A method for producing a thin gauge quenched and tempered Ti-containing steel

Abstract

The application discloses a production method of a thin specification quenched and tempered Ti-containing steel and relates to the technical field of Ti-containing steel production.The method comprises the following steps: S1: after rough rolling of a steel coil is completed, the final rolling FT7 temperature is controlled in the range of 810-840 DEG C; S2: the steel coil is cooled in a super-fast cooling and laminar cooling mode and is coiled after being cooled to the range of 300-400 DEG C; S3: after the steel coil is opened, critical quenching is carried out, and the steel coil is heated in a two-stage heating mode; and S4: the steel coil is tempered to obtain the thin specification quenched and tempered Ti-containing steel.The steel plate produced by the method has the advantages of the following aspects: the strength is generally increased by more than 200 MPa, the toughness is increased by more than 20 J, the performance of the steel plate is uniform, the strength and toughness are well matched, the process is simple, and the steel plate has a good popularization and application prospect.

Description

Technical Field

[0001] This invention relates to the field of Ti-containing steel production technology, and specifically to a method for producing thin-gauge quenched and tempered Ti-containing steel. Background Technology

[0002] Ti-containing microalloying refers to the addition of trace amounts of Ti, a strong carbonitriding compound-forming element, to the composition of low-alloy or plain carbon steel. Ti dissolves in the steel, forming solute atoms that exert a solute dragging effect. Furthermore, it can improve the microstructure of the steel by controlling the dissolution and precipitation of second-phase particles during heating, deformation, and cooling, thereby altering the steel's physical, chemical, and mechanical properties. However, excessively high Ti content often leads to decreased toughness, affecting subsequent processing and service life. How to improve the strength of Ti-containing steel without compromising its toughness, while also ensuring the steel plate meets the requirements for subsequent processing, forming, and service life, is a pressing problem that needs to be solved.

[0003] To improve the toughness and overall mechanical properties of Ti-containing steel, the literature "Improvement of Tensile Properties and Impact Toughness of Hot-Rolled Ti-Containing Microalloyed Steel" (Meng Jing et al., China Metallurgy, 2023) employs Ti-Nb composite strengthening, combined with low-temperature final rolling at 840℃ and low-temperature coiling at 560℃ to improve toughness. However, when the coiling temperature fluctuates downwards, insufficient precipitation of Ti in the steel can lead to insufficient strength. Furthermore, to improve the mechanical properties of Ti-containing steel, the literature "Study on the Precipitation Behavior of (Ti,Mo)C in Low-Carbon Martensitic Steel and Its Strength and Toughness" (Han Yun, 2013, Doctoral Dissertation) uses a direct quenching (DQ) + tempering (T) + quenching (RD) process. Although this can significantly improve the strength of the steel, direct quenching (DQ) makes it difficult to control the quality of thin-gauge plates, resulting in low yield and numerous, lengthy processes, making it difficult to promote in actual production. Furthermore, current research on Ti-containing steel, both domestically and internationally, mostly focuses on non-quenched and tempered steel, while research and products on quenched and tempered Ti-containing high-strength steel are relatively few. The price advantage of the microalloying element Ti has not been fully explored. Therefore, it is necessary to develop a production process for quenched and tempered Ti-containing steel. Summary of the Invention

[0004] The purpose of this invention is to provide a method for producing thin-gauge quenched and tempered Ti-containing steel, so as to solve the problems existing in the prior art as mentioned in the background section.

[0005] To achieve the above objectives, the present invention provides the following technical solution:

[0006] A method for producing thin-gauge quenched and tempered Ti-containing steel includes the following steps:

[0007] S1: After the rough rolling of the steel coil is completed, the temperature of the final rolling FT7 is controlled in the range of 810-840℃;

[0008] S2: The steel coil is cooled using ultra-fast cooling and laminar flow cooling methods, and then wound up when cooled to the range of 300-400℃.

[0009] S3: After the steel coil is leveled, it undergoes critical quenching. The quenching heating is carried out using a two-stage heating method.

[0010] S4: Tempering of steel coils yields thin-gauge tempered Ti-containing steel.

[0011] Preferably, the specific method for cooling the steel coil in S2 is as follows: the ultra-fast cooling section adopts a single-phase cooling mode, the water ratio CRT / CRB is 30-40 / 40-60, the cooling rate is 60-80℃ / s, the cooling water valve SVT / SVB is 1:1, and after the ultra-fast cooling is completed, symmetrical and uniform cooling is adopted from top to bottom, the CRT / CRB is 10-20 / 10-20, and the cooling rate is 10-20℃ / s.

