square root of the precipitate volume fraction, indicating that the most significant precipitation strengthening originates from TiNbV(C, N). Considering that Ti and N preferentially form TiN, the most significant precipitation strengthening actually comes from titanium-niobium-vanadium carbides (TiNbVC).

[0103] In this embodiment of the invention, the solution temperature is calculated using the solubility product formula of titanium niobium carbide in austenite, and the calculation results are as follows:

[0104] In high-Ti microalloyed high-strength structural steel, N preferentially reacts with Ti to form TiN. To avoid coarse liquid-precipitated TiN inclusions, high-Ti microalloyed steel requires the lowest possible N content, typically ≤60ppm (preferably ≤50ppm). Therefore, the most important strengthening precipitate is TiC. The main strengthening precipitates in high-Ti plus Nb composite strengthened microalloyed steel are TiC and NbC, with solid solubility product formulas of lg{[Ti]×[C]}=2.75-7000 / T and lg{[Nb]×[C]}=2.96-7510 / T, respectively. Since the Ti content is much higher than the Nb content, the solution temperature of niobium titanium carbide is calculated using the formula lg{[Ti+Nb]×[C]}=2.75-7000 / T, lg{[0.13+0.06]×[0.075]}=2.75-7000 / T (calculated in Kelvin), T=1523.002K=1250.002℃ (converted to Celsius), which is close to the result of 1257.4℃ calculated using JMatPro.

[0105] The solution temperature was calculated using the solubility product formula for titanium nitride in austenite, and the results are as follows:

[0106] Assuming all N in the steel forms TiN, the required Ti is 0.004 * (47.867 / 14) = 0.0137%. The solution temperature is calculated using the formula lg{[Ti] × [N]} = 0.32 - 8000 / T. Therefore, lg{0.0137] × [0.004]} = 0.32 - 8000 / T (calculated in Kelvin), and T = 1746.26 K = 1473.26 °C (converted to Celsius). This is close to the melting temperature, therefore, in actual production, TiN cannot be completely dissolved.

[0107] The solution temperature was calculated using the solubility product formula for vanadium carbide in austenite, and the results are as follows:

[0108] lg{[V]×[C]}=6.72-9500 / T, lg{[0.018]×[0.075]}=6.72-9500 / T (calculated as Kelvin temperature), T=990.6K=717.6℃ (converted to Celsius temperature), which is much lower than the heating temperature of a slab in normal rolling. Therefore, the solid solution temperature of vanadium carbide is usually not required to be considered during the heating process of high-titanium microalloy high-strength structural steel slabs before hot rolling.

[0109] Example 1

[0110] This embodiment describes a method for determining the solution treatment temperature required for heating high-titanium microalloyed high-strength structural steel slabs, comprising the following steps:

[0111] S1. Based on the chemical composition of low-carbon microalloyed high-strength steel, the solution temperature T (°C) required for the equilibrium state of microalloyed carbonitrides with the largest precipitation amount is calculated using JMatPro. In this embodiment, the solution temperature T for M (C, N) is 1257.4°C.

[0112] S2. The solution temperature calculated using the method in step S1 is normalized to have a unit digit of 0, resulting in the solution temperature T1 (1260℃).

[0113] S3. Adjust the temperature to 1230℃, 1240℃, 1250℃, 1260℃, 1270℃ and 1280℃. Control the solution treatment time within the range of 30min to 40min (see Tables 1 and 2 for specific solution treatment times), and ensure the uniformity of the longitudinal heating temperature of the slab.

[0114] Ignoring 6% of the length at the head and tail ends of the slab, and controlling the longitudinal temperature difference of the slab to be ≤10℃, the fluctuation of the rolling force in the first pass of the roughing mill with uniform speed rolling is evaluated.

[0115] After the hot-rolled coil is air-cooled to a temperature ≤80℃, samples are taken at the leveling line to test the mechanical properties of the hot-rolled coil at 1 / 2 length, 7m from the tail end, and other parts. The mechanical property test samples are located at 1 / 4 width.

