Angle-cleaning-free hot charging rolling method for high-strength automobile sheet

Abstract

The invention relates to the technical field of metallurgical plates, and discloses a corner-cleaning-free hot charging rolling method for a high-strength automobile sheet, which comprises the following steps: controlling the carbon equivalent lt of the high-strength automobile sheet; and carrying out smelting, continuous casting and corner-cleaning-free hot rolling. The high-strength automobile sheet comprises the following chemical components in percentage by mass: 0.050% < = C < = 0.075%, 0.20% < = Si < = 0.80%, 1.40% < = Mn < = 2.0%, 0.50% < = Cr, 0.0020% < = B, 0.040% < = Ti, 0.040% < = P, 0.010% < = Als < = 0.150%, 0.008% < = S, 0.0060% < = N, 0.0060% < = Ca and the balance of Fe and inevitable residual elements. Aiming at the industrial pain and difficulty that corner-cleaning-free hot charging rolling of the high-strength dual-phase steel is not easy to realize, restricting factors are deeply excavated from the sources of component design, a continuous casting process and a hot rolling process, and industrial practice of corner-cleaning-free hot charging rolling of the high-strength dual-phase steel is realized through integrated design; key indexes including the hot charging rate, the charging temperature, the yield and the yield are very bright.

Description

Technical Field

[0001] The invention relates to the technical field of metallurgical plates, in particular to a corner-clearing-free hot-loading rolling method for high-strength automobile plates. Background Art

[0002] Compared with cold rolling, hot rolling can reduce gas consumption, increase machine hour output, speed up billet turnover, reduce corner iron loss and oxidation and burning loss, which has very significant advantages. Under the guidance of extreme efficiency, extreme cost and extreme benefit, advanced steel companies represented by Baosteel attach great importance to the improvement of technical indicators of hot rolling.

[0003] Looking at the development history of high-strength dual-phase steel, the existing technology provides many solutions from the composition process design for the purpose of obtaining basic products to the composition process design for the purpose of improving product performance. For example: Chinese patents CN109943778B and CN112760463B disclose a graded rapid cooling innovation process, which introduces an appropriate amount of bainite into the traditional dual-phase steel of ferrite + martensite type, thereby improving the flanging performance without damaging the drawing performance of the dual-phase steel. Moreover, Chinese patent CN115386693B further upgrades the graded rapid cooling innovation process, by rationally designing the structural layout of rapid cooling I and rapid cooling II, optimizing the temperature and cooling rate of rapid cooling I and rapid cooling II, and accurately controlling the content of bainite and the morphology of bainite, thereby stabilizing the product strength and improving the product plasticity. However, the above schemes do not address the problems of corner cracks and edge defects (edge ​​warping + edge cracks) of the ingot, nor do they provide solutions for hot rolling. In order to eliminate corner cracks and edge defects, steel mills generally adopt the measure of clearing the corners of the ingot before hot rolling and then cold rolling. This measure not only increases production costs and wastes resources, but also increases production steps and reduces production efficiency. There are few solutions for hot rolling without corner clearing for high-strength automotive plates in the prior art, especially for high-strength dual-phase steel. Due to the large proportion of corner crack defects, it is extremely difficult to achieve hot rolling without corner clearing. Summary of the invention

[0004] In order to overcome the deficiencies of the above-mentioned technologies, the purpose of the present invention is to provide a method for hot-rolling high-strength automobile plates without corner cleaning, so as to solve the problems of corner cracks, edge warping, edge cracks, etc.

[0005] In order to achieve the above objectives, the technical ideas of the present invention are as follows:

[0006] Ingot cracks are one of the most common ingot quality defects, and have long been a difficult and important issue that metallurgical workers have paid great attention to. For a long time, duplex steels of 590MPa grade and above have been designed with the composition of hypoperitectic steels, and the C content generally falls within the range of 0.08-0.12%. The transformation of the δ phase to the γ phase of hypoperitectic steel during solidification is accompanied by a large volume shrinkage, which significantly reduces the high-temperature plasticity. For duplex steels designed with hypoperitectic steel composition, in order to achieve hot rolling without corner clearing, the requirements for casting equipment, casting process, hot rolling equipment, hot rolling process and surface inspection level are extremely high. Therefore, avoiding the peritectic zone / hyperitectic zone in the composition design is the key.

