High-strength automotive sheet metal hot-rolling method without corner clearing

Abstract

This invention relates to the field of metallurgical sheet metal technology, and discloses a method for hot-rolling high-strength automotive sheet metal without corner cleaning. The method includes controlling the carbon equivalent of the high-strength automotive sheet metal to <0.07% and performing smelting, continuous casting, and hot rolling without corner cleaning. The high-strength automotive sheet metal comprises the following chemical composition by mass percentage: 0.050%≤C≤0.075%, 0.20%≤Si≤0.80%, 1.40%≤Mn≤2.0%, Cr≤0.50%, B≤0.0020%, Ti≤0.040%, P≤0.040%, 0.010%≤Als≤0.150%, S≤0.008%, N≤0.0060%, Ca≤0.0060%, with the balance being Fe and unavoidable residual elements. This invention addresses the industry challenge of achieving corner-free hot-charging rolling of high-strength duplex steel. It delves into the limiting factors from the source of composition design, continuous casting process, and hot rolling process. Through integrated design, it realizes the industrial practice of corner-free hot-charging rolling of high-strength duplex steel, with outstanding key indicators including hot charging rate, furnace charging temperature, yield, and steel production rate.

Description

Technical Field

[0001] This invention relates to the field of metallurgical sheet technology, specifically to a method for hot-rolling high-strength automotive sheet without corner clearing. Background Technology

[0002] Compared to cold-charged rolling, hot-charged rolling offers significant advantages, including reduced gas consumption, increased machine-hour output, faster billet turnover, reduced corner iron removal losses, and lower oxidation losses. Guided by the pursuit of ultimate efficiency, cost-effectiveness, and profitability, leading steel companies like Baosteel place great emphasis on improving the technical specifications of hot-charged rolling.

[0003] Looking back at the development history of high-strength duplex steel, from composition and process design aimed at obtaining basic products to composition and process design aimed at improving product performance, existing technologies have provided numerous solutions. For example, Chinese patents CN109943778B and CN112760463B disclose a staged rapid cooling innovation process, which introduces an appropriate amount of bainite into traditional ferrite + martensite duplex steel, thereby improving the flanging performance without compromising the drawing performance of duplex steel. Furthermore, Chinese patent CN115386693B further upgrades the staged rapid cooling innovation process, by rationally designing the structural layout of rapid cooling I and rapid cooling II, and optimizing the design of the temperature and cooling rate of rapid cooling I and rapid cooling II, precisely controlling both the bainite content and the bainite morphology, thereby stabilizing product strength and improving product plasticity. However, the above solutions do not address the problems of corner cracks and edge defects (edge ​​peeling + edge cracks) in the cast billet, nor do they provide solutions for hot-charge rolling. To eliminate corner cracks and edge defects, steel mills commonly perform corner cleaning on cast billets before hot rolling. This method not only increases production costs and wastes resources but also adds production steps and reduces efficiency. Existing technologies rarely offer solutions for hot-charging high-strength automotive steel sheets without corner cleaning, especially for high-strength dual-phase steels, where the high proportion of corner crack defects makes corner-free hot-charging extremely difficult. Summary of the Invention

[0004] To overcome the shortcomings of the above-mentioned technologies, the purpose of this invention is to provide a hot rolling method for high-strength automotive steel sheets without corner cleaning, which solves problems such as corner cracks, edge peeling, and edge cracks.

[0005] To achieve the above objectives, the technical concept of this invention is as follows:

[0006] Ingot cracking is one of the most common ingot quality defects and has long been a major and challenging problem for metallurgists. For a long time, duplex steels of 590MPa grade and above have been designed with a peritectic composition, with carbon content generally falling within the range of 0.08% to 0.12%. During solidification, the transformation from the δ phase to the γ phase in peritectic steel is accompanied by significant volume shrinkage, thus significantly reducing high-temperature plasticity. Achieving corner-free hot-charging rolling of duplex steels with a peritectic composition places extremely high demands on casting equipment, casting processes, hot rolling equipment, hot rolling processes, and surface inspection levels. Therefore, avoiding peritectic / sub-pertectic regions in the composition design is crucial.

