Manufacturing method of an ultra-thin low-alloy hot-rolled steel sheet

Abstract

The present invention relates to a method for manufacturing an ultra-thin low-alloy hot-rolled steel plate. Using a continuous casting slab with a thickness ≤ 200 mm as the raw material, it is required that the carbon content in the slab is not more than 0.20% and the carbon equivalent is not more than 0.45%. The slab is rolled into a rough rolling plate with a thickness of 40 - 60 mm on a wide and heavy plate rolling mill, and then three rough rolling plates with the same thickness, width and length dimensions are welded to form a composite slab. The composite slab undergoes a reasonable heating process and a precise rolling process to be rolled into a composite steel plate with a thickness of 9.0 ± 0.3 mm. After that, it is straightened by a hot straightening machine and enters a cooling bed for cooling. Finally, the edges of the composite steel plate are trimmed and the three plates are separated to obtain a steel plate with a thickness of 3 ± 0.3 mm and a width of 2000 - 2400 mm. It solves the technical problems that a wide and heavy plate rolling mill cannot produce ultra-thin specification products and a hot strip mill cannot produce thin plates of such width, improves the product supply specifications of wide and heavy plate enterprises, has the ability to form mass production, ensures product quality, and meets market requirements.

Description

Technical Field

[0001] The present invention belongs to the hot rolling method of steel plates, and particularly relates to a manufacturing method of ultra-thin low-alloy hot-rolled steel plates. Background Art

[0002] Common low-alloy steel plates include structural steel plates represented by Q355B, marine plates represented by DH36, container steel plates represented by 16MnDR, and bridge plates represented by Q420qE, etc. The thickness of these steel plates in use generally concentrates on 8mm - 150mm. For some equipment, steel plates with smaller thicknesses are used, and the more common thickness specifications are 5mm, 6mm, etc. At present, large and medium-sized wide and heavy plate rolling enterprises in China can all produce steel plates of these thickness specifications. However, for steel plates with a thickness less than 5mm, it is difficult for some enterprises to produce. When the thickness of the steel plate required by users is 3mm, they are basically helpless.

[0003] Currently, 3mm thick and thin steel plates in China are usually rolled into coil plates by a hot continuous rolling mill and then obtained through leveling treatment. However, there are problems such as difficult plate shape control and narrow production width for the leveled plates. Summary of the Invention

[0004] The purpose of the present invention is to provide a method for wide and heavy plate rolling enterprises to produce low-alloy steel plates with a thickness as low as 3mm and a width of 2000 - 2400mm. It not only exceeds the continuous rolling steel plates in width but also solves the problem that some steel plate enterprises without continuous rolling equipment cannot produce such thin steel plates, expands the variety and specification of production, and helps to improve the comprehensive competitiveness of products.

[0005] The technical solution adopted by the present invention to solve the above problems is as follows: A manufacturing method of ultra-thin low-alloy hot-rolled steel plates. Prepare n slab billets. Each slab billet is first rolled into an initial rolled plate on a wide and heavy plate rolling mill, and then the n initial rolled plates are welded to form a composite slab billet. The composite slab billet is rolled into a composite steel plate with a thickness n times that of the final steel plate thickness through a heating process and a rolling process. After hot straightening treatment, the composite steel plate is sent to a cooling bed for cooling. Finally, the composite steel plate is trimmed to separate the n steel plates, obtaining ultra-thin specification steel plates.

[0006] Preferably, continuous casting slab billets with a thickness ≤ 200mm are used as raw materials, and it is required that the carbon content in the slab billet is not more than 0.20% and the carbon equivalent is not more than 0.45%. The slab billet is rolled into an initial rolled plate with a thickness of 40 - 60mm on a wide and heavy plate rolling mill, and then three initial rolled plates with the same thickness, width, and length dimensions are welded to form a composite slab billet. The composite slab billet is rolled into a composite steel plate with a thickness of 9.0 ± 0.3mm through a reasonable heating process and a precise rolling process. After being straightened by a hot straightening machine, it enters the cooling bed for cooling. Finally, after trimming the composite steel plate, the three plates are separated, obtaining steel plates with a thickness of 3 ± 0.3mm and a width of 2000 - 2400mm.

