CN115505817A - A kind of production method of thin thickness low-carbon B-containing hot-rolled steel plate - Google Patents

Abstract

The invention discloses a production method of a low-carbon B-containing hot rolled steel plate with a thin thickness, which mainly solves the technical problems of high cost and poor edge structure plasticity of the low-carbon B-containing hot rolled steel plate with the thickness of 1.2-2.5 mm in the conventional plate strip hot continuous rolling unit. The technical scheme is that the production method of the low-carbon B-containing hot rolled steel plate with the thin thickness comprises the following steps: continuously casting the molten steel to obtain a continuous casting plate blank; heating the continuous casting plate blank by a heating furnace at the temperature of 1190-1230 ℃; carrying out hot rolling on the continuous casting slab by using a slab band hot continuous rolling mill group, wherein the hot rolling adopts a two-stage controlled rolling process, after finish rolling, laminar cooling adopts a back-end cooling mode, and a hot rolled steel coil is obtained by coiling at the coiling temperature of 620-660 ℃; and transferring the hot rolled steel coil to a slow cooling area for heat preservation. The Vickers hardness value HV0.5 of the edge of the hot-rolled steel plate produced by the invention is less than or equal to 100, and the plasticity of the edge structure is good.

Description

A kind of production method of thin thickness low-carbon B-containing hot-rolled steel plate

technical field

The invention relates to a production method of a low-carbon B-containing hot-rolled steel plate, in particular to a production method of a thin-thickness low-carbon B-containing hot-rolled steel plate; The invention discloses a method for a 2.5mm low-carbon B-containing hot-rolled steel plate, belonging to the technical field of iron and steel material manufacturing.

Background technique

Hot-rolled low-carbon steel is a very common steel product, which is widely used in household appliances, packaging, automobiles, architectural decoration, hardware and other fields. It has good formability and can be processed into structural and appearance parts with complex shapes, as well as packaging containers.

The weight percent of the chemical composition of the existing hot-rolled low-carbon steel is: C: 0.01-0.10%, Si≤0.20%, Mn≤1.0%, P≤0.020%, S≤0.020%; the traditional cold-rolled low-carbon steel plate is generally It needs to be used after cold rolling and subsequent annealing on the basis of the hot-rolled original plate, and the thickness of the hot-rolled original plate is 2.5-4.0mm.

With the needs of green economy construction and production environment construction, reducing steel consumption has become the goal pursued by various industries. The product demand of "replacing cold with heat" and "high strength and thinning" has been paid more and more attention. On the one hand, the thickness of cold-rolled low-carbon steel sheets is getting thinner and thinner, requiring upstream to provide thinner and thinner hot-rolled steel sheets; what's more, it is hoped to directly replace cold-rolled steel sheets with thin-gauge hot-rolled sheets. All these changes require the ability to produce ultra-thin gauge hot-rolled low-carbon steel plates; therefore, it is necessary to produce ultra-thin gauge low-carbon hot-rolled steel plates, that is, low-carbon hot-rolled steel plates with a thickness of ≤2.5mm, to meet the social demand for ultra-thin gauge low-carbon hot-rolled steel plates. demand.

However, due to the technical requirements for improving formability, the content of carbon and other alloys in low-carbon steel is also getting lower and lower, so the phase transition temperature Ar ₃ of low-carbon steel for austenite to ferrite transformation is relatively high. However, during the hot rolling process, the temperature of the slab will inevitably drop during the rolling process. If the temperature of the steel plate drops below the Ar ₃ temperature during the rolling process, there will be a two-phase region of austenite and ferrite or even ferrite Deformation in the single-phase region, especially at the edge of the steel plate, where the temperature is low, there will be fibrous deformation structures.

Due to the above reasons, the edges of low-carbon hot-rolled steel sheets with a thickness ≤ 2.5mm retain a severe work-hardening state and fibrous features, and the shape of the edges of low-carbon hot-rolled steel sheets is very poor; The steel plate is processed into a low-carbon cold-rolled steel plate. After cold-rolling and rolling deformation, the edge of the low-carbon cold-rolled steel plate cracks to form edge cracks; , its deformation resistance and plasticity are very different from those of non-edge materials, and edge wave defects are easily formed during cold rolling. Therefore, in order to eliminate edge cracking and wavy defects of low-carbon cold-rolled steel sheets, before cold-rolling, it is necessary to trim the edges of low-carbon hot-rolled steel sheets, that is, cut the The edge portion of the hot-rolled steel plate is cut off, and then the low-carbon hot-rolled steel plate is cold-rolled to obtain a qualified low-carbon cold-rolled steel plate, resulting in waste of edge material of the low-carbon hot-rolled steel plate.

