Method for improving surface quality of automobile beam steel

Abstract

The invention discloses a method for improving the surface quality of automobile beam steel, and belongs to the technical field of steel and iron material manufacturing. The core improvement of the method is that the coiling temperature is controlled within the range of 585 DEG C to 595 DEG C. According to the method, by precisely reducing the coiling temperature and changing the surface oxidation kinetics conditions of the steel plate, the thickness of the oxide scale can be reduced by 30%-50% to be smaller than or equal to 9 microns, the adhesive force is improved to the first level of the GB / T 13232 standard, the surface defect rate is reduced to be 1% or below, and the subsequent acid pickling time is remarkably shortened. The method is simple in process, free of additional equipment, low in cost and particularly suitable for large-scale application and popularization on an existing hot continuous rolling production line.

Description

Technical Field

[0001] This invention relates to the field of steel material manufacturing technology, specifically to a method for producing hot-rolled automotive beam steel, and more particularly to a method for significantly improving its surface quality by optimizing the coiling process. Background Technology

[0002] Automotive frame steel is a key structural material for manufacturing automotive chassis (frames), requiring high strength, high toughness, good cold bending performance, and excellent surface quality. Defects such as excessively thick iron oxide scale, peeling, and inclusions on the surface will seriously affect subsequent painting processes and reduce the corrosion resistance and overall service life of the chassis.

[0003] In the traditional hot-rolling process of automotive frame steel, the coiling temperature is typically controlled at around 610℃. At this relatively high temperature, the iron atoms on the steel sheet surface react violently with oxygen in the air, producing a thick, loosely structured iron oxide scale with weak adhesion to the base metal. During subsequent pickling, straightening, and forming processes, this loose iron oxide scale easily detaches, causing defects such as pitting and cratering on the steel surface. Furthermore, the detached scale wears down processing equipment, increasing production costs and maintenance difficulties.

[0004] To improve the surface quality of steel beams, existing technologies have proposed several solutions. For example, some solutions attempt to control scale growth by adding extra descaling passes, optimizing complex cooling paths, or adjusting the slab heating regime. However, these methods often require increased equipment investment, extended production cycles, or higher energy consumption, making them unsuitable for economical and efficient implementation on existing large-scale continuous production lines. Other patented technologies optimize scale structure by significantly altering the coiling temperature (e.g., increasing it to 620-650℃ or decreasing it to 560-580℃) and combining it with specific heating, rolling, or slow cooling processes. However, these methods have narrow process windows, high requirements for supporting equipment, high energy consumption, and limited universality.

[0005] For example, patent publication number CN 102319742 A discloses a temperature control method for solving the black ash problem on the surface of hot-rolled automotive frame steel plates. This patent aims to solve the surface black ash problem by reducing the slab heating temperature, performing double descaling, and increasing the coiling temperature (approximately 620~650℃). Patent publication number CN 101906584 B discloses a production method for environmentally friendly, high-surface-quality, acid-free automotive frame steel. This patent aims to optimize the oxide scale and achieve acid-free production by using high-temperature heating, high-temperature rolling, and low-temperature coiling (approximately 560~580℃). Patent publication number CN 115011774 B discloses a method for producing high-surface-quality automotive frame steel using CSP (Continuous Processing) technology. This patent uses a CSP production line, induction heating to the coiling temperature, and slow cooling in a 110~130℃ insulated pit for ≥720 minutes.

[0006] Therefore, developing a method that is simple in process, low in modification cost, easy to implement on existing production lines, and can significantly improve the surface quality of automotive beam steel is a technical problem that urgently needs to be solved in this field. Summary of the Invention

[0007] The purpose of this invention is to overcome the shortcomings of existing technologies and provide a method for improving the surface quality of automotive frame steel. This method, through precise and minor adjustments to the coiling temperature, combined with reasonable composition design and basic process control, significantly reduces the thickness of the surface iron oxide scale and improves its structural density and adhesion without affecting the mechanical properties of the steel. This effectively reduces surface defects and lowers the burden of subsequent pickling.

[0008] To achieve the above objectives, the present invention adopts the following technical solution:

[0009] A method for improving the surface quality of automotive beam steel includes billet preparation, heating, rolling, cooling, coiling and subsequent processing steps, characterized in that the temperature of the coiling step is controlled within the range of 585°C to 595°C.

[0010] Furthermore, the chemical composition of the automobile beam steel billet, by weight percentage, is as follows: C: 0.06-0.10%, Si: ≤0.10%, Mn: 1.20-1.50%, P: ≤0.020%, S: ≤0.010%, Alt: 0.015-0.050%, Nb: 0.010-0.040%, Ti: 0.010-0.025%, with the balance being Fe and unavoidable impurities.

[0011] Furthermore, in the heating step, the slab is heated at a temperature of 1210℃~1240℃ to ensure uniform heating of the billet and avoid overheating.

[0012] Furthermore, in the rolling step, the finishing rolling temperature is 830℃~870℃ to ensure that the strip is rolled in the non-recrystallized austenite region and to refine the grains.