[0007] Specifically, the manufacturing method of the ultra-thin low-alloy hot-rolled steel plate of the present application includes the steps

[0008] I. Material selection

[0009] Using a low-alloy slab with a thickness ≤ 200 mm as the raw slab, and the hydrogen content of the slab not exceeding 1.2 ppm;

[0010] II. Heating and rolling

[0011] 2.1 Bloom heating

[0012] Heating process: Heating the slab to make the structure homogeneous and fully austenitized;

[0013] 2.2 Bloom rolling

[0014] Send the slab after high-pressure water descaling to a heavy plate rolling mill, roll for 5 - 10 passes, descale twice during the rolling process, roll into an initial rolled billet with a target thickness of 40 - 60 mm, straighten while still warm, then cool on a cooling bed, unload when the temperature ≥ 400 °C, and stack and slow-cool with a heat preservation cover;

[0015] 2.3 Finishing of the initial rolled billet

[0016] Inspect the appearance quality according to the YB / T2012 standard, store the qualified ones in the warehouse, and cut the initial rolled billet according to the production size requirements;

[0017] 2.4 Non-vacuum composite welding

[0018] Mill and process the surfaces of more than three initial rolled billets of the same size, and then perform non-vacuum composite welding up and down. The thickness of the welded composite billet is 110 - 175 mm;

[0019] 2.5 Heating of the composite billet

[0020] Send the composite billet into the heating furnace for reheating to make the structure homogeneous and fully austenitized;

[0021] 2.6 Finished product rolling

[0022] Roll the composite billet on a heavy plate rolling mill for 10 - 14 passes. The required starting rolling temperature is ≥ 950 °C, perform temperature chasing rolling, without controlled rolling, all longitudinal rolling, and the final rolling temperature is not less than 750 °C, roll into the composite steel plate with the target thickness. The reduction ratio of the first two passes is controlled at 10 - 15% to achieve effective welding of the welding position, and the reduction ratio of the subsequent three to six passes is not less than 20% to accelerate the forming speed of the material at the high temperature stage;

[0023] III. Finishing

[0024] The flame cutting edge of the finished low-alloy steel plate obtained by rolling is 40 - 50 mm wide. After cutting the edge, the upper and lower steel plates are separated to obtain the final steel plates with the same quantity as the number of slabs, and then the subsequent warm straightening, cooling, and offline operations are carried out according to the process requirements.

[0025] The carbon content of the slab described in this application is not more than 0.20 wt.%, and the carbon equivalent is not more than 0.45%.

[0026] The production thickness of the steel plate is less than or equal to 5 mm, and the production width is 2000 - 2400 mm. As one of the implementation manners of this application, the thickness of the composite steel plate is 9 ± 0.3 mm, the production thickness of the final steel plate is 3 mm, and the thickness tolerance is -0.3 to +0.3 mm.

[0027] As one of the implementation manners of this application, in step 2.1, the heating process of the slab: the temperature of the preheating section is 600 - 800 °C, the temperature of the heating section is 1050 - 1200 °C, and the temperature of the soaking section is 1200 - 1280 °C; the heating time is ≥ 8*H min / mm, where H is the thickness of the slab in mm.

[0028] Preferably, in step 2.2, the rolling thickness tolerance of the bloom is ±1.0 mm; the flatness of the bloom is not more than 2 mm / m and not more than 3 mm / 2m.

[0029] As one of the implementation manners of this application, in step 2.5, the heating process of the composite billet: the temperature of the preheating section is 600 - 800 °C, the temperature of the heating section is 1050 - 1200 °C, and the temperature of the soaking section is 1200 - 1280 °C; the heating time is ≥ 9*H min / mm, where H is the thickness of the billet in mm.

[0030] Compared with the prior art, the features or advantages of this application are as follows:

[0031] (1) The bloom is rolled into a bloom billet by blooming rolling. The purpose of blooming rolling is to control the total thickness of the subsequent composite billet to be not less than 100 mm and not more than 180 mm. If the total thickness of the billet is too small, there is a risk of buckling during heating in the furnace. If the total thickness of the billet is too large, when rolling the finished product (about 9 mm) thin plate, it will lead to too many rolling passes, and the billet temperature drops severely at the end of rolling, resulting in a significant increase in rolling force, and there is a risk of the steel plate being scrapped due to equipment pressure relief.

[0032] (2) The finished product rolling adopts single-phase zone rolling. When the heating process meets the requirements, the number of rolling passes varies according to the thickness of the composite billet. It is required that the reduction ratio of the first two passes be controlled at 10-15% to achieve effective welding at the welding position. The reduction ratio of the subsequent three to six passes is not less than 20%. The purpose is to accelerate the material forming speed at the high-temperature stage. A high final rolling temperature is an important means to ensure the plate shape of the material. When the final rolling temperature cannot meet the requirements, the resistance of the material to deformation increases significantly, making it difficult for rolling and subsequent temperature-assisted straightening. Moreover, when the temperature is low, the material enters the two-phase zone from the austenite zone, and the rolled steel plate is prone to warping.