[0007] The heat transfer, stress, and phase transformation processes during continuous casting solidification are highly complex, and the elements themselves also affect corner cracks. First, carbon (C) is a sensitive element for surface cracks in the billet. Excessive C content not only easily leads to peritectic reactions but may also increase the austenite grain size and even widen the banded structure and the hardness difference between the soft and hard phases in the finished product, severely affecting the formability of the duplex steel. Insufficient C content requires the addition of more alloying elements, which increases both cost and the difficulty of controlling corner cracks. In this invention, the C content is 0.050–0.075%, and can be further optimized to 0.050–0.070%. Second, AlN precipitation deteriorates the high-temperature plasticity of the billet in the second brittle zone. High nitrogen (N) content promotes AlN precipitation and may even increase the precipitation temperature. In this invention, N is limited to ≤0.0060%. Using a micro-Ti composition design can reduce the influence of N and further improve billet quality, but it also increases the fluctuation of the finished duplex steel's performance. When N ≤ 0.0050%, 0.010% ≤ Als ≤ 0.040%, and 0.015% ≤ Ti ≤ 0.040%, the billet quality is optimal, and it exhibits excellent tolerance to casting machine conditions, continuous casting processes, and hot rolling processes. During hot rolling, corner cracks in the billet can evolve into edge peeling defects. Furthermore, improper hot rolling processes can lead to edge cracking defects due to the upward flattening of edge metal under alternating side and flat pressure. Edge peeling and edge cracking defects typically occur within a 35mm range of the strip edge. Reducing and mitigating hot-rolled edge cracking defects is crucial by increasing edge temperature and minimizing edge metal uplift. This can be achieved through methods such as… Figure 1 The concave hammerhead shown and the control of the width reduction are important means.

[0008] Therefore, in order to achieve the objective of this invention, based on the above-mentioned ideas, the technical solution adopted by this invention is as follows:

[0009] A method for hot-rolling high-strength automotive steel sheets without corner clearing includes the following steps:

[0010] 1) Smelting: Smelting is carried out according to the chemical composition and mass percentage of high-strength automotive steel sheets; the carbon equivalent in the chemical composition of high-strength automotive steel sheets is...

[0011] CE=0.1C / (0.1+0.08Si-0.02Mn-0.04P-0.25S+0.03Cr+0.03Ti)<0.07%, the unit is %, the element symbols in the formula are the mass percentage of the corresponding elements, 0.050%≤C≤0.075%, N≤0.0060%;

[0012] 2) Continuous casting: A straight crystallizer arc continuous casting machine is used, with the arc accuracy controlled within ±0.5mm. The target temperature of the billet surface in the straightening zone is ≥900℃, and the billet casting speed is 1.1~1.8m / min.

[0013] 3) Hot rolling: The billet is not cleaned at the corners, the furnace charging temperature is 400~900℃, the heating temperature is 1230±40℃, the total width reduction is ≤200mm, and the side press adopts concave hammer head with a concave depth of 10~40mm;

[0014] 4) Pickling and rolling;

[0015] 5) Heat treatment: continuous annealing or hot-dip plating.

[0016] Furthermore, in step 1), the converter endpoint C ≤ 0.055%, and the refining process adopts a single-stage process.

[0017] Furthermore, in step 2), the liquid level fluctuation of the meniscus of the crystallizer is controlled within ±5mm, the superheat of the tundish is 15-35℃, and electromagnetic stirring and dynamic light pressure are applied.

[0018] Furthermore, in step 2), the billet casting speed is 1.4 to 1.8 m / min.

[0019] Furthermore, in step 3), the billet is a non-first-roll billet with a final rolling temperature of 890±40℃.

