An online heat treatment production process for medium and thick plates

Abstract

The present invention provides an online heat treatment production process for medium and thick plates, and relates to the technical field of steel plate production. The medium and thick plates of the present invention are manufactured through the steps of billet smelting, billet heating, rolling, cooling, online quenching, online tempering, etc. The present invention adjusts the microstructure morphology and distribution of the medium and thick plates by adjusting the chemical element ratio combined with a specific preparation process, thereby improving the strength and toughness of the medium and thick plates.

Description

Technical Field

[0001] The invention relates to the technical field of steel plate production, and in particular to an online heat treatment production process for medium and thick plates. Background Art

[0002] Medium and thick plates usually refer to steel plates with a thickness of 4.5 to 25 mm. Compared with ordinary steel plates, the uniformity of medium and thick steel plates is more difficult to control, which can easily cause problems such as strength differences and uneven performance between different parts of the steel plate.

[0003] The Chinese invention patent with the publication number CN115318844A discloses a cooling process for the production of medium and thick steel plates, including the following steps: the cooling control equipment receives the process information such as the size, water inlet temperature, and red-return temperature of the controlled cooling steel plate; sets the process parameters such as the cooling manifold water volume, the initial cooling roller speed, the head shielding length and mode, the cooling roller speed acceleration, the manifold water volume increment setting after acceleration, and the tail water volume compensation; the cooling steel plate head position is corrected, the steel plate position signal is fully tracked and turned on, the head shielding is turned on, and the water volume in the cooling area is adjusted to the right position; the cooling steel plate head passes through area A, the head shielding ends, and the cooling roller acceleration mode is turned on; the water volume in the cooling device B, C, and D areas is increased after the cooling roller is accelerated; the tail of the cooling steel plate enters the cooling area, and the tail flow compensation is turned on. This process is suitable for the production of controlled cooling low-alloy high-strength and high-toughness medium and thick plates, and has the advantage of uniform and stable performance for controlled cooling medium and thick plates with longer lengths.

[0004] The Chinese invention patent with publication number CN107119237A discloses a Q690D medium and thick steel plate and a production method for reducing the energy consumption of Q690D medium and thick steel plate. The method eliminates cracks on the surface of the steel plate when the Q690D steel slab is directly hot-charged and hot-delivered by rapidly cooling the 300mm thick slab after casting and cutting, optimizing the heating and rolling process; the surface temperature of the slab entering the furnace reaches above 670°C, and the core temperature is above 900°C, which greatly reduces the energy consumption of Q690D steel medium and thick steel plate production, simplifies the production process, and reduces production costs.

[0005] The Chinese invention patent with publication number CN104250681A discloses a slow cooling process for medium and thick steel plates. For medium and thick steel plates produced by non-thermomechanical rolling process, i.e. non-TMCP process: the slow cooling process is as follows: for steel plates with a thickness of 30 to 100 mm, the cooling bed is air-cooled to a surface temperature of 300 to 400°C after rolling, and then the plates are slow-cooled offline. The surface temperature of the steel plates is 300 to 400°C at the beginning of the slow cooling, and the surface temperature is controlled to be above 200°C at the end of the slow cooling. The process determines the temperature range that is prone to hydrogen-induced cracking based on the hydrogen diffusion and hydrogen-induced crack sensitivity characteristics of steel plates with different processes and thickness specifications, and optimizes the original slow cooling process accordingly, effectively improving the qualified rate of ultrasonic flaw detection of medium and thick plates.

[0006] It can be seen that the current process improvements for medium and thick plates are mainly focused on the cooling process, and the performance of medium and thick plates is improved by adjusting the cooling process. However, there are few records on the adjustment of chemical composition combined with other process improvements to adjust the performance of medium and thick plates. Summary of the invention

[0007] The object of the present invention is to provide an online heat treatment production method for improving the comprehensive properties of medium and thick plates, which specifically comprises the following steps:

[0008] (1) Billet smelting: molten steel is melted according to the design and cast into billets, wherein the chemical composition of the billets is as follows by weight: C: 0.40%-0.45%, Si: 0.02%-0.4%, Mn: 1.3%-1.5%, P: ≤0.005%, S: ≤0.002%, Cr: 2%-5%, Cu: 0.3%-0.5%, Nb: 0.01%-0.03%, Al: 0.05%-0.5%, Mo: 0.05%-0.35%, Sb: 0.01%-0.2%, Ti: 0.05%, B: 0.001%, Ni: 0.3%-1.5%, and the balance is Fe and unavoidable impurities;

[0009] (2) Billet heating: The billet is sent into a heating furnace for heating treatment;

[0010] (3) Rolling: The heated steel billet is descaled by high-pressure water, and then rough rolled. After the rough rolling, the billet is cooled and finished rolled.