[0116] The solution temperature at which the tensile strength of the hot-rolled coil is 5 MPa to 15 MPa greater than that of the section 7 m from the tail end is taken as the solution temperature required for heating the slab in actual production.

[0117] The effect of slab heating solution temperature on the strength of hot-rolled coil in this embodiment is shown in Tables 1 and 2. As can be seen from Table 1, the solution temperature required for slab heating in this embodiment is 1260℃~1280℃, that is, the solution temperature required for heating the high-titanium microalloy high-strength structural steel slab in this embodiment is 1270±10℃, and the solution time is 30min~40min.

[0118] Table 1: Heating and solution treatment parameters and strength test results of some slabs in Example 1

[0119]

[0120] Note: 1) The specifications of hot-rolled steel coils are 7mm (thickness) * 1500mm (width);

[0121] 2) Steel coil length 300m~310mm;

[0122] 3) The units for tensile strength and yield strength are MPa;

[0123] 4) The impact energy is Akv at -40℃, and the unit is J.

[0124] Table 2: Heating and solution treatment parameters and strength test results of the remaining slab in Example 1

[0125]

[0126] Note 1) Solution treatment temperature of No. 7 slab: 1260℃, solution treatment time: 38min;

[0127] 2) Solution treatment temperature of No. 8 slab: 1270℃; Solution treatment time: 40 min;

[0128] 3) The hot-rolled steel coils rolled from No. 7 and No. 8 slabs are both 7mm (thickness) * 1500mm (width), with lengths of 305m and 309m respectively.

[0129] Tables 1 and 2 show that when the slab heating and solution treatment temperature is ≥1260℃ and the solution treatment time is 30 min to 40 min, the hot-rolled coil has the highest strength at 15 m to 20 m from the head end, and the strength gradually decreases from 20 m from the head end to the tail end. The strength at 5 m from the head end is higher than that at 7 m from the tail end, and the strength at 4 m from the tail end is the lowest.

[0130] Microalloying elements need to be fully dissolved during the slab heating process in order to exert their strengthening effect. In this embodiment, the solution temperature needs to be no less than 1260℃.

[0131] The precipitation of microalloyed carbonitrides is a nucleation and growth process, requiring an incubation period. The PTT (precipitation amount-temperature-time) curve of microalloyed carbonitride precipitation in austenite is C-shaped. The nose-tip temperature of niobium-titanium carbonitride precipitation in austenite is approximately 950℃. Because the head of the intermediate slab after rough rolling is rolled into the finishing mill, and the temperature of the intermediate slab head is typically above 980℃, far from the nose-tip temperature, while the tail temperature is closer to the nose-tip temperature, the deformation-induced precipitation at the head of the intermediate slab after rough rolling is lower than that at the tail. This means that more precipitation occurs at the head during subsequent finishing rolling and coiling. Since lower precipitation temperatures result in finer precipitates and better strengthening effects, the strength of the hot-rolled coil gradually decreases from 20m from the head to the tail.

[0132] For high-Ti microalloyed high-strength structural steel, precipitation strengthening typically reaches its maximum at a temperature between 620℃ and 650℃. The head of the hot-rolled coil experiences faster cooling due to direct contact with the coiling mandrel and even faster cooling after uncoiling, thus suppressing precipitation strengthening. Therefore, the first 10m of the head is not the point of highest strength; the highest strength generally occurs 15m to 20m from the head.

[0133] In addition, the outermost ring of the hot-rolled coil experiences slower cooling after coiling, resulting in suppressed precipitation strengthening and the lowest strength of the entire coil. Therefore, the sampling points for mechanical testing of hot-rolled coils should avoid the outermost ring. Typically, the maximum diameter of a hot-rolled coil is 2m. When delivered as a coil, the sampling point for mechanical property testing can be 7m from the tail end. When the slab is heated to a solution temperature of 1260℃~1280℃ and the solution time is 30min~33min, the strength at this sampling point is slightly lower than or equal to the strength at half the length of the coil, but the difference does not exceed 15MPa, meaning the strengths are essentially the same.