In addition, in order to further improve the formability of cold-rolled low-carbon cold-rolled steel sheets, a certain amount of B element was added to the design of some low-carbon steel components. Due to the addition of B element, the fibrous structure of the edge of low-carbon B-containing hot-rolled steel sheets regenerated. Crystallization is more difficult, and the edge of the low-carbon B-containing hot-rolled steel sheet retains more work hardening, which leads to an increase in the deformation resistance of the edge of the low-carbon B-containing hot-rolled steel sheet during the cold rolling process. The problem of edge cracking of the steel plate is more prominent.

The cost of producing low-carbon and B-containing hot-rolled steel plates with a thickness below 2.5mm is high, and the edge microstructure and plasticity of low-carbon and B-containing hot-rolled steel plates are poor; before cold rolling, if the low-carbon and B-containing hot-rolled steel plates are not The B hot-rolled steel plate is subjected to edge cutting treatment, and the low-carbon B-containing hot-rolled steel plate is directly cold-rolled into a cold-rolled steel plate. Edge cracks and wave-shaped defects appear on the edge of the obtained low-carbon B-containing cold-rolled steel plate.

Contents of the invention

The purpose of the present invention is to provide a production method of thin-thickness low-carbon B-containing hot-rolled steel plate, which mainly solves the problem of high cost and low-carbon low-carbon B-containing hot-rolled steel plate produced by existing strip hot-rolling units. The technical problem of the poor plasticity of the edge structure of the B-containing hot-rolled steel plate; the 1.2-2.5mm thick low-carbon B-containing hot-rolled steel plate produced by the method of the present invention has good plasticity in the edge structure. Steel plate, before cold rolling, there is no need to cut the edge of the low-carbon B-containing hot-rolled steel plate, and directly cold-roll the low-carbon B-containing hot-rolled steel plate into a cold-rolled steel plate, and the obtained low-carbon B-containing cold-rolled steel plate There are no edge cracks and wave-shaped defects on the edge.

The technical idea of the present invention is that the applicant added a certain amount of B element to the design of some existing low-carbon steel components. Due to the addition of B element, the recrystallization of the fibrous structure at the edge of the low-carbon B-containing hot-rolled steel plate is more difficult. , the edge of the low-carbon B-containing hot-rolled steel plate retains more work hardening, which leads to an increase in the deformation resistance of the edge of the low-carbon B-containing hot-rolled steel plate during the cold rolling process, and the edge of the low-carbon B-containing cold-rolled steel plate The cracking problem is more prominent than the technical problem.

In the process of producing low-carbon B-containing hot-rolled steel sheets below 2.5mm in thickness by the existing hot-rolled hot-rolling units, it is common for low-carbon B-containing hot-rolled steel sheets to enter the two-phase region and single-phase region due to the low temperature of the hot-rolling process. The plasticity of the edge structure is poor; subsequently, when the low-carbon B-containing hot-rolled steel plate is processed into a low-carbon B-containing cold-rolled steel plate, after cold-rolling and rolling deformation, edge cracks and wave shapes appear on the edge of the low-carbon B-containing cold-rolled steel plate defect. The cost of producing low-carbon B-containing hot-rolled steel plates with a thickness below 2.5 mm by the existing strip hot rolling mills is high.

Using the newly developed hot rolling process, through the setting of laminar cooling speed and coiling temperature, the fibrous deformation structure at the edge of the hot-rolled steel plate can be recovered to a certain extent in the subsequent slow cooling process, reducing the work hardening of the hot-rolled steel plate To improve the plasticity of the edge of the hot-rolled steel sheet, avoid cold-rolled processing into cold-rolled steel sheet, edge cracks and wave-shaped defects appear in the cold-rolled steel sheet.