[0011] (4) Cooling: Cooling the steel plate after finish rolling;

[0012] (5) Online quenching: online quenching of the cooled steel plate;

[0013] (6) Online tempering: The steel plate after online quenching is tempered and heated, and after tempering is completed, it is cooled to room temperature to obtain a finished medium and thick plate.

[0014] Furthermore, the chemical composition of the steel billet is as follows by weight: C: 0.45%, Si: 0.02%, Mn: 1.5%, P: ≤0.005%, S: ≤0.002%, Cr: 2%, Cu: 0.5%, Nb: 0.01%, Al: 0.35%, Mo: 0.15%, Sb: 0.2%, Ti: 0.05%, B: 0.001%, Ni: 0.5%, and the remainder is Fe and unavoidable impurities.

[0015] Furthermore, in step (1), the furnace entry temperature of the steel billet is 450°C to 500°C, the heating treatment temperature of the steel billet in the heating furnace is 1200°C to 1300°C, the soaking temperature is 1250°C, the soaking rate is 6 to 8 min / cm, and the furnace exit temperature is 1000°C to 1200°C.

[0016] Furthermore, the rough rolling temperature is 980° C. to 1000° C., the single-pass reduction rate is 18% to 20%, and the rough rolling passes are 3 to 5 passes. The present invention promotes the refinement of austenite grains through rough rolling.

[0017] Furthermore, the finishing rolling temperature is 850° C. to 920° C., the single-pass reduction rate is 6% to 10%, and the finishing rolling passes are 5 to 8 passes.

[0018] Furthermore, the cooling in step (4) is carried out in two steps. First, the steel plate is cooled to 600-650°C at a rate of 20-35°C / s, kept at that temperature for 6-8 minutes, and then cooled to room temperature at a rate of 40-50°C / s.

[0019] Furthermore, the quenching temperature is 895°C to 945°C, the cooling method is water cooling, and the water cooling speed is 20-35°C / s.

[0020] Furthermore, the tempering is carried out in two steps. First, the temperature is increased to 350°C-400°C at a rate of 5°C-6°C / min and kept warm for 30 minutes. Then, the temperature is increased to 600°C-650°C at a rate of 1°C-2°C / min and kept warm for 90-150 minutes.

[0021] Furthermore, the cooling method after tempering is air cooling.

[0022] Furthermore, the thickness of the medium and thick plate is 4 to 25 mm.

[0023] The present invention forms high-density dislocations in the medium and thick plates by adjusting the ratio of Cr, Mo, Sb, and B, and combines two pressing and quenching processes to improve the strength and toughness of the medium and thick plates. The medium and thick plates of the present invention fully diffuse carbon through quenching, but also lead to larger cementite particles, and then adopt an online tempering process to diffuse bainite in the medium and thick plates to the grain boundaries and intracrystalline defects, thereby increasing the cementite nucleation rate and forming dispersed cementite. In addition, the present invention converts austenite into MA components through rapid cooling to adjust the mechanical properties of the medium and thick plates.

[0024] Compared with the prior art, the beneficial technical effects of the present invention are:

[0025] The present invention adjusts the microstructure morphology and distribution of the medium and thick plates through an online heat treatment process combined with the adjustment of the chemical element ratio, rolling and cooling processes, thereby improving the strength and toughness of the medium and thick plates. DETAILED DESCRIPTION

[0026] The technical solution provided by the present invention is further described below in conjunction with embodiments.

[0027] Example 1

[0028] (1) Billet smelting: molten steel is melted according to the design and cast into billets, the chemical composition of the billets is as follows by weight: C: 0.45%, Si: 0.02%, Mn: 1.5%, P: ≤0.005%, S: ≤0.002%, Cr: 2%, Cu: 0.5%, Nb: 0.01%, Al: 0.35%, Mo: 0.15%, Sb: 0.2%, Ti: 0.05%, B: 0.001%, Ni: 0.5%, and the balance is Fe and unavoidable impurities;

[0029] (2) Billet heating: The billet is sent into a heating furnace for heating treatment. The billet entry temperature is 450°C, the heating treatment temperature is 1200°C, the soaking temperature is 1250°C, the soaking speed is 8 min / cm, and the furnace exit temperature is 1000°C.