[0134] When the slab heating solution temperature is equal to 1260℃, the strength of the hot-rolled coil at half its length is the same as the strength 7m from the tail end; when the slab heating solution temperature is greater than or equal to 1270℃, the strength of the hot-rolled coil at half its length is higher than that 7m from the tail end; when the slab heating solution temperature is less than or equal to 1250℃, the strength of the hot-rolled coil at half its length is lower than that 7m from the tail end.

[0135] The head and tail ends of the slab have 5-sided heat transfer, while the remaining parts have 4-sided heat transfer. Therefore, the heating temperature of approximately 5% of the length at the head and tail ends of the slab is usually higher than that in the middle. Furthermore, the shorter the heating time and the lower the slab temperature, the greater the temperature difference between the head / tail and the middle. This results in the strength of the middle section of the hot-rolled coil being lower than the strength of the tail sampling section when the slab heating temperature is less than or equal to a certain value (1250℃ in this example). As the slab heating temperature increases, the solid solution content of microalloyed carbonitrides continuously increases, and the strength of the hot-rolled coil continuously increases. However, the temperature drop rate in the middle section of the hot-rolled strip is significantly slower than that at the head and tail ends. Therefore, the middle section of the hot-rolled coil is more conducive to the precipitation of microalloyed carbonitrides, which is beneficial for precipitation strengthening. Consequently, the increase in precipitation strengthening in the middle section of the hot-rolled coil is significantly greater than that at the head and tail ends. As a result, when the solution temperature is equal to a certain value (1260℃ in this example), the strength of the half-length section of the hot-rolled coil is equal to that of the section 7m from the end of the coil. When the solution temperature is greater than or equal to a certain value (1270℃ in this example), the strength of the half-length section of the hot-rolled coil is greater than that of the section 7m from the end of the coil. See Tables 1 to 3 below for details.

[0136] Niobium carbonitride titanium exhibits an approximately linear increase in solid solution content with increasing temperature within the temperature range of 1190℃ to 1250℃, see [reference needed]. Figure 1 As the slab temperature rises, TiNbV (C, N) gradually dissolves, and the TiNbV (C, N) content in the slab gradually decreases. Within this temperature range, every 10°C increase in the slab's solution temperature leads to an increase of approximately 15 MPa in the strength of the middle section of the hot-rolled coil, while the strength increase at the head and tail sections is only about half that of the middle section. When the temperature reaches 1257.4°C, all TiNbV (C, N) in the slab dissolves. Therefore, when the slab temperature rises from 1250°C to 1260°C, the amount of TiNbV (C, N) dissolved in the slab increases sharply, resulting in a tensile strength increase of up to 30 MPa at the half-length section of the hot-rolled coil. The strength increase at the head and tail ends, however, is significantly lower. At higher temperatures, niobium-titanium carbonitride is already fully dissolved in advance. The increase in strength at the beginning and end of the hot-rolled coil mainly comes from a small amount of TiN and Ti4C2S2 dissolved in the solution. Furthermore, the precipitation strengthening at the beginning and end of the hot-rolled coil is suppressed after the strip is coiled, resulting in a significantly lower increase in strength compared to the middle section. Consequently, when the slab heating solution temperature is equal to 1260℃, the strength of the half-length section of the hot-rolled coil is the same as the strength 7m from the end. When the slab heating solution temperature is greater than or equal to 1270℃, the strength of the half-length section of the hot-rolled coil is higher than that 7m from the end. When the slab heating solution temperature is less than or equal to 1250℃, the strength of the half-length section of the hot-rolled coil is lower than that 7m from the end.