The technical solution adopted in the present invention is a production method of thin-thickness low-carbon B-containing hot-rolled steel plate, the method comprising:

The molten steel is continuously cast to obtain a continuous casting slab, wherein the weight percentage of the chemical composition of the molten steel is: C: 0.01%-0.08%, Si: 0.01%-0.10%, Mn: 0.10%-0.40%, P: 0.005%-0.018 %, S≤0.018%, Al: 0.015%～0.050%, B: 0.0010%～0.0050%, the balance is Fe and other alloys and unavoidable impurities;

The continuous casting slab is heated with a heating furnace, and the heating temperature is 1190-1230°C; the continuous casting slab is hot-rolled with a strip hot rolling mill, and the hot rolling adopts a two-stage controlled rolling process, and the rough rolling It is 5-pass rolling, the finishing temperature of rough rolling is 1050-1070℃, and the finishing temperature of finishing rolling is 860-900℃; The flow cooling rate is 5-20°C/s, and the coiling temperature is 620-660°C to obtain hot-rolled steel coils;

Transfer the hot-rolled steel coils to the slow cooling zone, and use the slow cooling wall to slowly cool the hot-rolled steel coils. After slow cooling for 36-72 hours, move the hot-rolled steel coils out of the slow cooling zone for air cooling.

Further, heat preservation covers are set on the hot rolling table between the rough rolling and the finish rolling of the strip hot rolling mill; in the rough rolling stage, the rough rolling descaling is 4 to 6 passes; in the finish rolling stage, the finish rolling descaling is 2 to 6 passes. 4 passes.

For the hot-rolled steel plate produced by the method of the present invention, the metallographic structure of the middle part of the hot-rolled steel plate along the width direction of the plate is equiaxed ferrite + a small amount of pearlite, and the grain size grade of ferrite is 7-9; The metallographic structure of the edge of the hot-rolled steel plate with a distance of 0-20mm from the edge is fine phase transformation ferrite + coarse recrystallized coarse ferrite; the lower yield strength R _eL of the 1.2-2.5mm thick hot-rolled steel plate is 200-280MPa, the tensile strength _Rm is 300-380MPa, the elongation after fracture A is 35-50%, and the Vickers hardness value of the edge of the hot-rolled steel plate is 0-20mm away from the edge along the width direction of the plate HV0.5≤ 100.

The hot rolling process is the technical key to realize the present invention. Through calculation, the transformation temperature Ar _of the composition system of the present invention austenite to ferrite transformation is 800～815 DEG C; the hot rolling process adopted in the present invention is all based on this Invented compositional systems and calculated phase transition points.

The reason of the manufacturing process system that the present invention takes is as follows:

1. Setting of continuous casting slab heating temperature

During the hot rolling process of the slab, in order to obtain suitable properties and reduce deformation resistance, a suitable rolling temperature is required. If the temperature is too high, excess energy will be consumed, and the phenomenon of austenite coarsening and growth is serious, and the surface of the hot-rolled slab is severely oxidized, with thicker iron oxide scale remaining on the final product, affecting the appearance of the product and subsequent use . If the temperature is too low, the temperature of the steel plate will drop below the Ar ₃ temperature during subsequent rolling, resulting in fibrous structures at the edges, edge cracks and wave-shaped defects. Therefore, the heating temperature of the slab in the present invention is 1190-1230°C.

2. Setting of finish temperature of rough rolling

If the finishing temperature of the rough rolling is too low, it will also lead to a low temperature in the finish rolling process, which is lower than the Ar ₃ temperature, resulting in fibrous structures at the edges, edge cracks and wave-shaped defects. If the finishing temperature of rough rolling is too high, the surface temperature of the same steel plate will also be oxidized and neutralized, which will remain on the final product, affecting the appearance and subsequent use of the product. Therefore, the finishing temperature of the rough rolling of the slab of the present invention is 1050 to 1070°C.