[0030] (3) Rolling: After the billet leaves the heating furnace, it is descaled with high-pressure water and then subjected to rough rolling. The rough rolling temperature is 980°C, the single-pass reduction rate is 20%, and the number of rough rolling passes is 3. After the rough rolling, the temperature is lowered to 900°C, the single-pass reduction rate is 8%, and the number of finishing passes is 8.

[0031] (4) Cooling: The steel after finish rolling is cooled to 650°C at a rate of 20°C / s, kept at this temperature for 6 min, and then cooled to room temperature at a rate of 50°C / s;

[0032] (5) Online quenching: The steel plate is quenched online at a quenching temperature of 925°C and a cooling method of water at a cooling rate of 25°C / s;

[0033] (6) Online tempering: The steel plate is tempered and heated. First, the temperature is raised to 350°C at a rate of 6°C / min and kept at this temperature for 30 min. Then, the temperature is raised to 650°C at a rate of 2°C / min and kept at this temperature for 90 min. After tempering, the steel plate is air-cooled to room temperature to obtain a medium and thick plate with a thickness of 25 mm.

[0034] Example 2

[0035] (1) Billet smelting: molten steel is melted according to the design and cast into billets, the chemical composition of the billets is as follows by weight: C: 0.40%, Si: 0.02%, Mn: 1.5%, P: ≤0.005%, S: ≤0.002%, Cr: 2%, Cu: 0.3%, Nb: 0.02%, Al: 0.35%, Mo: 0.15%, Sb: 0.05%, Ti: 0.05%, B: 0.001%, Ni: 1.5%, and the balance is Fe and unavoidable impurities;

[0036] (2) Billet heating: The billet is sent into a heating furnace for heating treatment. The billet entry temperature is 500°C, the heating treatment temperature is 1300°C, the soaking temperature is 1250°C, the soaking speed is 6 min / cm, and the furnace exit temperature is 1100°C.

[0037] (3) Rolling: After the billet leaves the heating furnace, it is descaled with high-pressure water and then subjected to rough rolling. The rough rolling temperature is 1000°C, the single-pass reduction rate is 20%, and the number of rough rolling passes is 3. After the rough rolling, the temperature is lowered to 920°C, the single-pass reduction rate is 6%, and the number of finishing passes is 6.

[0038] (4) Cooling: The steel after finish rolling is cooled to 600°C at a rate of 35°C / s, kept at this temperature for 8 min, and then cooled to room temperature at a rate of 45°C / s;

[0039] (5) Online quenching: The steel plate is quenched online at a quenching temperature of 900°C and a cooling method of water at a cooling rate of 35°C / s;

[0040] (6) Online tempering: The steel plate is tempered and heated. First, the temperature is raised to 400°C at a rate of 6°C / min and kept at this temperature for 30 min. Then, the temperature is raised to 600°C at a rate of 2°C / min and kept at this temperature for 100 min. After tempering, the steel plate is air-cooled to room temperature to obtain a medium and thick plate with a thickness of 25 mm.

[0041] Example 3

[0042] (1) Billet smelting: molten steel is melted according to the design and cast into billets, the chemical composition of the billets is as follows by weight: C: 0.45%, Si: 0.04%, Mn: 1.5%, P: ≤0.005%, S: ≤0.002%, Cr: 3.5%, Cu: 0.4%, Nb: 0.01%, Al: 0.35%, Mo: 0.25%, Sb: 0.2%, Ti: 0.05%, B: 0.001%, Ni: 0.5%, and the balance is Fe and unavoidable impurities;

[0043] (2) Billet heating: The billet is sent into a heating furnace for heating treatment. The billet entry temperature is 480°C, the heating treatment temperature is 1200°C, the soaking temperature is 1250°C, the soaking speed is 8 min / cm, and the furnace exit temperature is 1000°C.

[0044] (3) Rolling: After the billet leaves the heating furnace, it is descaled with high-pressure water and then subjected to rough rolling. The rough rolling temperature is 980°C, the single-pass reduction rate is 20%, and the number of rough rolling passes is 5. After the rough rolling, the temperature is lowered to 895°C, the single-pass reduction rate is 8%, and the number of finishing passes is 5.