[0033] (3) The production of the 3mm thin steel plate in this application adopts non-vacuum composite of three thin billets. Except for the welding at the edge, other positions are not welded. After trimming the edges of the rolled steel plate, it can be split into three 3mm thick steel plates. The purpose of rapid heating rolling is to avoid excessive temperature loss at the end of rolling, which causes difficulties in rolling. The difficulties are reflected in the need to comprehensively consider the thickness of the composite billet and the setting of the rolling passes to avoid cracking in the non-vacuum slab rolling; it is also reflected in the precise control of the rolling temperature to achieve the purpose of controlling the consistency of the plate shape and thickness of the steel plate.

[0034] This application realizes a method for producing 3mm thick ultra-thin low-alloy hot-rolled steel plates on a wide and heavy plate rolling mill. By designing a billet with a reasonable composition, it is required that the carbon content in the billet is not more than 0.20% and the carbon equivalent is not more than 0.45%. After blooming, the billet forms an ingot slab. The thickness and length of the intermediate billet are reasonably selected and then compound welded. The welded billet is heated, rolled, finished, inspected, and tested to produce a low-alloy hot-rolled steel plate with a thickness of 3mm, a thickness tolerance of -0.3 to +0.3mm, and a width of 2000 - 2400mm. Its product performance, dimensions, and surface quality meet the requirements of relevant standards. This method can fill the gap in the production of low-alloy hot-rolled steel plates with such thicknesses that cannot be produced by wide and heavy plate rolling at home and abroad. Specific Embodiments

[0035] The following further describes the present invention in detail with specific embodiments. Embodiment

[0036] The 3mm thick low-alloy steel type involved in this embodiment is Q355B with a width of 2400mm. Its chemical composition by mass percentage is: C: 0.15, Si: 0.25, Mn: 1.50, Al: 0.03, P: 0.005, S: 0.002, and the rest is Fe and unavoidable impurities. The carbon equivalent is 0.40%.

[0037] The manufacturing process of this steel plate is as follows:

[0038] Steelmaking → Continuous casting → Ingot slab → Composite of three billets → Rolling → Cutting of upper, middle, and lower plates → Finishing → Inspection → Storage.

[0039] The specific operations of the main processes are as follows:

[0040] Steelmaking: During the steelmaking process, harmful elements in the steel are strictly controlled. Among them, the content of H is not more than 1.2 ppm. Calcium treatment needs to be carried out on the molten steel during the refining and degassing processes to control the size and composition of inclusions.

[0041] Continuous casting: The molten steel is continuously cast into a continuous casting slab with a thickness of 150 mm and a width of 2,800 mm by a straight-arc continuous caster. The macrostructure of the slab is as follows: the center segregation is class C level 1.0; the center porosity is level 0.5; there are no other defects such as triangular zone cracks.

[0042] Rolling of the bloom: The continuous casting slab is rolled longitudinally to 40 mm. The rolled bloom is straightened by a hot straightening machine and then cooled in a cooling bed. Heating process of the continuous casting billet: the temperature of the preheating section is 600 - 800 °C, the temperature of the heating section is 1,050 - 1,200 °C, and the temperature of the soaking section is 1,200 - 1,280 °C; the heating time is ≥ 8 * H min / mm, where H is the thickness of the billet in mm. The bloom is taken off the line and cooled to room temperature with a heat preservation cover when the temperature is about 400 °C. The flatness of the bloom is not more than 2 mm / m and not more than 3 mm / 2 m.

[0043] Non-vacuum composite welding: Cut three blooms with the same size from the bloom for surface milling (the upper and lower plates are milled on one side, and the middle plate is milled on both sides), and then perform composite welding around. The middle is non-vacuum. The composite welding position is within 30 - 40 mm from the edge of the billet. The thickness of the composite billet is 112 ± 2 mm.

[0044] Rolling of the composite billet: The temperature of each heating section of the composite billet is the same as the heating requirements of the continuous casting billet, and the heating time is slightly extended, with the requirement that the heating time is ≥ 9 * H min / mm, where H is the thickness of the billet in mm. There is no controlled rolling during the rolling process of the composite billet. The starting rolling temperature is 965 °C, and the final rolling temperature is 800 °C. There is no width spreading rolling, and all are longitudinal rolling, with a total of 12 rolling passes. The reduction of the first two passes is 15 mm and 12 mm respectively. The rolled steel plate is straightened by a hot straightening machine and then cooled in a cooling bed without obvious camber. The thickness of the rolled composite steel plate is 9.1 mm.