[0137] When the slab heating solution temperature is increased from 1250℃ to 1260℃, the half-length portion of the hot-rolled coil exhibits the largest strength increase. This is directly related to the sudden and significant increase in the TiNbV(C,N) solution content. Figure 1The increase in strength of the hot-rolled coil's half section is second only to the increase in temperature of slab heating solution treatment temperature from 1230℃ to 1250℃, which is related to the gradual solution treatment of TiNbV(C,N). (See...) Figure 1 The strength increase at half the length of the hot-rolled coil is minimal for every 10°C increase in the slab heating solution temperature from 1260°C to 1280°C. This is related to the fact that TiNbV(C,N) is completely dissolved at 1260°C. Further increasing the solution temperature will gradually cause some dissolution of TiN and Ti4C2S2, but the amount of dissolution is very small. Figure 2 and Figure 3 This results in a significant reduction in strength increment; the effect of solution temperature on the tensile strength and yield strength of the half-length portion of the hot-rolled coil is shown below. Figure 4 With below Figure 5 .

[0138] Example 2

[0139] This embodiment describes a method for determining the solution treatment parameters required for heating high-titanium microalloyed high-strength structural steel slabs, which consists of the following steps:

[0140] High-titanium microalloyed high-strength structural steel slabs were heated and solution-treated at a solution temperature of 1270℃ for 60 min (slab number 8) and 90 min (slab number 9), and then rolled into hot-rolled coils.

[0141] The mechanical property test results of the hot-rolled steel coil in this embodiment are shown in Table 3.

[0142] Table 3: Performance test results of some slabs in Example 1 and slabs in Example 2

[0143]

[0144]

[0145] Note 1) The solution treatment temperature for all slabs is 1270℃.

[0146] 2) All hot-rolled coils are 7mm (thickness) * 1500mm (width);

[0147] 3) The length of the hot-rolled coil is 300m to 310m;

[0148] 4) The units for tensile strength and yield strength are both MPa;

[0149] 5) The impact energy is Akv at -40℃, and the unit is J.

[0150] In Examples 1 and 2 of this invention, the solution treatment temperature of the slab was 1270℃, but the mechanical property test results of the hot-rolled coils rolled with different solution treatment times are shown in Table 3. Table 3 shows that extending the solution treatment time from 30 min to 90 min results in a slight increase in the strength (tensile strength and yield strength) of the hot-rolled steel coil, but the increase is very limited. The strength increase is greatest at the half-length section of the steel coil, but the increase in tensile strength is only 11 MPa, and the increase in yield strength is only 10 MPa. Therefore, increasing the strength by extending the solution treatment time is uneconomical (significantly increases fuel consumption) and also significantly reduces the production line efficiency.

[0151] When the slab solution temperature is 1270℃, the impact toughness of the hot-rolled coil decreases as the solution time increases from 30 min to 90 min. This is mainly due to the coarsening of the austenite grains in the slab with increasing solution time. Therefore, the preferred solution time range is 30 min to 40 min.

[0152] However, the narrower the control range of solution treatment time, the greater the difficulty in organizing and implementing production. Therefore, the solution treatment time for products with relatively difficult impact toughness can be controlled at 30-60 minutes; the solution treatment time for products with relatively easy impact toughness can be controlled at 30-90 minutes; and the solution treatment time for products with easily easy impact toughness can be controlled at ≥30 minutes.

[0153] In summary, this invention, in its research on the hot rolling process of high-Ti microalloyed high-strength structural steel (with microalloying elements such as Ti), found that for a given composition of high-Ti microalloyed high-strength structural steel, since the mechanical property sampling point for the hot-rolled coil of high-Ti microalloyed high-strength structural steel during production inspection is located 7m from the tail end of the hot-rolled steel coil, to ensure that the longitudinal temperature uniformity of the slab and the CT temperature uniformity meet the requirements, and to ensure that the mechanical properties of the entire coil are qualified, the slab heating and solution treatment temperature needs to make the tensile strength of the middle part (1 / 2 length) of the hot-rolled coil 5MPa to 15MPa greater than the tensile strength of the sampling point at the tail end.