[0012] Preferably, in step S3, the first stage heating temperature is 520-560℃, and the heating time is 36-48 minutes; the second stage heating temperature is A. r3 +(5-10)℃, heating time 24-32min, then water quenched to room temperature.

[0013] Preferably, in step S4, different tempering processes are used depending on the application of the steel. For low-temperature tempering below 300°C, the tempering time is set to 50-64 minutes. For high-temperature tempering above 500°C, the tempering time is set to 45-60 minutes.

[0014] Compared with the prior art, the beneficial effects of the present invention are:

[0015] 1. The steel plate produced by the present invention using ultra-fast cooling, low-temperature coiling, segmented heating, critical quenching and flexible tempering processes generally has a strength that is more than 200 MPa higher and a toughness that is more than 20 J higher than that of the same composition and tempered at the same temperature. Moreover, the steel plate has uniform properties, good strength and toughness matching, and simple process, and has good prospects for promotion and application.

[0016] 2. Compared with tempered steel plates of the same strength level, this invention is a high-strength steel production method that saves on alloys due to the lower content of precious alloying elements such as Ni and Mo, thus reducing the production cost of thin-gauge tempered Ti-containing steel. Detailed Implementation

[0017] To make the technical means, creative features, objectives and effects of this invention easier to understand, the invention will be further described below in conjunction with specific embodiments.

[0018] This invention provides the following technical solutions:

[0019] Example 1

[0020] The selected steel is a 3mm tempered steel plate with a yield strength of 700MPa (standard Rel≥700MPa, -20℃ KV2≥40J). Its chemical composition is: 0.09% C, 1.85% Mn, 0.08% Si, P≤0.010%, S<0.005%, 0.05% Nb, 0.13% Ti, with the balance being Fe. r3 The temperature was 830℃. The steel plate was rolled to a specification of 3×1500mm. After final rolling, the specific processing method was as follows:

[0021] S1: After the rough rolling of the steel coil is completed, the temperature of the final rolling FT7 is controlled at 840℃.

[0022] S2: The steel coil is cooled using ultra-fast cooling and laminar flow cooling methods, and then coiled at 400℃. The specific cooling method for the steel coil is as follows: the ultra-fast cooling section adopts a single-phase cooling mode with a water ratio of CRT / CRB of 30 / 40, a cooling rate of 80℃ / s, and a cooling water valve ratio of SVT / SVB of 1:1. After the ultra-fast cooling is completed, symmetrical and uniform cooling is adopted from top to bottom with a CRT / CRB ratio of 10 / 10 and a cooling rate of 20℃ / s.

[0023] S3: After the steel coil is leveled, it undergoes critical quenching. The quenching heating is carried out in two stages. The first stage heating temperature is 520℃ and the heating time is 36min. The second stage heating temperature is 840℃ and the heating time is 24min. Then, it is water quenched to room temperature.

[0024] S4: Temper the steel coil at a temperature of 520℃ for 45 minutes to obtain thin-gauge tempered Ti-containing steel.

[0025] Example 2

[0026] The steel selected is an 8mm tempered plate with a yield strength of 700MPa (standard Rel≥700MPa, -20℃ KV2≥40J), with the following chemical composition: 0.12% C, 1.80% Mn, 0.10% Si, P≤0.010%, S<0.005%, 0.06% Nb, 0.15% Ti, 0.15% Cr, and the balance being Fe. Its A... r3 The temperature was 825℃. The steel plate was rolled to a specification of 8×1700mm. After final rolling, the specific processing method was as follows:

[0027] S1: After the rough rolling of the steel coil is completed, the temperature of the final rolling FT7 is controlled at 830℃.

[0028] S2: The steel coil is cooled using ultra-fast cooling and laminar flow cooling methods, and then coiled at 380℃. The specific cooling method for the steel coil is as follows: the ultra-fast cooling section adopts a single-phase cooling mode with a water ratio of CRT / CRB of 30 / 50, a cooling rate of 70℃ / s, and a cooling water valve ratio of SVT / SVB of 1:1. After the ultra-fast cooling is completed, symmetrical and uniform cooling is adopted from top to bottom with a CRT / CRB ratio of 15 / 15 and a cooling rate of 18℃ / s.

[0029] S3: After the steel coil is leveled, it undergoes critical quenching. The quenching heating is carried out in two stages. The first stage heating temperature is 540℃ and the heating time is 40min. The second stage heating temperature is 830℃ and the heating time is 28min. Then, it is water quenched to room temperature.

[0030] S4: Temper the steel coil at a temperature of 540℃ for 60 minutes to obtain thin-gauge tempered Ti-containing steel.

