Production method of low-alloy structural steel Q355

Abstract

The invention discloses a production method of low-alloy structural steel Q355, which comprises the step of realizing continuous rolling of blanks with different thicknesses by controlling the heating temperature, the heating time and the heating rate of the blanks with different thicknesses in a heating process and the isothermal temperature, the rolling temperature, the self-tempering temperature and the rolling reduction of the blanks with different thicknesses in a rolling process. According to the invention, a set of available effective method for producing the low-alloy plate is finally summarized by testing the Q355 mark, and by adopting the method provided by the invention, the continuous rolling of blanks with different thicknesses can be realized, and the rolling efficiency and the rolling quality are improved.

Description

Technical Field

[0001] The invention relates to the technical field of metallurgy, and in particular to a production method of low alloy structural steel Q355. Background Art

[0002] Low alloy structural steel Q355 plate has good processability and welding performance. Due to its high hardness and good wear resistance, it is of great significance in the field of processing and is mainly used in engineering machinery, shipbuilding and steel structure. However, the production cost of this type of structural steel is high, and there is a lack of rolling control methods for Q355 steel plates of different thicknesses. Summary of the invention

[0003] The purpose of the present invention is to overcome the defects of the above-mentioned prior art and provide a production method of low alloy structural steel Q355.

[0004] The present invention solves the technical problem by adopting the following technical solutions.

[0005] The present invention provides a production method of low alloy structural steel Q355, comprising: controlling the heating temperature, heating time and heating rate of billets with different thicknesses in a heating process, as well as the isothermal temperature, rolling temperature, red-returning temperature and reduction amount of billets with different thicknesses in a rolling process, so as to realize continuous rolling of billets with different thicknesses.

[0006] The present invention has the following beneficial effects:

[0007] The present invention provides a production method of low alloy structural steel Q355. The present invention provides a production method of low alloy structural steel Q355, comprising: controlling the heating temperature, heating time and heating rate of billets of different thicknesses in a heating process, and the isothermal temperature, rolling temperature, red-returning temperature and reduction amount of billets of different thicknesses in a rolling process, so as to achieve continuous rolling of billets of different thicknesses. The present invention finally summarizes a set of available effective methods for producing low alloy plates by testing the Q355 brand. The method provided by the present invention can achieve continuous rolling of billets of different thicknesses, and improve rolling efficiency and rolling quality. BRIEF DESCRIPTION OF THE DRAWINGS

[0008] In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the drawings required for use in the embodiments are briefly introduced below. It should be understood that the following drawings only show certain embodiments of the present invention and therefore should not be regarded as limiting the scope. For ordinary technicians in this field, other related drawings can be obtained based on these drawings without creative work.

[0009] Figure 1 This is a schematic diagram of the production process of low alloy structural steel Q355. DETAILED DESCRIPTION

[0010] In order to make the purpose, technical scheme and advantages of the embodiments of the present invention clearer, the technical scheme in the embodiments of the present invention will be described clearly and completely below. If the specific conditions are not specified in the embodiments, they are carried out according to conventional conditions or conditions recommended by the manufacturer. If the manufacturer of the reagents or instruments used is not specified, they are all conventional products that can be purchased commercially.

[0011] The following is a specific description of a production method of low alloy structural steel Q355 provided in an embodiment of the present invention.

[0012] An embodiment of the present invention provides a production method of low alloy structural steel Q355, wherein the yield strength of Q355 is 355 MPa, and the chemical composition and mass percentage are as follows: C: ≤0.24, Si: ≤0.55, Mn: ≤1.60, P: ≤0.035, S: ≤0.035, Cr: ≤0.30, Ni: ≤0.30, Cu: ≤0.40.

[0013] The following are the key points for controlling the heating temperature, heating time and heating rate of steel plates of different thicknesses in the heating furnace.

[0014] Direct charging means that the billet is put into the furnace at a temperature greater than or equal to 500°C, hot charging means that the billet is put into the furnace at a temperature greater than or equal to 400°C and less than 500°C, and cold charging means that the billet is put into the furnace at a temperature less than 400°C. Zhongnan Co., Ltd. Plate has two heating furnaces, 1# furnace is a walking beam heating furnace, and 2# furnace is a pusher heating furnace.

[0015]

[0016] 1. When the rolling is stopped for troubleshooting or maintenance for more than 60 minutes, the furnace temperature must be lowered and the upper limit temperature cannot exceed 1200℃.

