800MPa-grade girder steel suitable for EPS (Expandable Polystyrene) treatment and production method of 800MPa-grade girder steel

Abstract

The invention discloses 800 MPa-grade girder steel suitable for EPS (Expandable Polystyrene) treatment and a production method of the 800 MPa-grade girder steel. The girder steel comprises the following chemical components: 0.05%-0.10% of C, 1.40%-1.80% of Mn, 0.08%-0.18% of Si, less than or equal to 0.003% of S, less than or equal to 0.018% of P, 0.015%-0.035% of Als, 0.03%-0.06% of Nb, 0.06%-0.11% of Ti, less than or equal to 0.0040% of N and the balance of Fe and other inevitable residual elements. The production method comprises the steps of continuous casting, heating, rough rolling, finish rolling, laminar cooling and coiling. According to the girder steel, the tensile strength is larger than or equal to 800 MPa, the yield strength is larger than or equal to 700 MPa, the ductility A is larger than or equal to 18%, and the-20 DEG C impact property is larger than or equal to 80 J; and in the later EPS treatment process, the efficiency is high, and the strip steel quality is excellent.

Description

Technical Field

[0001] The present invention belongs to the technical field of manufacturing hot-rolled automobile beam steel, and particularly relates to an 800MPa grade beam steel suitable for EPS treatment and a production method thereof. Background Art

[0002] The longitudinal beams, reinforcement plates, cross beams, connection plates and some brackets of the truck frame assembly are all made of hot-rolled steel plates. During the rolling process of hot-rolled steel plates, a black scale composed of ferrous oxide (FeO, gray), magnetite (Fe3O4, blue-black) and ferric oxide (Fe2O3, red) is formed on the surface due to the action of high temperature; before painting, the scale and rust on the steel surface should be completely removed, otherwise it will affect the adhesion, decoration and service life of the paint film.

[0003] Common rust removal methods mainly include chemical rust removal and mechanical rust removal. In the past, chemical methods were mostly used for rust removal of large parts - pickling rust removal. However, due to the large environmental pollution caused by pickling, it has gradually been replaced by mechanical rust removal methods. The most common mechanical rust removal is shot blasting rust removal. By using a high-speed rotating impeller, steel shots are thrown onto the steel surface to achieve the purpose of rust removal. Shot blasting rust removal is a common rust removal method, which has the characteristics of high production efficiency, low cost, high automation degree, and can realize assembly line operation, and is widely used by major automobile manufacturing companies. However, with the increasing prominence of environmental problems, a large amount of dust generated during shot blasting pollutes the workshop environment to a certain extent.

[0004] Both pickling and shot blasting rust removal methods will have a certain impact on the environment. Therefore, a new green and environmentally friendly rust removal method EPS (EcoPickledSurface) appears and gradually replaces the traditional rust removal methods. EPS can effectively improve the surface quality and corrosion resistance of the frame assembly after painting; however, the EPS treatment cost is expensive, about 300 yuan per ton. Therefore, it is urgent to start from the material and improve the thickness of the oxide scale on the surface of hot-rolled steel plates to increase the EPS processing speed and thus reduce the processing cost. Summary of the Invention

[0005] The technical problem to be solved by the present invention is to provide an 800MPa grade beam steel suitable for EPS treatment and a production method thereof, which solve the problems of high EPS processing cost and poor surface quality of strip steel in the prior art while ensuring good strength and plasticity of the strip steel.

[0006] To solve the above technical problems, the technical solutions adopted by the present invention are as follows: An 800MPa grade girder steel suitable for EPS treatment, the chemical composition and its mass percentage are as follows: C: 0.05% - 0.10%, Mn: 1.40% - 1.80%, Si: 0.08% - 0.18%, S ≤ 0.003%, P ≤ 0.018%, Als: 0.015% - 0.035%, Nb: 0.03% - 0.06%, Ti: 0.06% - 0.11%, N ≤ 0.0040%, and the balance is Fe and other inevitable residual elements.

[0007] For the girder steel described in the present invention, the tensile strength Rm ≥ 800MPa, the yield strength ReL ≥ 700MPa, the elongation A ≥ 18%, and the impact performance at -20°C ≥ 80J.

[0008] The metallographic structure of the girder steel described in the present invention is ferrite + pearlite.

[0009] The thickness of the girder steel described in the present invention is 4.0 - 12.0mm, and the width is 1250 - 1800mm.

[0010] The production method of the 800MPa grade girder steel suitable for EPS treatment described in the present invention includes smelting, slab continuous casting, heating, rough rolling, finish rolling, laminar flow cooling and coiling processes.

[0011] In the slab continuous casting process described in the present invention, the thickness of the continuous casting billet is 200 - 230mm.

[0012] In the heating process described in the present invention, the heating and holding temperature is 1210 - 1260°C, and the holding time is 190 - 230min. The 800Mpa grade girder steel is mainly strengthened by precipitation, and the main precipitate is TiC. To ensure the effective precipitation of TiC, a relatively high heating temperature is required to fully dissolve TiC; however, if the temperature is too high, it will lead to a relatively high FET inlet temperature and a relatively thick surface scale formed, affecting the final EPS treatment. Therefore, considering comprehensively, the heating and holding temperature is limited to 1210 - 1260°C, and the holding time is 190 - 230min.