[0020] In step 3), in order to control edge defects, the concave depth is further 25-40mm; in order to increase the edge temperature during rolling, an insulation cover and an edge heater are further used; in order to reduce the upward pressing and flattening of the edge metal during rolling, the total width reduction is further ≤150mm.

[0021] This invention also provides a high-strength automotive sheet prepared using the above-mentioned heat-resistant rolling method without cleaning, wherein its chemical composition and mass percentage are: 0.050%≤C≤0.075%, 0.20%≤Si≤0.80%, 1.40%≤Mn≤2.0%, Cr≤0.50%, B≤0.0020%, Ti≤0.040%, P≤0.040%, 0.010%≤Als≤0.150%, S≤0.008%, N≤0.0060%, Ca≤0.0060%, with the balance being Fe and unavoidable residual elements; the carbon equivalent CE of the high-strength automotive sheet is <0.07%; wherein the carbon equivalent CE is calculated as follows:

[0022] CE = 0.1C / (0.1 + 0.08Si - 0.02Mn - 0.04P - 0.25S + 0.03Cr + 0.03Ti), where the unit is %. The element symbols in the formula represent the mass percentage of the corresponding element.

[0023] Furthermore, in the above chemical composition, N≤0.0050%, 0.010%≤Als≤0.040%, and 0.015%≤Ti≤0.040%.

[0024] Furthermore, the high-strength automotive sheet is a duplex steel with a tensile strength ≥590MPa.

[0025] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0026] This invention effectively avoids the crack-sensitive subperitectic region by designing the composition with carbon equivalent CE < 0.07%, 0.050% ≤ C ≤ 0.075%, and N ≤ 0.0060%, thus eliminating the tendency for corner cracks in the cast billet from the source of composition design.

[0027] This invention reduces and mitigates corner crack defects in billets from the source of the continuous casting process by rationally designing the target surface temperature of the billet in the straightening zone, the arc alignment accuracy, the overheating of the tundish, the billet casting speed, and the liquid level fluctuation.

[0028] This invention, through the rational design of the hammer head shape, side pressure reduction amount, heating temperature, and furnace charging temperature of the side press, reduces, mitigates, or even eliminates edge defects (edge ​​peeling and edge cracks) from the source of the hot rolling process. The edge defects can be controlled within 10mm of the edge, which facilitates removal in subsequent processes.

[0029] This invention adopts an integrated design of chemical composition, continuous casting process, and hot rolling process, realizing the industrial-scale hot-rolling of high-strength duplex steel without corner cleaning. The yield rate of edge defects (edge ​​peeling + edge cracks) within 10mm of the hot-rolled edge can reach more than 90%, the hot charging rate can reach more than 90%, the furnace time can be reduced by more than 20 minutes compared with cold charging rolling, and the hot-rolled yield is increased by more than 0.3%. Attached Figure Description

[0030] Figure 1 This is a photograph of the concave hammerhead used in the heat-resistant rolling method of the present invention.

[0031] Figure 2 This is a pickling diagram of the corner of the cast billet in Example 1;

[0032] Figure 3 This is a diagram of hot-rolled edge defects for Comparative Example 4;

[0033] Figure 4 This is a diagram of the edge defects of the finished product in Comparative Example 4;

[0034] Figure 5 This is a metallographic diagram of Example 1. Detailed Implementation

[0035] To better explain the present invention, the main contents of the present invention are further illustrated below with reference to the accompanying drawings and specific embodiments, but the contents of the present invention are not limited to the following embodiments.

[0036] In Examples 1-4 (S1-S4) and Comparative Examples 1-4 (D1-D4), the high-strength automotive steel sheets were dual-phase steels, and their chemical compositions are shown in Table 1. The target thickness of the cast billet was 230 mm, and the corners of the cast billet were right angles.