[0017] 2. Because the heating capacity of the dual furnace production is greater than the rolling capacity, the furnace temperature must be lowered when the total heating time exceeds the upper limit. When the heating time exceeds the standard time of 30 minutes, the heating temperature of each section shall be lowered and controlled according to the lower and middle limits of the process target temperature.

[0018] 3. When the gas pressure fluctuates greatly, the temperature range of some measuring points in the heating furnace is allowed to fluctuate by less than 30°C beyond the allowable range, but it must be adjusted to the allowable range within 5 minutes each time.

[0019] During the production process of low-alloy structural steel Q355, it is necessary to heat the plate strictly according to the given heating temperature and time of the heating furnace according to different plate thicknesses. If the heating temperature is too high, it will affect the surface quality, and if the temperature is too low, it will affect the rolling stability, and the rolling mill will easily trip, affecting the production rhythm, and the target thickness will not be achieved.

[0020] The following are the key points for controlling the rolling temperature and time of steel plates of different thicknesses after they are heated in the heating furnace.

[0021] 1. For steel plates with a thickness of ≤12mm, the rolling temperature of the last three passes is used instead of the finishing rolling start temperature;

[0022]

[0023] Note: h in the isothermal thickness is the plate thickness. 2. Principles for determining the start temperature of finishing rolling: When the length of the rolling plate is greater than 45m, it can be controlled close to the upper limit; when the length of the rolling plate is ≤30m, it should be controlled close to the lower limit (not higher than the center line); when the length of the rolling plate is greater than 30-45m, it should be controlled close to the center line; 3. After rolling, the steel plate with a thickness of greater than 40mm should adopt a uniform cooling process, and the cooling rate should be controlled within the range of 3-10℃ / s; the steel plate with a thickness of ≥55mm should be cooled in a reciprocating manner.

[0024]

[0025] 1. Due to the limitation of the billet, when the isothermal thickness conflicts with the reduction rate, it is necessary to ensure that the two non-widening passes before rough rolling meet the average reduction requirements.

[0026] 2. When the length of the controlled rolling intermediate billet calculated according to the isothermal thickness coefficient in the above table is less than 4.0m, it is not necessary to roll according to the minimum isothermal thickness in the above table, and the intermediate billet can be produced as (4±0.2)m.

[0027] During the production process of low alloy structural steel Q355, rolling needs to be carried out according to the given rolling temperature, time and rolling reduction. The rolling mill can only move when the temperature of the steel plate reaches a sufficient temperature. Otherwise, the rolling force will be too large and the circuit breaker will trip, causing production interruption. The steel plate will not be able to continue rolling after the temperature of the roller table drops. High rolling temperature may cause grain growth and form coarse grains. This will affect the strength and plasticity of the steel and reduce its mechanical properties.

[0028] The following are the key points for controlling the descaling pressure and pass number of steel plates of different thicknesses after they are heated in the heating furnace and then rolled.

[0029] 3. The descaling system pressure is ≥18Mpa, and the descaling box descales 1-2 times. Descaling must be done in the first pass of the rough rolling stage, and the first pass after steel transfer must be descaled. The number of descaling passes in the rough rolling stage shall not be less than 2. Descaling must be done in the first pass of the finishing rolling stage, and the number of descaling passes in the finishing rolling stage shall not be less than 2. The number of descaling passes can be appropriately increased according to the descaling effect.

[0030] During the production process of low alloy structural steel Q355, it is necessary to comply with the given descaling passes and pressure for descaling. Descaling will affect the surface quality. More passes will result in a greater temperature drop and the plate will be easily bent. Fewer passes will result in unclean descaling, affecting the surface quality. Through the above-mentioned process control and optimization, qualified low alloy structural steel Q355 slabs that meet the mechanical properties and organizational requirements can be obtained.

[0031] The present invention will be further described below in conjunction with the embodiments.

[0032] Example 1

[0033] A production method of low alloy structural steel Q355, see Figure 1 , including the following steps:

[0034] The yield strength of Q355 is 355MPa, and the chemical composition and mass percentage are as follows: C: ≤0.24, Si: ≤0.55, Mn: ≤1.60, P: ≤0.035, S: ≤0.035, Cr: ≤0.30, Ni: ≤0.30, Cu: ≤0.40. Direct charging means that the billet is put into the furnace at a temperature greater than or equal to 500°C, hot charging means that the billet is put into the furnace at a temperature greater than or equal to 400 and less than 500°C, and cold charging means that the billet is put into the furnace at a temperature less than 400°C. Zhongnan Co., Ltd. Plate has two heating furnaces, 1# furnace is a walking heating furnace. 2# furnace is a pusher heating furnace.