[0045] Finishing: After the steel plate is cooled, the welded areas on the four sides are cut off and the upper, middle, and lower plates are separated to obtain a steel plate with a thickness of 3.0 ± 0.3 mm and a width of 2,400 mm. The flatness of the steel plate is not more than 3 mm / m.

[0046] The Q355B steel plate with a thickness of 3 mm and a width of 2,400 mm obtained by the above manufacturing process has a good plate shape, and its main properties are shown in Table 1. Example

[0047] The 3mm-thick low-alloy steel grade involved in this embodiment is DH36, with a width of 2000mm. Its chemical composition by mass percentage is as follows: C: 0.14, Si: 0.30, Mn: 1.45, Cr: 0.20, Nb: 0.015, Al: 0.03, P: 0.005, S: 0.003, and the rest is Fe and inevitable impurities, and the carbon equivalent is 0.42%.

[0048] The manufacturing process of this steel plate is as follows:

[0049] Steelmaking → Continuous casting → Bloomer → Tri-billet composite → Rolling → Top, middle and bottom plate splitting → Finishing → Inspection → Storage.

[0050] The specific operations of the main processes are as follows:

[0051] Steelmaking: During the steelmaking process, harmful elements in the steel are strictly controlled. Among them, H is not more than 1.2ppm. Calcium treatment needs to be carried out on the molten steel in the refining and degassing processes to control the size and composition of inclusions.

[0052] Continuous casting: The molten steel is continuously cast into a continuous casting slab with a thickness of 150mm and a width of 2400mm by a straight-arc continuous caster. The macrostructure of the slab is as follows: center segregation class C is 0.5 level; center porosity is 0.5 level; there are no other defects such as triangular zone cracks.

[0053] Bloomer rolling: The continuous casting slab is fully longitudinally rolled to 60mm. The rolled bloomer enters the hot straightening machine for straightening and then enters the cooling bed for cooling. Heating process of continuous casting billet: preheating section temperature 600 - 800°C, heating section temperature 1050 - 1200°C, soaking section temperature 1200 - 1280°C; heating time ≥ 8*H min / mm, where H is the billet thickness in mm. The bloomer temperature is taken offline and cooled to room temperature with a heat preservation cover when it is about 400°C. The flatness of the bloomer is not more than 2mm / m and not more than 3mm / 2m.

[0054] Non-vacuum composite welding: Cut three bloomer plates with the same size from the bloomer for surface milling (the upper and lower plates are milled on one side, and the middle plate is milled on both sides), and then perform composite welding around. The middle is non-vacuum. The composite welding position is within 30 - 40mm from the edge of the billet. The thickness of the composite billet is 172 ± 2mm.

[0055] Rolling of composite billet: The heating section temperatures of the composite billet are the same as the heating requirements of the continuous casting billet, and the heating time is slightly extended, requiring the heating time ≥ 9*H min / mm, where H is the billet thickness in mm. There is no controlled rolling during the rolling process of the composite billet. The starting rolling temperature is 980°C, the final rolling temperature is 790°C, there is no width spreading rolling, all are longitudinal rolling, and a total of 14 passes are rolled. Among them, the reduction ratios of the first two passes are 19mm and 20mm. The rolled composite steel plate is straightened by the hot straightening machine and then cooled on the cooling bed without obvious camber. The thickness of the rolled composite steel plate is 9.0mm.

[0056] Finishing: After the steel plate is cooled, the welded areas on the four sides are cut off and the upper, middle and lower plates are separated to obtain a steel plate with a thickness of 3.0 ± 0.3 mm, a width of 2000 mm, and the flatness of the steel plate is not more than 3 mm / m.

[0057] The 3-mm-thick and 2000-mm-wide DH36 steel plate produced by the above manufacturing process has a good plate shape, and its main properties are shown in Table 1.

[0058] Table 1 Performance of the steel plates produced in each example

[0059]

[0060] Although the preferred embodiments of the present invention have been described in detail above, it should be clearly understood that various changes and modifications can be made to the present invention for those skilled in the art. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principle of the present invention shall be included within the protection scope of the present invention.