[0037] Table 1: Major chemical composition (wt%, balance being Fe and unavoidable residual elements)

[0038] serial number C Si Mn Cr Als P S Ca N Ti B CE S1 0.065 0.45 1.80 0.20 0.025 0.016 0.004 0.0040 0.0050 / / <0.07 S2 0.055 0.33 1.90 0.35 0.080 0.030 0.003 0.0026 0.0060 0.025 / <0.07 S3 0.075 0.68 1.50 0.15 0.020 0.012 0.005 0.0034 0.0055 0.030 0.0016 <0.07 S4 0.060 0.50 1.75 0.30 0.030 0.010 0.005 0.0045 0.0048 0.020 / <0.07 D1 0.085 0.40 1.70 / 0.040 0.012 0.004 0.0028 0.0042 / / >0.07 D2 0.085 0.40 1.70 / 0.040 0.012 0.004 0.0028 0.0042 / / >0.07 D3 0.065 0.45 1.80 0.20 0.025 0.016 0.004 0.0040 0.0050 / / <0.07 D4 0.065 0.45 1.80 0.20 0.025 0.016 0.004 0.0040 0.0050 / / <0.07

[0039] The high-strength automotive steel sheets in Examples 1-4 were prepared using the method of the present invention. The difference between Comparative Examples 1-4 and Examples 1-4 lies in the different related processes and parameters; the related processes and parameters are shown in Tables 2 and 3.

[0040] Table 2: Main Smelting and Continuous Casting Process Parameters

[0041]

[0042] Table 3: Main Hot Rolling Process Parameters

[0043]

[0044] Table 4: Comparison of the effects of the proposed solutions

[0045]

[0046]

[0047] Note: Hot-rolled coil yield refers to the percentage of steel coils with edge defects located within 10mm of the edge.

[0048] The corners of the billets produced using the method of this invention (Examples 1-4) are of good quality, and no corner cracks were observed after low-magnification pickling. Figure 2 As shown, its surface quality is good. Furthermore, Examples 1-4 exhibit the microstructure characteristics of duplex steel after heat treatment, consisting of ferrite + martensite + bainite, with the martensite having a larger area ratio than the bainite. Figure 5 As shown, the tensile strength also reaches 590MPa.

[0049] The effects of the schemes in Examples 1-4 and Comparative Examples 1-4 are shown in Table 4. Comparative Examples 1-2 have a high C content (CE > 0.07%) and do not avoid the subperitectic region. Despite using the continuous casting process of this invention, the billet still exhibits severe corner cracks. When the corner cracks are severe, regardless of the hot rolling process used, the yield rate for edge defects within 10mm of the hot-rolled edge is very low. Comparative Examples 3-4 have the same chemical composition as Example 1, but the continuous casting process of Comparative Example 3 differs from that of Example 1. Comparative Example 3 has lower arc-alignment accuracy and a lower billet casting speed, directly affecting the surface quality of the billet. The hot rolling process of Comparative Example 4 differs from that of Example 1, as it does not use a concave hammer, resulting in edge defects in the billet, such as... Figure 3 As shown, the yield rate of edge defects within 10mm of the hot-rolled edge is very low, and even after trimming in subsequent production processes, edge defects that affect usability still exist. Figure 4 As shown. In all embodiments of the present invention, the yield of hot-rolled coils is above 90%, the hot-rolled yield is not less than 90%, and the furnace time is less than 180 minutes. Compared with Comparative Example 2, which uses cold-rolled coils, the hot-rolled yield is increased by 0.5%, and the furnace time is reduced by more than 20 minutes.

[0050] In summary, the method of this invention adopts an integrated design of chemical composition, continuous casting process and hot rolling process, which can realize the industrial-scale hot rolling of high-strength duplex steel without corner cleaning. It has the advantages of low cost, good product surface quality, simple process flow, reduced corner cleaning iron loss and reduced oxidation burn loss.