[0013] Furthermore, the cooling step employs laminar flow cooling at a rate of 20°C / s to 50°C / s, rapidly cooling the strip from the final rolling temperature to the target coiling temperature and reducing oxidation time.

[0014] Furthermore, the automotive beam steel produced by the method has an iron oxide scale thickness of ≤9μm.

[0015] Furthermore, the proportion of Fe3O4 phase in the surface iron oxide scale is ≥60%.

[0016] Furthermore, the automotive beam steel produced by the method exhibits surface iron oxide scale adhesion that meets Level 1 of the GB / T13232-2008 standard.

[0017] On the other hand, the present invention also provides automotive beam steel, which is prepared by the above-described method.

[0018] The core principle of this invention:

[0019] This invention creatively reduces the hot-rolling coiling temperature of automotive beam steel from the conventional 610℃ to a range of 590℃±5℃. This specific "medium-low temperature" coiling window is key to achieving a breakthrough in surface quality. At this temperature, the kinetics of the oxidation reaction on the steel plate surface change favorably, the oxidation rate decreases significantly, and the resulting iron oxide scale is thinner. More importantly, this temperature promotes the formation of a large amount of dense, strongly adhering Fe3O4 (magnetite) phase in the iron oxide scale, increasing its proportion to over 60%, while the proportion of the loosely structured FeO (Virtue) phase decreases accordingly. This thin and dense oxide scale layer, dominated by Fe3O4, bonds firmly to the steel substrate and is not easily peeled off during subsequent processing, thus fundamentally solving the problems of surface pitting, peeling, and other defects.

[0020] Meanwhile, this invention, through a compositional design that matches low silicon (Si≤0.10%) with Nb / Ti microalloying, suppresses the formation of a loose SiO2 layer and provides a favorable matrix structure for oxide scale control through fine grain strengthening. Combined with optimized heating, rolling, and cooling processes, it ensures that the internal microstructure of the steel (such as grain size and phase composition) remains well controlled even after the coiling temperature is reduced. This allows the product's key mechanical properties, such as yield strength, tensile strength, elongation, and cold bending performance, to fully meet the requirements of high-standard automotive beam steel.

[0021] Compared with the prior art, the outstanding advantages of the present invention are as follows:

[0022] 1. Significantly improved surface quality: Reducing the coiling temperature to 590℃±5℃ (e.g., 585℃ to 590℃) can significantly reduce the rate and extent of oxidation reaction on the steel surface. The thickness of the iron oxide scale is reduced by 30%~50% compared to conventional processes. The thickness of the surface iron oxide scale can be reduced from the traditional 12-15μm to below 8μm, a reduction of 30%-50%. The Fe3O4 phase ratio is increased from about 40% to ≥60%. The oxide scale adhesion reaches GB / T 13232 standard level 1 (best level).

[0023] 2. Reduced product defect rate: After rolling, the surface defect rate of the finished product (such as pitting and peeling) is reduced from more than 5% to less than 1%.

[0024] 3. The process is extremely simple and the cost is low: the core only requires adjusting one process parameter of the existing production line - the coiling temperature. There is no need to add descaling equipment, induction heating, long-term slow cooling, high-temperature rolling and other complex supporting processes. The transformation cost is almost zero, and it is very easy to achieve large-scale promotion and application on existing hot continuous rolling production lines.

[0025] 4. Excellent overall performance: While achieving excellent surface quality, it fully maintains the core mechanical properties such as high strength and high toughness required for automotive beam steel, achieving the best balance between surface quality and mechanical properties.

[0026] 5. Good environmental and economic benefits: The thinner and easier-to-remove iron oxide scale reduces the subsequent pickling time by about 50%, reduces acid consumption and wastewater treatment load, and lowers production costs and environmental pressure. Attached Figure Description

[0027] Figure 1 This is a thickness measurement diagram of the iron oxide scale on the surface of the steel plate produced in Example 1 of the present invention (coiling temperature 590°C). Detailed Implementation

[0028] The present invention will be described in detail below through specific embodiments. These embodiments are intended to help understand the present invention and are not intended to limit the scope of the present invention.

[0029] This invention provides a method for improving the surface quality of automotive frame steel, the process flow of which includes: hot metal pretreatment → converter smelting → LF refining → continuous casting → slab heating → rough rolling → finish rolling → laminar flow cooling → coiling → finished product. The core lies in the precise control of the coiling temperature (CT) and its combination with specific chemical composition and basic process windows.

[0030] Example 1

[0031] Producing 510L grade automotive beam steel with a target thickness of 6.0mm.

[0032] Smelting and continuous casting: According to the target composition (C: 0.08%, Si: 0.07%, Mn: 1.35%, P: 0.013%, S: 0.006%, Nb: 0.025%, Ti: 0.018%, Alt: 0.030%), converter smelting, LF refining and continuous casting were carried out to obtain a continuously cast slab with a thickness of 230mm.

[0033] Heating: The slab is sent into the heating furnace and heated to 1220℃. The total heating time is ≥180 minutes to ensure that the slab is heated evenly.