[0045] (4) Cooling: The steel after finish rolling is cooled to 600°C at a rate of 30°C / s, kept at this temperature for 6 min, and then cooled to room temperature at a rate of 50°C / s;

[0046] (5) Online quenching: The steel plate is quenched online at a quenching temperature of 915°C and a cooling method of water at a cooling rate of 30°C / s;

[0047] (6) Online tempering: The steel plate is tempered and heated. First, the temperature is raised to 400°C at a rate of 6°C / min and kept at this temperature for 30 min. Then, the temperature is raised to 600°C at a rate of 1°C / min and kept at this temperature for 150 min. After tempering, the steel plate is air-cooled to room temperature to obtain a medium and thick plate with a thickness of 25 mm.

[0048] Example 4

[0049] (1) Billet smelting: molten steel is melted according to the design and cast into billets, the chemical composition of the billets is as follows by weight: C: 0.45%, Si: 0.02%, Mn: 1.5%, P: ≤0.005%, S: ≤0.002%, Cr: 2%, Cu: 0.5%, Nb: 0.01%, Al: 0.35%, Mo: 0.15%, Sb: 0.2%, Ti: 0.05%, B: 0.001%, Ni: 0.5%, and the balance is Fe and unavoidable impurities;

[0050] (2) Billet heating: The billet is sent into a heating furnace for heating treatment. The billet entry temperature is 450°C, the heating treatment temperature is 1200°C, the soaking temperature is 1250°C, the soaking speed is 8 min / cm, and the furnace exit temperature is 1000°C.

[0051] (3) Rolling: After the billet leaves the heating furnace, it is descaled with high-pressure water and then subjected to rough rolling. The rough rolling temperature is 980°C, the single-pass reduction rate is 20%, and the number of rough rolling passes is 3. After the rough rolling, the temperature is lowered to 900°C, the single-pass reduction rate is 10%, and the number of finishing passes is 8.

[0052] (4) Cooling: The steel after finish rolling is cooled to 650°C at a rate of 20°C / s, kept at this temperature for 6 min, and then cooled to room temperature at a rate of 50°C / s;

[0053] (5) Online quenching: The steel plate is quenched online at a quenching temperature of 925°C and a cooling method of water at a cooling rate of 25°C / s;

[0054] (6) Online tempering: The steel plate is tempered and heated. First, the temperature is raised to 350°C at a rate of 6°C / min and kept at this temperature for 30 min. Then, the temperature is raised to 650°C at a rate of 2°C / min and kept at this temperature for 90 min. After tempering, the steel plate is air-cooled to room temperature to obtain a medium and thick plate with a thickness of 20 mm.

[0055] Example 5

[0056] (1) Billet smelting: molten steel is melted according to the design and cast into billets, the chemical composition of the billets is as follows by weight: C: 0.45%, Si: 0.02%, Mn: 1.5%, P: ≤0.005%, S: ≤0.002%, Cr: 2%, Cu: 0.5%, Nb: 0.01%, Al: 0.35%, Mo: 0.15%, Sb: 0.2%, Ti: 0.05%, B: 0.001%, Ni: 0.5%, and the balance is Fe and unavoidable impurities;

[0057] (2) Billet heating: The billet is sent into a heating furnace for heating treatment. The billet entry temperature is 450°C, the heating treatment temperature is 1200°C, the soaking temperature is 1250°C, the soaking speed is 8 min / cm, and the furnace exit temperature is 1000°C.

[0058] (3) Rolling: After the billet leaves the heating furnace, it is descaled with high-pressure water and then subjected to rough rolling. The rough rolling temperature is 980°C, the single-pass reduction rate is 20%, and the number of rough rolling passes is 3. After the rough rolling, the temperature is lowered to 900°C, the single-pass reduction rate is 8%, and the number of finishing passes is 8.

[0059] (4) Cooling: The steel after finish rolling is cooled to 650°C at a rate of 20°C / s, kept at this temperature for 6 min, and then cooled to room temperature at a rate of 50°C / s;

[0060] (5) Online quenching: The steel plate is quenched online at a quenching temperature of 925°C and a cooling method of water at a cooling rate of 25°C / s;

[0061] (6) Online tempering: The steel plate is tempered and heated. First, the temperature is raised to 350°C at a rate of 6°C / min and kept at this temperature for 30 min. Then, the temperature is raised to 650°C at a rate of 2°C / min and kept at this temperature for 90 min. After tempering, the steel plate is air-cooled to room temperature to obtain a medium and thick plate with a thickness of 15 mm.