Claims

1. A method for hot-rolling high-strength automotive steel sheets without corner clearing, characterized in that: Includes the following steps: 1) Smelting: Smelting is carried out according to the chemical composition and mass percentage of high-strength automotive steel sheet, wherein the chemical composition and mass percentage of the high-strength automotive steel sheet are: 0.050%≤C≤0.075%, 0.20%≤Si≤0.80%, 1.40%≤Mn≤2.0%, Cr≤0.50%, B≤0.0020%, Ti≤0.040%, P≤0.040%, 0.010%≤Als≤0.150%, S≤0.008%, N≤0.0060%, Ca≤0.0060%, with the balance being Fe and unavoidable residual elements; the carbon equivalent of the high-strength automotive steel sheet is controlled to be <0.07%; wherein, the carbon equivalent CE is calculated as follows: CE = 0.1C / (0.1 + 0.08Si - 0.02Mn - 0.04P - 0.25S + 0.03Cr + 0.03Ti), the unit is %, and the element symbols in the formula represent the mass percentage content of the corresponding elements; 2) Continuous casting: A straight crystallizer arc continuous casting machine is used, with the arc accuracy controlled within ±0.5mm. The target temperature of the billet surface in the straightening zone is ≥900℃, and the billet casting speed is 1.1~1.8m / min. 3) Hot rolling: The billet is not clear at the corners, the furnace charging temperature is 400~900℃, the heating temperature is 1230±40℃, the total width reduction is ≤200mm, and the side press adopts a concave hammer head with a concave depth of 10~40mm.

2. The corner-free hot-charging rolling method according to claim 1, characterized in that: In step 1), the converter endpoint C ≤ 0.055%, and the refining process adopts a single-stage process.

3. The corner-free hot-charging rolling method according to claim 1, characterized in that: In step 2), the liquid level fluctuation on the meniscus of the crystallizer is controlled within ±5mm, and the superheat of the tundish is 15~35℃; electromagnetic stirring and dynamic light pressure are used.

4. The corner-free hot-charging rolling method according to claim 1, characterized in that: In step 2), the billet casting speed is 1.4~1.8m / min.

5. The corner-free hot-charging rolling method according to claim 1, characterized in that: In step 3), the billet is a non-head billet, and the final rolling temperature is 890±40℃.

6. The corner-free hot-charging rolling method according to claim 1, characterized in that: In step 3), an insulation cover and a side heater are put into use.

7. The hot-charging rolling method without corner clearing according to any one of claims 1 to 6, characterized in that: In step 3), the concave depth is 25~40mm; the total width reduction is ≤150mm.

8. A high-strength automotive sheet prepared using the corner-free hot-rolling method described in claim 1, characterized in that: The chemical composition and mass percentage of the high-strength automotive steel sheet are as follows: 0.050%≤C≤0.075%, 0.20%≤Si≤0.80%, 1.40%≤Mn≤2.0%, Cr≤0.50%, B≤0.0020%, Ti≤0.040%, P≤0.040%, 0.010%≤Als≤0.150%, S≤0.008%, N≤0.0060%, Ca≤0.0060%, with the balance being Fe and unavoidable residual elements; the carbon equivalent CE of the high-strength automotive steel sheet is <0.07%; the calculation method for carbon equivalent CE is as follows: CE = 0.1C / (0.1 + 0.08Si - 0.02Mn - 0.04P - 0.25S + 0.03Cr + 0.03Ti), where the unit is %. The element symbols in the formula represent the mass percentage of the corresponding element.

9. The high-strength automotive steel sheet according to claim 8, characterized in that: In the chemical composition, N≤0.0050%, 0.010%≤Als≤0.040%, and 0.015%≤Ti≤0.040%.

10. The high-strength automotive steel sheet according to claim 8 or 9, characterized in that: The high-strength automotive steel sheet is a duplex steel with a tensile strength ≥590MPa.

Images