[0034] Rolling: The slab is rolled to a 40mm thick intermediate slab by the roughing mill, and then continuously rolled to the target thickness of 6.0mm by the finishing mill. The finishing rolling temperature is controlled at 870℃.

[0035] Cooling and coiling: The strip immediately enters the laminar flow cooling system after rolling, and is rapidly cooled from the final rolling temperature to 590°C at a cooling rate of about 20°C / s. It is then coiled at 590°C to obtain hot-rolled steel coils. Figure 1 The thickness of the iron oxide scale on the surface of the steel plate obtained in Example 1 of the present invention (winding temperature 590°C) is shown.

[0036] Example 2

[0037] The steps are basically the same as in Example 1, except that the winding temperature is adjusted to 585°C.

[0038] Comparative Example 1

[0039] The steps are basically the same as in Example 1, but conventional processes are used, and the winding temperature is adjusted to 610°C.

[0040] The steel coils obtained in Examples 1 and 2 and Comparative Example 1 were sampled and their mechanical properties and surface iron oxide scale quality were tested. The results are shown in Table 1.

[0041] Table 1: Comparison of effects before and after improvement

[0042]

[0043] Results analysis:

[0044] The data in Table 1 clearly shows that:

[0045] 1. Mechanical Properties: The yield strength, tensile strength, and elongation of Examples 1 and 2 are comparable to those of Comparative Example 1, and all meet the mechanical property requirements for 510L beam steel in GB / T 3273-2015. This indicates that the process adjustments of the present invention did not adversely affect the core mechanical properties of the steel.

[0046] 2. Surface Quality: The oxide scale thickness of the embodiment of this invention (≤9μm) is significantly thinner than that of the comparative example (12-15μm). Simultaneously, the proportion of dense Fe3O4 phase in the oxide scale is significantly increased to over 63%, far exceeding the 42% of the comparative example. This directly results in the oxide scale adhesion rating reaching the optimal level 1, while traditional processes only achieve levels 3-4. Reflected in actual production, the surface defect rate of the embodiment is less than 1%, while that of the comparative example is as high as 5.2%.

[0047] 3. Subsequent processing: The thinner iron oxide scale with better adhesion greatly improves pickling efficiency and reduces pickling time by nearly 50%, resulting in significant economic and environmental benefits.

[0048] In summary, this invention, by precisely controlling the coiling temperature within the lower range of 590℃±5℃ and combining it with appropriate components and processes, successfully obtained high-quality automotive beam steel with a thin surface iron oxide scale, dense structure, and strong adhesion without sacrificing mechanical properties. This method is clear in principle, simple to operate, and highly effective, possessing significant industrial application value. This process promotes the formation of a thin, dense, and highly adhesive iron oxide scale (mainly composed of an inner layer of Fe3O4) on the steel plate surface, effectively solving problems such as thick oxide scale, easy detachment leading to surface pitting, and low pickling efficiency under traditional processes. This invention significantly improves the surface quality and subsequent processing performance of the product without significantly altering existing production lines or increasing costs, while ensuring excellent mechanical properties of the steel. This method is applicable to the production of 510L, 610L, and higher grade automotive beam steel.

[0049] Finally, it should be noted that the above descriptions are merely preferred embodiments of the present invention and are not intended to limit the present invention. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art can still modify the technical solutions described in the foregoing embodiments or make equivalent substitutions for some of the technical features. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for improving the surface quality of automotive frame steel, comprising the steps of billet preparation, heating, rolling, cooling, coiling, and subsequent processing, characterized in that, The temperature of the winding step is controlled within the range of 585°C to 595°C.

2. The method according to claim 1, characterized in that, The chemical composition of the steel billet for the automobile frame, by weight percentage, is as follows: C: 0.06-0.10%, Si: ≤0.10%, Mn: 1.20-1.50%, P: ≤0.020%, S: ≤0.010%, Alt: 0.015-0.050%, Nb: 0.010-0.040%, Ti: 0.010-0.025%, with the balance being Fe and unavoidable impurities.

3. The method according to claim 1 or 2, characterized in that, In the heating step, the heating temperature of the slab is 1210℃~1240℃.

4. The method according to claim 1 or 2, characterized in that, In the rolling process, the finishing rolling temperature is 830℃~870℃.

5. The method according to claim 1 or 2, characterized in that, The cooling step employs laminar flow cooling with a cooling rate of 20℃ / s to 50℃ / s.

6. The method according to claim 1 or 2, characterized in that, The automotive beam steel produced by the method has an iron oxide scale thickness of ≤9μm.

7. The method according to claim 6, characterized in that, The proportion of Fe3O4 phase in the surface iron oxide scale is ≥60%.

8. The method according to claim 1 or 2, characterized in that, The automotive beam steel produced by the method described above has an iron oxide scale adhesion that meets the Class 1 standard specified in GB / T 13232-2008.

9. A type of steel for automobile beams, characterized in that, Prepared by the method according to any one of claims 1 to 8.

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