3. The heat preservation cover is set on the hot rolling roller table between the rough rolling and the finishing rolling of the strip hot rolling unit, and the descaling pass in the rough rolling stage and the descaling pass in the finishing rolling stage are set

Extremely thin specifications, that is, hot-rolled steel plates with a thickness of ≤2.5mm, because of their thin specifications, they dissipate heat quickly and have a large temperature drop during the rolling process. Therefore, it is necessary to use insulation covers on the hot-rolling roller table. The temperature will drop below the phase transition point Ar ₃ temperature. The purpose of rough rolling and finish rolling descaling is to remove the iron oxide scale produced during the heating process and rolling process of the slab to obtain a good surface. However, too much descaling water for ultra-thin hot-rolled low-carbon steel will cause the temperature drop on the surface of the steel plate to be excessive, especially because the descaling water flows to both sides of the steel plate, which has a great impact on the temperature drop at the edge of the hot-rolled steel plate. Therefore, in the strip hot continuous rolling mill of the present invention, heat preservation covers are set on the hot rolling table between rough rolling and finish rolling, and 4-6 times of rough rolling descaling are set, and 2-4 passes of finishing rolling are descaled.

4. Setting of finishing temperature

The finishing temperature of finish rolling is the key factor to ensure the state of the edge structure, taking into account the temperature difference in the width direction of the steel plate and the transformation point of low carbon steel containing B. If the end temperature is too high, it will inevitably require a higher slab heating temperature, which will aggravate surface defects and surface oxide scale. The end temperature is too low, as low as below the Ar ₃ temperature, resulting in fibrous tissue at the edge, edge cracks and wave-shaped defects. Therefore, the finishing temperature of the present invention is 860-900°C.

5. Setting of laminar cooling speed and coiling temperature

During the rolling process of the hot-rolled steel plate, there will be a temperature difference in the width direction of the steel plate, and the temperature at the edge of the steel plate will be lower than that of other parts. When it is lower than the Ar ₃ temperature, it enters the two-phase region or single-phase region for rolling, and the ferrite in it will be deformed into a fibrous structure. The fibrous deformed structure will recover to a certain extent in the subsequent slow cooling process, reducing the degree of work hardening and improving the plasticity of the material. Similarly, if a high coiling temperature is adopted, the steel plate is kept at a high temperature for a long time, which is also conducive to the recovery of the fibrous structure. Therefore, a slower laminar cooling rate and a higher coiling temperature after finishing rolling are beneficial to improve the plasticity of the edge structure and eliminate edge cracks and wave-shaped defects. However, if the coiling temperature is too high, the grains of the steel plate will be coarse, which will affect the final formability of the steel plate. Therefore, the present invention assumes that the laminar flow cooling adopts the post-stage cooling method, the laminar flow cooling rate is 5-20°C/s, and the coiling temperature is 620-660°C.

6. Setting of the slow cooling time of hot-rolled steel coils in the slow cooling zone

For the reasons mentioned above, when the rolling temperature at the edge of the hot-rolled steel plate is lower than the Ar ₃ temperature, it enters the two-phase region or single-phase region for rolling, and the ferrite in it will be deformed into a fibrous structure. In order to further promote the recovery and recrystallization of these fibrous structures, the hot-rolled steel coils are slowly cooled with a slow cooling wall. If the slow cooling time is too short, the degree of recovery and recrystallization is weak, and some fibrous structures are still retained; If it is too long, it will affect the production turnaround time of the steel plate, and the oxide scale on the surface of the steel plate will thicken during the slow cooling stage, which will affect the surface quality. The present invention sets slow cooling time as 36-72 hours.