[0013] In the rough rolling process described in the present invention, the heated billet is rough rolled in the austenite recrystallization zone in a 3 + 3 mode, and the descaling at the outlet and inlet of R1 and R2 is fully opened, and the descaling pressure ≥ 20MPa.

[0014] In the finish rolling process described in the present invention, the finish rolling inlet temperature is 1000°C - 1050°C, all the cooling water between the finish rolling stands is opened, and the final rolling temperature is 860°C - 900°C.

[0015] In the laminar flow cooling process described in the present invention, the front - stage centralized cooling is adopted, the cooling water is opened sequentially from the back to the front, and the cooling rate is 60 - 100°C / s.

[0016] In the coiling process of the present invention, the coiling temperature is 580 - 620 °C.

[0017] The thickness of the scale on the surface of the girder steel provided by the present invention is ≤ 10 mm, and it is loose and easy to process. During the subsequent EPS treatment, the EPS processing speed can reach 12 m / min, with high efficiency and excellent strip quality.

[0018] The inventive principle of the technical solution of the present invention lies in: At present, the scale on the surface of hot-rolled products is in two extreme states. One is the state that is difficult to remove, and the other is the powdered state. During the slitting process of customers, serious powder dropping occurs, which seriously affects the use of customers. For EPS treatment, the scale needs to be loose and easy to process without powder dropping of hot-rolled products. Currently, controlling the FET temperature is through intermediate billet cooling and steel placement measures, but most production lines do not have intermediate billet cooling devices, and steel placement reduces production efficiency and the FET temperature is difficult to accurately control. The present invention reduces the FET temperature and controls the scale thickness through rough rolling descaling and cooling; in laminar flow cooling, the cooling water controls the strip cooling speed by opening from the back to the front in sequence, achieving the purpose of controlling the scale structure. Finally, the thickness of the scale on the surface of the girder steel produced is ≤ 10 mm, which is loose and easy to process. During the subsequent EPS treatment, the EPS processing speed can reach 12 m / min, with high efficiency and excellent strip quality.

[0019] The beneficial effects of adopting the above technical solution are as follows: (1) By reasonably controlling the controlled cooling process, the present invention develops an 800 MPa grade girder steel suitable for EPS treatment, with a scale thickness of ≤ 10 mm, which is loose and easy to process. During the subsequent EPS treatment, the EPS processing speed can reach more than 12 m / min, with high efficiency and excellent strip quality, effectively reducing the EPS treatment cost.

[0020] (2) The girder steel provided by the present invention has a tensile strength Rm ≥ 800 MPa, a yield strength ReL ≥ 700 MPa, an elongation A ≥ 18%, and an impact performance at -20 °C ≥ 80 J, with good strength and toughness matching. Description of the Drawings

[0021] Figure 1 The metallographic structure of the girder steel in Example 1; Figure 2 The morphology and thickness of the scale on the surface of the girder steel in Example 3 under an optical microscope; Figure 3 The morphology of the scale on the surface of the girder steel in Example 3 under a scanning electron microscope. Detailed Description of the Invention

[0022] The present invention will be further described in detail below in conjunction with the drawings and specific embodiments, but the protection scope of the present invention is not limited thereto.

[0023] An 800MPa grade girder steel suitable for EPS treatment, its chemical composition and mass percentage are as follows: C: 0.05% - 0.10%, Mn: 1.40% - 1.80%, Si: 0.08% - 0.18%, S ≤ 0.003%, P ≤ 0.018%, Als: 0.015% - 0.035%, Nb: 0.03% - 0.06%, Ti: 0.06% - 0.11%, N ≤ 0.0040%, and the balance is Fe and other inevitable residual elements. The chemical compositions of the girder steel in each example are shown in Table 1.

[0024] Table 1 Chemical compositions of the girder steel in each example (wt%)

[0025] In Table 1, the balance is Fe and other inevitable residual elements.

[0026] The production method of the 800MPa grade girder steel suitable for EPS treatment includes smelting, slab continuous casting, heating, rough rolling, finish rolling, cooling and coiling processes; specifically as follows: (1) Smelting and slab continuous casting processes: The steel plate composition undergoes KR hot metal desulfurization pretreatment, top and bottom blowing in the converter, LF refining and continuous casting processes to form a high-purity continuous casting billet, and the thickness of the continuous casting billet is 200 - 230mm.

[0027] (2) Heating: The heating and holding temperature is 1210 - 1260°C, and the holding time is 190 - 230min.

[0028] (3) Rough rolling: The rough rolling is carried out in a 3 + 3 mode, and the descaling at the inlet and outlet of R1 and R2 is fully opened, and the descaling pressure ≥ 20MPa.