Claims

1. A manufacturing method of an ultra-thin low-alloy hot-rolled steel plate, characterized in that: Prepare n slabs. Each slab is first rolled into an ingot slab on a heavy plate mill, and then the n ingot slabs are welded to form a composite slab. The composite slab is rolled into a composite steel plate with a thickness n times that of the final steel plate thickness through a heating process and a rolling process. After hot straightening treatment, the composite steel plate is sent to a cooling bed for cooling. Finally, the composite steel plate is trimmed to separate the n steel plates, and an ultra-thin specification steel plate is obtained. The method includes the following steps: I. Material selection Use low-alloy slabs with a thickness ≤ 200 mm as raw material slabs, and the hydrogen content of the billets is not more than 1.2 ppm. II. Heating and rolling 2.1 Ingot slab heating Heating process: Heat the slab to make the structure homogeneous and fully austenitized. 2.2 Ingot slab rolling Send the slab that has undergone high-pressure water descaling to a heavy plate mill and roll it for 5 - 10 passes. During the rolling process, descale twice. Roll it into an ingot slab with a target thickness of 40 - 60 mm. After hot straightening, cool it on a cooling bed and unload it when the temperature ≥ 400 °C, and stack it with a heat preservation cover for slow cooling. 2.3 Finishing of ingot slab Inspect the appearance quality according to the YB / T2012 standard. Those meeting the standard are stored in the warehouse. Cut the ingot slab according to the production size requirements. 2.4 Non-vacuum composite welding Mill and process the surfaces of more than three ingot slabs of the same size, and then perform non-vacuum composite welding up and down, that is, the welding joints at the edges of the ingot slabs are welded together, and other positions are not welded. The thickness of the welded composite blank is 100 - 180 mm. 2.5 Heating of composite blank Send the composite blank into a heating furnace for reheating to make the structure homogeneous and fully austenitized. 2.6 Finished product rolling Roll the composite blank on a heavy plate mill for 10 - 14 passes. The required starting rolling temperature is ≥ 950 °C. Roll while seizing the temperature, without controlled rolling, all longitudinal rolling. The final rolling temperature is not less than 750 °C. Roll it into a composite steel plate with the target thickness. Control the reduction ratio of the first two passes at 10 - 15% to achieve effective welding of the welding position. The reduction ratio of the subsequent three to six passes is not less than 20%, and accelerate the forming speed of the material at the high-temperature stage. III. Finishing Flame cut 40 - 50 mm wide from the edges of the finished low-alloy steel plate obtained by rolling. After cutting the edges, separate the upper and lower steel plates to obtain the same number of final steel plates as the number of slabs. Subsequently, perform hot straightening, cooling, and unloading according to the process requirements.

2. The manufacturing method of the ultra-thin low-alloy hot-rolled steel sheet according to claim 1, characterized in that: The carbon content of the slab is not more than 0.20 wt.%, and the carbon equivalent is not more than 0.45%.

3. The manufacturing method of the ultra-thin low-alloy hot-rolled steel sheet according to claim 1, characterized in that: The production thickness of the steel plate is less than or equal to 5 mm, and the production width is 2000 - 2400 mm.

4. The manufacturing method of the ultra-thin low-alloy hot-rolled steel sheet according to claim 3, characterized in that: The thickness of the composite steel plate is 9 ± 0.3 mm, the production thickness of the final steel plate is 3 mm, and the thickness tolerance is -0.3 to +0.3 mm.

5. The manufacturing method of the ultra-thin low-alloy hot-rolled steel sheet according to claim 1, characterized in that: In step 2.1, for the heating process of the slab: the temperature of the preheating section is 600 - 800 °C, the temperature of the heating section is 1050 - 1200 °C, and the temperature of the soaking section is 1200 - 1280 °C; the heating time is ≥ 8 * H min / mm, where H is the thickness of the slab in mm.

6. The manufacturing method of the ultra-thin low-alloy hot-rolled steel sheet according to claim 1, characterized in that: In step 2.2, the rolling thickness tolerance of the ingot slab is ±1.0 mm; the flatness of the ingot slab is not more than 2 mm / m and not more than 3 mm / 2m.

7. The manufacturing method of the ultra-thin low-alloy hot-rolled steel sheet according to claim 1, characterized in that: In Step 2.5, the heating process of the composite blank: preheating section temperature 600 - 800 °C, heating section temperature 1050 - 1200 °C, soaking section temperature 1200 - 1280 °C; heating time ≥ 9 * H min / mm, where H is the blank thickness in mm.

8. The manufacturing method of the ultra-thin low-alloy hot-rolled steel sheet according to claim 1, characterized in that: In Step 2.4, the thickness of the composite blank is 110 - 175 mm.

Images