Compared with the prior art, the present invention has the following positive effects: 1. Through reasonable process design, the present invention can effectively solve the common problems in the prior art when producing ultra-thin low-carbon steel due to the low temperature of the hot rolling process. From the two-phase region and the single-phase region, the plasticity of the edge structure is poor, and the problems of edge cracks and waves appear in the subsequent cold rolling. 2. Through the setting of the laminar cooling rate and the coiling temperature, the present invention makes the fibrous deformation structure at the edge of the steel plate recover to a certain extent in the subsequent slow cooling process, reduces the degree of work hardening, and is beneficial to improve the plasticity of the material. Eliminate edge cracks and wave-shaped defects. 3. For the 1.2-2.5 mm thick low-carbon B-containing hot-rolled steel plate produced by the present invention, the metallographic structure in the middle of the hot-rolled steel plate along the width direction is equiaxed ferrite + a small amount of pearlite, ferrite grains The hardness level is 7-9; the metallographic structure of the edge of the hot-rolled steel plate 0-20mm away from the edge along the width direction of the plate is fine phase transformation ferrite + coarse recrystallized coarse ferrite; 1.2-2.5mm The lower yield strength R _eL of the thick low-carbon B-containing hot-rolled steel plate is 200-280 MPa, the tensile strength R _m is 300-380 MPa, the elongation A after fracture is 35-50%, and the distance from the edge along the width direction of the plate is 0-20mm The Vickers hardness value of the edge of the hot-rolled steel plate is HV0.5≤100; the edge of the hot-rolled steel plate with a distance of 0-20mm from the edge along the width direction of the plate has good plasticity. 4. The 1.2～2.5mm thick low-carbon B-containing hot-rolled steel plate produced by the method of the present invention is processed into a low-carbon B-containing cold-rolled steel plate through cold rolling. The edge part of the rolled steel plate is cut off, and the low-carbon B-containing cold-rolled steel plate has no edge cracks and wave-shaped defects, which improves the yield of the low-carbon B-containing cold-rolled steel plate and reduces the production cost of the low-carbon B-containing cold-rolled steel plate. 5. The method of the present invention utilizes the existing strip hot rolling unit to improve the microstructure plasticity of the edge of the low-carbon B-containing hot-rolled steel plate, and controls the Vickers hardness value HV0.5≤100 of the edge of the hot-rolled steel plate, without the need for The edge heating equipment of the hot-rolled steel plate is added to the existing strip hot-rolling unit, which saves equipment investment and reduces the production cost of the low-carbon B-containing hot-rolled steel plate.

Description of drawings

Fig. 1 is the metallographic structure photograph of the edge portion of the hot-rolled steel plate 0-20mm away from the edge portion along the plate width direction of Example 2 of the present invention, and the magnification is 100 times;

Fig. 2 is the metallographic structure photograph of the middle part of the hot-rolled steel plate along the plate width direction of embodiment 2, and the magnification is 100 times;

Fig. 3 is the metallographic structure photograph of the edge portion of the low-carbon B-containing hot-rolled steel sheet produced by the prior art along the plate width direction from the edge portion 0-20mm, the chemical composition and thickness of the hot-rolled steel sheet are the same as in Example 2 of the present invention, and the magnification 100 times.

detailed description

The present invention will be further described below in conjunction with Examples 1-5, as shown in Tables 1-3.

Table 1 is the chemical composition (by weight percentage) of the steel of the embodiment of the present invention, and the balance is Fe and unavoidable impurities.

Table 1 Chemical composition of the steel of the embodiment of the present invention, unit: weight percent.

According to the requirements of the material composition design of the present invention, pre-desulfurization of molten iron is adopted, combined top-bottom blowing of converter, Ar blowing station (or LF furnace) ensures bottom blowing Ar stirring time is more than 5 minutes, and fine-tuning of composition is carried out, and Ar blowing protects casting throughout the whole process. Cast into continuous casting slabs. The thickness of the continuous casting slab is 230mm, the width is 800-1630mm, and the length is 8000-11000mm.

The cut-to-length slab produced by steelmaking is sent to the heating furnace for reheating, and then sent to the hot continuous rolling mill for rolling after being descaled. The strip hot rolling mill sets a thermal insulation cover on the hot rolling table between the rough rolling and the finishing rolling, and the rolling is controlled by the rough rolling and the finishing rolling mill. Scale 2-4 passes; coiling after laminar cooling, the laminar cooling adopts the rear cooling method, the laminar cooling speed is 5-20 ℃/s, and the qualified hot-rolled steel coil is produced. The thickness of the hot-rolled steel plate is It is 1.2-2.5mm.

Transfer the hot-rolled steel coils to the slow cooling zone, and use the slow cooling wall to slowly cool the hot-rolled steel coils. After slow cooling for 36-72 hours, move the hot-rolled steel coils out of the slow cooling zone for air cooling. The control parameters of the hot rolling process are shown in Table 2.

Table 2 Embodiment of the present invention hot rolling process control parameters

Example 1. The low-carbon B-containing hot-rolled steel plate with a thickness of 2-2.5mm, the phase transition temperature Ar3 of austenite to ferrite transformation is 800-810°C.