[0035]

[0036] 1. When the rolling is stopped for troubleshooting or maintenance for more than 60 minutes, the furnace temperature must be lowered and the upper limit temperature cannot exceed 1200℃.

[0037] 2. Because the heating capacity of the dual furnace production is greater than the rolling capacity, the furnace temperature must be lowered when the total heating time exceeds the upper limit. When the heating time exceeds the standard time of 30 minutes, the heating temperature of each section shall be lowered and controlled according to the lower and middle limits of the process target temperature.

[0038] 3. When the gas pressure fluctuates greatly, the temperature range of some measuring points in the heating furnace is allowed to fluctuate by less than 30°C, but it must be adjusted to the allowable range within 5 minutes each time. 1. For steel plates with a thickness of ≤12mm, the rolling temperature of the last three passes is used instead of the finishing rolling start temperature;

[0039]

[0040] Note: h in isothermal thickness is the plate thickness.

[0041] 2. Principles for determining the starting temperature of finishing rolling: when the length of the rolled plate is greater than 45m, it can be controlled close to the upper limit; when the length of the rolled plate is ≤30m, it should be controlled close to the lower limit (not higher than the center line); when the length of the rolled plate is greater than 30-45m, it should be controlled close to the center line; 3. Steel plates with a thickness of greater than 40mm should adopt a uniform cooling process after rolling, and the cooling rate should be controlled within the range of 3-10℃ / s; steel plates with a thickness of ≥55mm should be cooled in a reciprocating manner.

[0042]

[0043] 1. Due to the limitation of the billet, when the isothermal thickness conflicts with the reduction rate, it is necessary to ensure that the two non-widening passes before rough rolling meet the average reduction requirements.

[0044] 2. When the length of the controlled rolling intermediate billet calculated according to the isothermal thickness coefficient in the above table is less than 4.0m, it is not necessary to roll according to the minimum isothermal thickness in the above table, and the intermediate billet can be produced as (4±0.2)m.

[0045] 3. The descaling system pressure is ≥18Mpa, and the descaling box descales 1-2 times. Descaling must be done in the first pass of the rough rolling stage, and the first pass after steel transfer must be descaled. The number of descaling passes in the rough rolling stage shall not be less than 2. Descaling must be done in the first pass of the finishing rolling stage, and the number of descaling passes in the finishing rolling stage shall not be less than 2. The number of descaling passes can be appropriately increased according to the descaling effect.

[0046] Example 2, during the production process of the heating furnace, due to the slow rolling rhythm of the rolling mill, the billet is heated for too long, and the temperature of the heating furnace is controlled at 1220°C to effectively ensure the thickness of the oxide scale on the surface of the billet. During the rolling process, the surface oxide scale can be cleaned using a descaling box to ensure the stability of the rolling process and the control of surface quality.

[0047] Example 3, during the production process, the starting rolling temperature of the steel plate is controlled at 1100°C. During the rolling process, the first three non-widening passes of rough rolling can meet the reduction requirements. The isothermal thickness is controlled according to the isothermal thickness requirements in the table, and the required mechanical properties can be achieved. The grains are refined, and the finishing rolling start temperature is controlled to ensure that the reduction amount of each finishing rolling pass meets the requirements, and finally the specified thickness required by the contract is reached. For example: 30mm thickness Q355, width 2250mm, total rolling passes 13, start rolling temperature 1050℃, average reduction of the first three passes 27.621mm, (27.724mm, 27.662mm, 27.478mm), finishing rolling start rolling temperature 886℃, reduction rates are: 16.59%, 15.15%, 13.01%, 12.36%, 10.61%, 8.04%, final rolling temperature 778℃, after rapid cooling, the red return temperature is 620℃. There are 5 descaling passes, and the descaling passes are 1, 4, 7, 8, and 13, which all meet the control requirements.

[0048] Comparative Example 1

[0049] The steps are similar to those in Example 1, except that: during the heating process in the heating furnace, due to production interruption or double-furnace production, the total heating time exceeds the upper limit and the temperature is not cooled down in time, causing the slab to continue to be intensively heated in the heating furnace, and the surface oxide scale thickens. After being taken out of the furnace, the oxide scale cannot be completely removed by the descaling box, causing the oxide scale to remain on the surface of the steel plate during rolling, affecting the product quality.