[0031] Example 3

[0032] The selected steel grade is NM400 steel or similar steel (standard Rm≥1200MPa, -20℃ KV2≥24J), with the following chemical composition: 0.16% C, 1.50% Mn, 0.10% Si, P≤0.010%, S<0.005%, 0.15% Ti, 0.30% Cr, and the balance being Fe. Its A... r3 The temperature was 820℃. The steel plate was rolled to a specification of 10×1800mm. After final rolling, the specific processing method was as follows:

[0033] S1: After the rough rolling of the steel coil is completed, the temperature of the final rolling FT7 is controlled at 820℃.

[0034] S2: The steel coil is cooled using ultra-fast cooling and laminar flow cooling methods, and then coiled at 350℃. The specific cooling method for the steel coil is as follows: the ultra-fast cooling section adopts a single-phase cooling mode with a water ratio of CRT / CRB of 40 / 50, a cooling rate of 75℃ / s, and a cooling water valve ratio of SVT / SVB of 1:1. After the ultra-fast cooling is completed, symmetrical and uniform cooling is adopted from top to bottom with a CRT / CRB ratio of 18 / 18 and a cooling rate of 15℃ / s.

[0035] S3: After the steel coil is leveled, it undergoes critical quenching. The quenching heating is carried out in two stages. The first stage heating temperature is 540℃ and the heating time is 45min. The second stage heating temperature is 825℃ and the heating time is 30min. Then, it is water quenched to room temperature.

[0036] S4: Temper the steel coil at a temperature of 250°C for 50 minutes to obtain thin-gauge tempered Ti-containing steel.

[0037] Example 4

[0038] The selected steel grade is NM450 steel or similar steel (standard Rm≥1250MPa, -20℃ KV2≥24J), with the following chemical composition: 0.20% C, 1.50% Mn, 0.10% Si, P≤0.010%, S<0.005%, 0.16% Ti, 0.50% Cr, 0.003% B, and the balance being Fe. r3 The temperature was 815℃. The steel plate was rolled to a specification of 16×2000mm. After final rolling, the specific processing method was as follows:

[0039] S1: After the rough rolling of the steel coil is completed, the temperature of the final rolling FT7 is controlled at 820℃.

[0040] S2: The steel coil is cooled using ultra-fast cooling and laminar flow cooling methods, and then coiled at 320℃. The specific cooling method for the steel coil is as follows: the ultra-fast cooling section adopts a single-phase cooling mode with a water ratio of CRT / CRB of 35 / 55, a cooling rate of 68℃ / s, and a cooling water valve ratio of SVT / SVB of 1:1. After the ultra-fast cooling is completed, symmetrical and uniform cooling is adopted from top to bottom with a CRT / CRB ratio of 16 / 16 and a cooling rate of 12℃ / s.

[0041] S3: After the steel coil is leveled, it undergoes critical quenching. The quenching heating is carried out in two stages. The first stage heating temperature is 550℃ and the heating time is 45min. The second stage heating temperature is 825℃ and the heating time is 30min. Then, it is water quenched to room temperature.

[0042] S4: Temper the steel coil at a temperature of 200℃ for 60 minutes to obtain thin-gauge tempered Ti-containing steel.

[0043] Example 5

[0044] The selected steel grade has the same chemical composition as in Example 4. The steel plate is rolled to a specification of 12×1800mm, and its specific processing method after final rolling is as follows:

[0045] S1: After the rough rolling of the steel coil is completed, the temperature of the final rolling FT7 is controlled at 810℃.

[0046] S2: The steel coil is cooled using ultra-fast cooling and laminar flow cooling methods, and then coiled at 300℃. The specific cooling method for the steel coil is as follows: the ultra-fast cooling section adopts a single-phase cooling mode with a water ratio of CRT / CRB of 40 / 60, a cooling rate of 60℃ / s, and a cooling water valve ratio of SVT / SVB of 1:1. After the ultra-fast cooling is completed, symmetrical and uniform cooling is adopted from top to bottom with a CRT / CRB ratio of 20 / 20 and a cooling rate of 10℃ / s.

[0047] S3: After the steel coil is leveled, it undergoes critical quenching. The quenching heating is carried out in two stages. The first stage heating temperature is 560℃ and the heating time is 48min. The second stage heating temperature is 820℃ and the heating time is 32min. Then, it is water quenched to room temperature.

[0048] S4: Temper the steel coil at a temperature of 180℃ for 64 minutes to obtain thin-gauge tempered Ti-containing steel.