The low-carbon B-containing hot-rolled steel plate obtained by the above invention, see Figure 1 and Figure 2, the metallographic structure in the middle of the hot-rolled steel plate along the width direction is equiaxed ferrite + a small amount of pearlite, ferrite grains The hardness level is 7-9; the metallographic structure of the edge of the hot-rolled steel plate 0-20mm away from the edge along the width direction of the plate is fine phase transformation ferrite + coarse recrystallized coarse ferrite; 1.2-2.5mm The lower yield strength R _eL of the thick low-carbon B-containing hot-rolled steel plate is 200-280 MPa, the tensile strength R _m is 300-380 MPa, the elongation A after fracture is 35-50%, and the distance from the edge along the width direction of the plate is 0-20mm The Vickers hardness value of the edge of the hot-rolled steel plate is HV0.5≤100; the edge of the hot-rolled steel plate with a distance of 0-20mm from the edge along the width direction of the plate has good plasticity.

The hot-rolled steel sheet obtained in the present invention is subjected to a tensile test according to "GB/T228.1-2010 Metal Material Tensile Test Part 1: Room Temperature Test Method", and the edge of the hot-rolled steel sheet obtained in the present invention is subjected to GB T 4340.1- 2009 "Vickers Hardness Test of Metallic Materials Part 1 - Test Method" for Vickers hardness test, its mechanical properties are shown in Table 3.

Table 3 The mechanical properties of the hot-rolled steel plate of the embodiment of the present invention

Referring to Fig. 3, the metallographic structure of the edge of the low-carbon B-containing hot-rolled steel sheet produced in the prior art along the width direction of the sheet is 0-20mm away from the edge is fibrous ferrite, and the plasticity of the edge of the hot-rolled steel sheet is poor, High hardness; before cold rolling, if the hot-rolled steel sheet is not edge-cut, the low-carbon B-containing hot-rolled steel sheet is directly cold-rolled into a cold-rolled steel sheet, and the edge of the obtained low-carbon B-containing cold-rolled steel sheet appears Edge cracks and wave defects.

In addition to the above-mentioned embodiments, the present invention can also have other implementations. All technical solutions formed by equivalent replacement or equivalent transformation fall within the scope of protection required by the present invention.

Claims (3)

1. a kind of production method of thin thickness low carbon containing B hot-rolled steel plate, it is characterized in that, the method may further comprise the steps: The molten steel is continuously cast to obtain a continuous casting slab, wherein the weight percentage of the chemical composition of the molten steel is: C: 0.01%-0.08%, Si: 0.01%-0.10%, Mn: 0.10%-0.40%, P: 0.005%-0.018 %, S≤0.018%, Al: 0.015%～0.050%, B: 0.0010%～0.0050%, the balance is Fe and other alloys and unavoidable impurities; The continuous casting slab is heated with a heating furnace, and the heating temperature is 1190-1230°C; the continuous casting slab is hot-rolled with a strip hot rolling mill, and the hot rolling adopts a two-stage controlled rolling process, and the rough rolling It is 5-pass rolling, the finishing temperature of rough rolling is 1050-1070℃, and the finishing temperature of finishing rolling is 860-900℃; The flow cooling rate is 5-20°C/s, and the coiling temperature is 620-660°C to obtain hot-rolled steel coils; Transfer the hot-rolled steel coils to the slow cooling zone, and use the slow cooling wall to slowly cool the hot-rolled steel coils. After slow cooling for 36-72 hours, move the hot-rolled steel coils out of the slow cooling zone for air cooling. 2. the production method of thin-thickness low carbon containing B hot-rolled steel plate as claimed in claim 1, it is characterized in that, strip hot-rolling unit is provided with insulation cover on the hot-rolling roller table between rough rolling and finish rolling; 4-6 passes for descaling; 2-4 passes for finishing rolling. 3. the production method of thin-thickness low-carbon B-containing hot-rolled steel plate as claimed in claim 1, is characterized in that, along the metallographic structure of hot-rolled steel plate middle part in plate width direction is equiaxed ferrite+a small amount of pearlite, The grain size grade of ferrite is 7-9; the metallographic structure of the edge of the hot-rolled steel plate 0-20mm away from the edge along the width direction of the plate is fine phase-transformed ferrite + coarse recrystallized coarse ferrite body; the lower yield strength R _eL of the hot-rolled steel plate is 200-280 MPa, the tensile strength R _m is 300-380 MPa, and the elongation A after fracture is 35-50%. The Vickers hardness value of the edge of the steel plate is HV0.5≤100.

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