[0050] Comparative Example 2

[0051] The steps are similar to those in Example 1, except that the starting rolling temperature is less than 1050°C, the slab temperature is low during the preparation process, and a certain parameter of the rolling reduction amount affected by the temperature during the rolling process is ≤22mm, which cannot meet the reduction requirements of the first three non-widening passes of rough rolling. As a result, the thickness of the steel plate after rolling cannot reach the thickness required by the contract, resulting in failure to meet the contract. For example: 25mmQ355, width 2242mm, rolling temperature 1026℃, does not meet the rolling temperature requirements, the average reduction of the first three passes is 26.86mm (27.21mm, 26.84mm, 26.54mm), in order to ensure that the temperature meets the requirements of finishing rolling, the steel plate is not isothermal, and finishing rolling is started directly. The finishing rolling temperature is 780℃, and the reduction rates of each pass are 16.70mm, 15.28mm, 14.09mm, 12.52mm, 10.54mm, 7.61mm, and the final thickness is 24.97mm, which is significantly greater than the national standard (25±0.75mm).

[0052] The above are only preferred embodiments of the present invention and are not intended to limit the present invention. For those skilled in the art, the present invention may have various modifications and variations. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. A method for producing low alloy structural steel Q355, characterized in that: include: By controlling the heating temperature, heating time and heating rate of billets of different thicknesses in the heating process, as well as the isothermal temperature, rolling temperature, red-returning temperature and reduction amount of billets of different thicknesses in the rolling process, continuous rolling of billets of different thicknesses can be achieved.

2. The production method according to claim 1, characterized in that The chemical composition of low alloy structural steel Q355 is as follows: C: ≤0.24, Si: ≤0.55, Mn: ≤1.60, P: ≤0.035, S: ≤0.035, Cr: ≤0.30, Ni: ≤0.30, Cu: ≤0.40, the balance is iron and other inevitable impurities.

3. The production method according to claim 1, characterized in that Two heating furnaces, 1# furnace and 2# furnace, are used for heating. The heating process control of the casting is shown in the following table: Among them: direct charging means that the blank is put into the furnace at a temperature greater than or equal to 500℃, hot charging means that the blank is put into the furnace at a temperature greater than or equal to 400℃ and less than 500℃, and cold charging means that the blank is put into the furnace at a temperature less than 400℃.

4. The production method according to claim 3, characterized in that: The slab heating process control also includes: (1) When the rolling is stopped for more than 60 minutes due to fault handling or maintenance, the furnace temperature must be lowered and the upper limit temperature cannot exceed 1200℃; (2) When the total heating time exceeds the upper limit requirement, the furnace temperature must be lowered. When the heating time exceeds the standard time by 30 minutes, the heating temperature of each section shall be lowered and controlled according to the lower and middle limits of the process target temperature; (3) When the gas pressure fluctuates greatly, the temperature range of some measuring points in the heating furnace is allowed to fluctuate by less than 30°C beyond the allowable range. Each time, it must be adjusted to the allowable range within 5 minutes.

5. The production method according to claim 1, characterized in that: The rolling process control is shown in the following table:

6. The production method according to claim 5, characterized in that Rolling process control also includes: (1) For steel plates with a thickness of ≤12 mm, the rolling temperature of the last three passes is used instead of the start rolling temperature of the finishing rolling; (2) Principles for determining the start temperature of finishing rolling: when the length of the rolling plate is greater than 45m, it can be controlled close to the upper limit; when the length of the rolling plate is ≤30m, it should be controlled close to the lower limit, that is, it should not be higher than the center line; when the length of the rolling plate is greater than 30-45m, it should be controlled close to the center line; (3) Steel plates with a thickness of more than 40 mm should be cooled uniformly after rolling, and the cooling rate should be controlled within the range of 3-10°C / s; steel plates with a thickness of ≥55 mm should be cooled in a reciprocating manner.

7. The production method according to claim 1, characterized in that: The rolling reduction control is shown in the following table:

8. The production method according to claim 7, characterized in that: Rolling reduction control also includes: (1) Due to the limitation of the billet, when the isothermal thickness conflicts with the reduction rate, it should be ensured that the two non-widening passes before rough rolling meet the average reduction requirements; (2) When the length of the controlled rolling intermediate billet calculated according to the isothermal thickness coefficient in the above table is less than 4.0m, the minimum isothermal thickness in the above table may not be used for rolling, and the intermediate billet may be produced at 4±0.2m; (3) The descaling system pressure is ≥18Mpa, and the descaling box descales 1-2 times. The first pass in the rough rolling stage must be descaled, and the first pass after steel transfer must be descaled. The number of descaling passes in the rough rolling stage shall not be less than 2 times. The first pass in the finishing rolling stage must be descaled, and the number of descaling passes in the finishing rolling stage shall not be less than 2 times. The number of descaling passes can be appropriately increased according to the descaling effect.

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