[0029] (4) Finish rolling: The finish rolling inlet temperature is 1000°C - 1050°C, the cooling water between the finish rolling stands is fully opened, and the finish rolling temperature is 860°C - 900°C.

[0030] (5) Laminar flow cooling and coiling: The laminar flow cooling adopts concentrated cooling in the front section, as Figure 1 shown, the cooling water is opened sequentially from the back to the front, the cooling rate is 60 - 100°C / s, and the coiling temperature is 580 - 620°C.

[0031] The production process parameters of the steel plates in each example are shown in Table 2 and Table 3.

[0032] Table 2 Parameters of continuous casting, heating and rough rolling processes of the steel plates in each example

[0033] Table 3 Parameters of finish rolling, laminar flow cooling and coiling processes of the steel plates in each example

[0034] The metallographic structure of the girder steel produced in Example 1 is as Figure 1 shown. It can be seen from Figure 1 that its structure is ferrite + pearlite. The metallographic structures of the girder steels in the remaining examples are the same as those in Example 1 and will not be shown one by one.

[0035] The specifications, mechanical properties, scale thickness, and EPS treatment speed of the girder steels in each example are shown in Table 4.

[0036] Table 4 Specifications, mechanical properties, scale thickness, and EPS treatment speed of the steel plates in each example

[0037] It can be seen from Table 4 that the girder steel provided by the present invention has a tensile strength Rm≥800 MPa, a yield strength ReL≥700 MPa, an elongation A≥18%, an impact property at -20°C≥80 J; the surface scale thickness≤10 mm, and the processing speed can reach more than 12 m / min during the later EPS processing.

[0038] An optical microscope was used to observe the scale morphology on the surface of the girder steel in Example 3 and measure its thickness. The results are as Figure 2 shown; in addition, a scanning electron microscope was used to observe the scale morphology on the surface of the girder steel in Example 3. The results are as Figure 3 shown; it can be seen from Figure 2 , Figure 3 that the scale on its surface is loose and easy to handle. The scale morphologies on the surfaces of the girder steels in the remaining examples are the same as those in Example 3 and will not be shown one by one.

[0039] The above examples are only used to illustrate rather than limit the technical solutions of the present invention. Although the present invention has been described in detail with reference to the above examples, those of ordinary skill in the art should understand that: the present invention can still be modified or equivalently replaced, and any modification or partial replacement without departing from the spirit and scope of the present invention shall be covered by the scope of the claims of the present invention.

Claims

1. An 800 MPa grade girder steel suitable for EPS treatment, characterized in that, Its chemical composition and mass percentage are as follows: C: 0.05% - 0.10%, Mn: 1.40% - 1.80%, Si: 0.08% - 0.18%, S ≤ 0.003%, P ≤ 0.018%, Als: 0.015% - 0.035%, Nb: 0.03% - 0.06%, Ti: 0.06% - 0.11%, N ≤ 0.0040%, and the balance is Fe and other inevitable residual elements.

2. An 800 MPa grade girder steel applicable to EPS treatment according to claim 1, characterized in that, The tensile strength Rm of the girder steel is ≥ 800 MPa, the yield strength ReL is ≥ 700 MPa, the elongation A is ≥ 18%, and the impact property at - 20 °C is ≥ 80 J.

3. An 800 MPa grade girder steel applicable to EPS treatment according to claim 1, characterized in that, The thickness of the scale on the surface of the girder steel is ≤ 10 mm.

4. An 800 MPa grade girder steel applicable to EPS treatment according to claim 1, characterized in that, The metallographic structure of the girder steel is ferrite + pearlite.

5. A production method of an 800MPa grade girder steel applicable to EPS treatment according to any one of claims 1-4, characterized in that, It includes smelting, slab continuous casting, heating, rough rolling, finish rolling, laminar flow cooling and coiling processes.

6. The production method of an 800MPa grade girder steel applicable to EPS treatment according to claim 5, characterized in that, In the slab continuous casting process, the thickness of the continuous casting billet is 200 - 230 mm.

7. The production method of an 800MPa grade girder steel applicable to EPS treatment according to claim 5, characterized in that, In the heating process, the heating and holding temperature is 1220 - 1260 °C, and the holding time is 190 - 230 min.

8. The production method of an 800MPa grade girder steel applicable to EPS treatment according to claim 5, characterized in that In the rough rolling process, the 3 + 3 mode is adopted, and the descaling at the inlet and outlet of R1 and R2 is fully opened, and the descaling pressure is ≥ 20 MPa.

9. The production method of an 800MPa grade girder steel applicable to EPS treatment according to claim 5, characterized in that, In the finish rolling process, the finish rolling inlet temperature is 1000 °C - 1050 °C, the cooling water between the finish rolling stands is fully opened, and the final rolling temperature is 860 °C - 900 °C.

10. The production method of an 800MPa grade girder steel applicable to EPS treatment according to claim 5, characterized in that, In the laminar flow cooling process, the front - end centralized cooling is adopted, the cooling water is opened from the back to the front in sequence, and the cooling rate is 60 - 100 °C / s.

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