[0154] The specific embodiments described above further illustrate the purpose, technical solution, and beneficial effects of the present invention. It should be understood that the above descriptions are merely specific embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for determining the solid solution temperature required for heating a high-titanium microalloyed high-strength structural steel slab, characterized in that, Includes the following steps: S1. Calculate the solution solution temperature of niobium titanium carbide; The calculation method described in step S1 includes the following formula: lg{[Ti+Nb] [C]}=2.75-7000 / T; In the formula: Ti represents the mass percentage content of Ti in the high-titanium microalloyed high-strength structural steel; Nb represents the mass percentage content of Nb in the high-titanium microalloyed high-strength structural steel. C represents the mass percentage content of C in the high-titanium microalloyed high-strength structural steel; T represents the absolute solution temperature, and the unit is K; Converting T to Celsius temperature yields the Celsius solid solution temperature of niobium titanium carbide; S2. The Celsius solution temperature of niobium titanium carbide calculated in step S1 is normalized to obtain the normalization temperature T1. The normalization process involves resetting the units digit and decimal part of the solution temperature in step S1 to zero. If the units digit and the decimal part of the solution temperature mentioned in step S1 are both zero, then the normalization treatment temperature is the same as the solution temperature in step S1. If the tens digit or decimal part of the solution temperature mentioned in step S1 is non-zero, then the tens digit of the normalization temperature is increased by one, and the units digit and decimal part are reduced to zero. S3. Using T1 temperature as the center temperature and ±10℃ as the gradient, several adjustment solution temperatures are set to heat and solution treat the high-titanium microalloy high-strength structural steel slab. After heating and solution treatment, rough rolling and fine rolling are performed to obtain hot-rolled steel coils. The heating and solution treatment time is 30 min to 90 min; The adjustment temperature at which the difference between the tensile strength at the middle position and the tensile strength at a position 7m from the tail end of the hot-rolled high-titanium microalloy high-strength structural steel coil is 5MPa~15MPa is the solution treatment temperature required for heating the high-titanium microalloy steel slab. The microalloying elements in the high-titanium microalloyed steel include Ti.

2. The method of claim 1, wherein the method is characterized by: The mass fraction of Ti element in the high-titanium microalloy high-strength structural steel is 0.06%~0.2%.

3. The method of claim 1, wherein the method is characterized by: The microalloying elements in the high-titanium microalloyed high-strength structural steel also include at least one of V and Mo.

4. The method for determining the solution treatment temperature required for heating the high-titanium microalloyed high-strength structural steel slab according to claim 1, wherein the mass fraction of Nb element in the high-titanium microalloyed high-strength structural steel is 0.01%~0.08%.

5. The method of claim 3, wherein the method is characterized by: The mass fraction of V in the high-titanium microalloy high-strength structural steel is between 0.01% and 0.2%.

6. The method of claim 1, wherein the method is characterized by: The mass fraction of carbon in the high-titanium microalloy high-strength structural steel is between 0.03% and 0.12%.

7. The method of claim 1, wherein the method is characterized by: The high-titanium microalloy high-strength structural steel contains nitrogen element with a mass fraction of ≤0.01%.

8. The method of claim 1, wherein the method is characterized by: The high-titanium microalloy high-strength structural steel contains 0.2% to 2% Mn by mass.

9. The method of determining the solution temperature required for heating a high titanium microalloyed high strength structural steel slab according to any one of claims 1 to 8, characterized in that, The heating and solution treatment time in step S3 is 30 min to 60 min.

10. The method of determining the solution temperature required for heating a high titanium microalloyed high strength structural steel slab according to any one of claims 1 to 8, characterized in that, The length of the hot-rolled steel coil is 70m to 1600m.

11. The method of determining the solution temperature required for heating a high titanium microalloyed high strength structural steel slab according to any one of claims 1 to 8, characterized in that, The calculation method described in step S1 also includes using JMatPro software to calculate the solution temperature and solution temperature curve.

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