[0062] Example 6

[0063] (1) Billet smelting: molten steel is melted according to the design and cast into billets, the chemical composition of the billets is as follows by weight: C: 0.45%, Si: 0.02%, Mn: 1.5%, P: ≤0.005%, S: ≤0.002%, Cr: 2%, Cu: 0.5%, Nb: 0.01%, Al: 0.35%, Mo: 0.15%, Sb: 0.2%, Ti: 0.05%, B: 0.001%, Ni: 0.5%, and the balance is Fe and unavoidable impurities;

[0064] (2) Billet heating: The billet is sent into a heating furnace for heating treatment. The billet entry temperature is 450°C, the heating treatment temperature is 1200°C, the soaking temperature is 1250°C, the soaking speed is 8 min / cm, and the furnace exit temperature is 1000°C.

[0065] (3) Rolling: After the billet leaves the heating furnace, it is descaled with high-pressure water and then subjected to rough rolling. The rough rolling temperature is 980°C, the single-pass reduction rate is 20%, and the number of rough rolling passes is 3. After the rough rolling, the temperature is lowered to 900°C, the single-pass reduction rate is 10%, and the number of finishing passes is 8.

[0066] (4) Cooling: The steel after finish rolling is cooled to 650°C at a rate of 20°C / s, kept at this temperature for 6 min, and then cooled to room temperature at a rate of 50°C / s;

[0067] (5) Online quenching: The steel plate is quenched online at a quenching temperature of 925°C and a cooling method of water at a cooling rate of 25°C / s;

[0068] (6) Online tempering: The steel plate is tempered and heated. First, the temperature is raised to 350°C at a rate of 6°C / min and kept at this temperature for 30 min. Then, the temperature is raised to 650°C at a rate of 2°C / min and kept at this temperature for 90 min. After tempering, the steel plate is air-cooled to room temperature to obtain a medium and thick plate with a thickness of 10 mm.

[0069] Example 7

[0070] (1) Billet smelting: molten steel is melted according to the design and cast into billets, the chemical composition of the billets is as follows by weight: C: 0.45%, Si: 0.02%, Mn: 1.5%, P: ≤0.005%, S: ≤0.002%, Cr: 2%, Cu: 0.5%, Nb: 0.01%, Al: 0.35%, Mo: 0.15%, Sb: 0.2%, Ti: 0.05%, B: 0.001%, Ni: 0.5%, and the balance is Fe and unavoidable impurities;

[0071] (2) Billet heating: The billet is sent into a heating furnace for heating treatment. The billet entry temperature is 450°C, the heating treatment temperature is 1200°C, the soaking temperature is 1250°C, the soaking speed is 8 min / cm, and the furnace exit temperature is 1000°C.

[0072] (3) Rolling: After the billet leaves the heating furnace, it is descaled with high-pressure water and then subjected to rough rolling. The rough rolling temperature is 980°C, the single-pass reduction rate is 20%, and the number of rough rolling passes is 3. After the rough rolling, the temperature is lowered to 900°C, the single-pass reduction rate is 10%, and the number of finishing passes is 8.

[0073] (4) Cooling: The steel after finish rolling is cooled to 650°C at a rate of 20°C / s, kept at this temperature for 6 min, and then cooled to room temperature at a rate of 50°C / s;

[0074] (5) Online quenching: The steel plate is quenched online at a quenching temperature of 925°C and a cooling method of water at a cooling rate of 25°C / s;

[0075] (6) Online tempering: The steel plate is tempered and heated. First, the temperature is raised to 350°C at a rate of 6°C / min and kept at this temperature for 30 min. Then, the temperature is raised to 650°C at a rate of 2°C / min and kept at this temperature for 90 min. After the tempering is completed, the steel plate is air-cooled to room temperature to obtain a medium-thick plate with a thickness of 4.5 mm.

[0076] Comparative Example 1

[0077] The same as Example 1, except that the steel billet does not contain Cr, Mo, Sb, and B elements.

[0078] Comparative Example 2

[0079] The same as Example 1, except that the steel billet does not contain Cr element.

[0080] Comparative Example 3

[0081] The same as Example 1, except that the steel billet does not contain the Mo element.

[0082] Comparative Example 4

[0083] The same as Example 1, except that the steel billet does not contain Sb element.

[0084] Comparative Example 5

[0085] The same as Example 1, except that the steel billet does not contain element B.

[0086] Comparative Example 6

[0087] Same as Example 1, except that finish rolling was not performed.

[0088] Comparative Example 7

[0089] The same as Example 1, except that only one cooling was performed, and the cooling rate was 20° C. / s.