[0049] The steel plate production process parameters for Examples 1-5 are shown in Table 1:

[0050] Table 1 Steel Plate Production Process Parameters

[0051]

[0052] The mechanical properties of the steel plates in Examples 1-5 were tested, and the test results are shown in Table 2:

[0053] Table 2 Mechanical properties of steel plates in the examples

[0054]

[0055] By controlling the final rolling temperature of the steel coil, the main purpose is that when the final rolling temperature is low, a large number of deformation bands and dislocation substructures generated by deformation in the amorphous region are retained in the subsequent ultra-fast cooling process. This can, on the one hand, hinder the growth of grains during bainitic phase transformation, because the growth of grains during shear phase transformation will stop when they encounter deformation bands and dislocation substructures; on the other hand, these deformation bands and dislocations provide a large number of nucleation sites for the subsequent precipitation of TiC and other particles, thus accelerating their nucleation.

[0056] Immediate ultra-rapid cooling and low-temperature coiling after final rolling are employed because this method prevents Ti dissolved in austenite from precipitating during cooling and growing during subsequent laminar cooling and coiling, thus failing to effectively pin grain boundaries. It also ensures the coil is coiled within the 300-400℃ range, resulting in a non-diffusional phase transformation structure, i.e., lower bainite or lower bainite with a small amount of granular bainite, while retaining some dislocations generated during austenite rolling deformation. Below 300℃, the coiling temperature is too low, leading to a greater combined effect of phase transformation stress (especially martensitic transformation stress) and thermal stress, making it difficult to control the coil shape quality. Above 400℃, the coil structure is a diffusive phase transformation structure of ferrite and pearlite. Re-austenitization requires prolonged holding to homogenize the alloying elements, which extends the holding time for subsequent heat treatment.

[0057] The first stage of quenching heating for steel plates involves a relatively low temperature but a long heating time. This is because ultra-rapid cooling is performed after hot rolling to obtain a bainitic structure. Due to the short cooling time, the amount of Ti precipitation during cooling is negligible. Holding at this temperature range allows TiC particles, whose precipitation nose temperature is around 540℃, to rapidly precipitate in the form of TiC particles in dislocations and deformation bands. These particles, with a size of about ten nanometers, effectively hinder the growth of austenite grains during subsequent austenitization. The second stage heating temperature is A... r3 +(5-10)℃, i.e., critical quenching, is higher than the conventional quenching temperature A for hypoeutectoid steel. r3 The temperature is 25-50℃ lower than +(30-50)℃. This is mainly because the structure before quenching is lower bainite or lower bainite with a small amount of granular bainite, which is a non-diffusion structure. The distribution of elements in the steel is relatively uniform, and there is no need for high temperature and long time to homogenize it. On the other hand, the heating temperature is lower, the tendency of austenite grain growth is reduced, and the large number of fine TiC particles precipitated during the first stage of heating pin the grain boundaries. Therefore, the austenite grains are controlled below 10um and are fully refined, thereby improving the strength and toughness of the steel plate.

[0058] Tempering is performed using different flexible tempering processes depending on the intended use of the steel plate. For example, if the quenched steel plate is used as wear-resistant steel, it needs to be tempered at low temperature for a long time to eliminate residual stress. If it is used for engineering machinery steel, it can be tempered at high temperature to further improve the toughness of the steel plate.

[0059] According to the mechanical property test results of the steel plates in Examples 1-5, the steel plates produced by the production method of the present invention have a strength that is 200 MPa higher and a toughness that is more than 20 J higher than that of the same composition and tempered at the same temperature. Moreover, the steel plates have uniform properties and a good balance between strength and toughness.

[0060] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A method for producing thin-gauge quenched and tempered Ti-containing steel, characterized in that, Includes the following steps: S1: After the rough rolling of the steel coil is completed, the temperature of the final rolling FT7 is controlled in the range of 810-840℃; S2: The steel coil is cooled using ultra-fast cooling and laminar flow cooling methods, and then wound up when cooled to the range of 300-400℃. S3: After the steel coil is leveled, it undergoes critical quenching. The quenching heating is carried out using a two-stage heating method. In S3, the first stage heating temperature is 520-560℃, and the heating time is 36-48 minutes; the second stage heating temperature is A. r3 +(5-10)℃, heating time 24-32min, then water quench to room temperature; S4: Tempering of steel coils yields thin-gauge tempered Ti-containing steel.

2. The method for producing thin-gauge quenched and tempered Ti-containing steel according to claim 1, characterized in that, The specific method for cooling the steel coil in S2 is as follows: The ultra-fast cooling section adopts a single-phase cooling mode with a water ratio of CRT / CRB of 30-40 / 40-60, a cooling rate of 60-80℃ / s, and a cooling water valve ratio of SVT / SVB of 1:

1. After the ultra-fast cooling is completed, symmetrical and uniform cooling is adopted from top to bottom with a CRT / CRB ratio of 10-20 / 10-20 and a cooling rate of 10-20℃ / s.

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