[0090] Comparative Example 8

[0091] Same as Example 1, except that tempering treatment is performed directly after cooling.

[0092] The performance of the products obtained in Example 1 and Comparative Examples 1 to 8 was tested, and the results were as follows:

[0093] <![CDATA[R p 0.2 / MPa]]> Rm / MPa A / ％ Z / ％ Akv / J(0℃) Akv / J(20℃) Example 1 1150 1380 13.5 70 190 135 Comparative Example 1 1000 1180 18 58 165 119 Comparative Example 2 1050 1260 16 67 173 119 Comparative Example 3 1030 1300 14 65 179 125 Comparative Example 4 1000 1320 13.5 69 182 122 Comparative Example 5 1065 1290 15 63 181 128 Comparative Example 6 1100 1330 13 70 185 127 Comparative Example 7 1130 1280 16 64 190 121 Comparative Example 8 975 1090 18 60 160 110

[0094] The principles and implementation methods of the present invention are described in this article using specific examples. The description of the above embodiments is only used to help understand the method and core idea of ​​the present invention. At the same time, for those skilled in the art, according to the idea of ​​the present invention, there will be changes in the specific implementation methods and application scope. In summary, the content of this specification should not be understood as limiting the present invention.

Claims

1. An online heat treatment production process for medium and thick plates, characterized in that: The following steps are involved: (1) Billet smelting: molten steel is melted according to the composition design and cast into billets, the chemical composition of the billets is as follows by weight: C: 0.40%-0.45%, Si: 0.02%-0.4%, Mn: 1.3%-1.5%, P: ≤0.005%, S: ≤0.002%, Cr: 2%-5%, Cu: 0.3%-0.5%, Nb: 0.01%-0.03%, Al: 0.05%-0.5%, Mo: 0.05%-0.35%, Sb: 0.01%-0.2%, Ti: 0.05%, B: 0.001%, Ni: 0.3%-1.5%, and the balance is Fe and unavoidable impurities; (2) Billet heating: The billet is sent into a heating furnace for heating treatment; (3) Rolling: The heated billet is descaled with high-pressure water and then rough rolled. After the rough rolling, the billet is cooled and finished. (4) Cooling: Cooling the steel plate after finish rolling; (5) Online quenching: online quenching of the cooled steel plate; (6) Online tempering: The steel plate after online quenching is tempered and heated, and after tempering is completed, it is cooled to room temperature to obtain a finished medium and thick plate; In step (2), the furnace temperature of the steel billet is 450°C to 500°C, the heating temperature of the steel billet in the heating furnace is 1200°C to 1300°C, the soaking temperature is 1250°C, the soaking rate is 6 to 8 min / cm, and the furnace exit temperature is 1000°C to 1200°C; The rough rolling temperature is 980°C to 1000°C, the single-pass reduction rate is 18% to 20%, and the rough rolling passes are 3 to 5 passes; The finishing rolling temperature is 850°C to 920°C, the single-pass reduction rate is 6% to 10%, and the finishing rolling passes are 5 to 8 passes; The cooling in step (4) is carried out in two steps. First, the steel plate is cooled to 600-650°C at a rate of 20-35°C / s, kept at that temperature for 6-8 minutes, and then cooled to room temperature at a rate of 40-50°C / s. The quenching temperature is 895°C to 945°C, the cooling method is water cooling, and the water cooling speed is 20-35°C / s; The tempering is carried out in two steps. First, the temperature is raised to 350°C~400°C at a rate of 5°C~6°C / min and kept at this temperature for 30 min. Then, the temperature is raised to 600°C~650°C at a rate of 1°C~2°C / min and kept at this temperature for 90~150 min.

2. The online heat treatment production process for medium and thick plates according to claim 1 is characterized in that: The chemical composition of the steel billet is as follows by weight: C: 0.45%, Si: 0.02%, Mn: 1.5%, P: ≤0.005%, S: ≤0.002%, Cr: 2%, Cu: 0.5%, Nb: 0.01%, Al: 0.35%, Mo: 0.15%, Sb: 0.2%, Ti: 0.05%, B: 0.001%, Ni: 0.5%, and the balance is Fe and unavoidable impurities.

3. The online heat treatment production process for medium and thick plates according to claim 1 is characterized in that: The cooling method after tempering is air cooling.

4. The online heat treatment production process for medium and thick plates according to claim 1 is characterized in that: The thickness of the medium and thick plate is 4-25 mm.