[0007] The heat transfer, stress and phase change processes during continuous casting solidification are very complicated, and the elements themselves also have an impact on corner cracks. First, C is a sensitive element for surface cracks on the ingot. If the C content is too high, it is not only easy to cause peritectic reaction, but also may increase the size of austenite grains, and even increase the hardness difference between the soft and hard phases of the banded structure and the finished product, which seriously affects the forming performance of the dual-phase steel; if the C content is too low, more alloy content needs to be added, which not only increases the cost, but also increases the difficulty of controlling corner cracks. In the present invention, the C content is 0.050-0.075%, and further, it can be optimized to 0.050-0.070%. Secondly, the precipitation of AlN will deteriorate the high-temperature plasticity of the ingot in the brittle zone II. A high N content will promote the precipitation of AlN and even increase the precipitation temperature. In the present invention, it is limited to N≤0.0060%. The composition design of micro-Ti can reduce the influence of N and further improve the quality of the ingot, but it will also increase the performance fluctuation of the finished product of the dual-phase steel. When N≤0.0050%, 0.010%≤Als≤0.040%, 0.015%≤Ti≤0.040%, the ingot quality is optimal, and it is also very tolerant to the casting machine state, continuous casting process, and hot rolling process. During hot rolling production, cracks at the corners of the ingot are inherited and evolve into edge warping defects. In addition, if the hot rolling process is improper, the ingot will be flattened and the edge metal will be turned up and flattened under the alternating action of lateral pressure and flat pressure, which will cause edge crack defects. Edge warping and edge crack defects are usually within 35mm of the edge of the strip. To reduce and alleviate hot rolling edge crack defects, it is key to increase the edge temperature and reduce the upward turning of the edge metal. Use the following methods: Figure 1 The concave hammer head shown and the controlled width reduction are important means.

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

[0009] A method for hot-charging and rolling high-strength automobile plates without corner clearing comprises the following steps:

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

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

[0012] 2) Continuous casting: Use straight mold arc continuous casting machine, the arc accuracy is controlled at ±0.5mm, the target temperature of the billet surface in the straightening zone is ≥900℃, and the billet pulling speed is 1.1~1.8m / min;

[0013] 3) Hot rolling: The billet is not clear, the furnace temperature is 400-900℃, the heating temperature is 1230±40℃, the total width reduction is ≤200mm, the side press uses a concave hammer head, and the concave depth is 10-40mm;

[0014] 4) Acid rolling;

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

[0016] Furthermore, in the step 1), the converter end point C is ≤ 0.055%, and the refining adopts a single-link process.

[0017] Furthermore, in the step 2), the liquid level fluctuation of the meniscus of the crystallizer is controlled at ±5 mm, the superheat of the tundish is 15-35° C., and electromagnetic stirring and dynamic soft pressure are used.

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

[0019] Furthermore, in the step 3), the cast billet is not a head billet, and the final rolling temperature is 890±40°C.

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

[0021] The present invention also provides a high-strength automobile sheet prepared by the above-mentioned hot-rolling method without cleaning, wherein the chemical composition and mass percentage 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%, and the remainder is Fe and unavoidable residual elements; the carbon equivalent CE of the high-strength automobile sheet is less than 0.07%; wherein the calculation method of the carbon equivalent CE is as follows:

[0022] CE=0.1C / (0.1+0.08Si-0.02Mn-0.04P-0.25S+0.03Cr+0.03Ti), the unit is %, and the element symbol in the formula represents 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 automobile plate is dual-phase steel with a tensile strength of ≥590MPa.

[0025] Compared with the prior art, the present invention has the following beneficial effects:

[0026] The present invention effectively avoids the crack-sensitive hypoperitectic region through the composition design of carbon equivalent CE<0.07%, 0.050%≤C≤0.075%, and N≤0.0060%, and eliminates the tendency of corner cracks in the casting billet from the source of composition design.

[0027] The present invention reduces and alleviates the defects of corner cracks of the billet from the source of the continuous casting process by reasonably designing the target temperature of the billet surface in the straightening zone, the arc accuracy, the superheat of the tundish, the billet drawing speed and the liquid level fluctuation.

[0028] The present invention reduces, alleviates or even eliminates edge defects (edge ​​warping + edge cracks) from the source of the hot rolling process through the reasonable design of the hammer head shape, side pressure width reduction, heating temperature and furnace loading temperature of the side press, and can control the edge defects within 10mm of the edge, which is convenient for removal in subsequent processes.

[0029] The present invention adopts an integrated design of chemical composition, continuous casting process and hot rolling process to realize industrial hot rolling without corner cleaning of high-strength dual-phase steel. The yield rate of edge defects (edge ​​warping + edge cracks) within 10 mm of the hot rolled edge can reach more than 90%, the hot charging rate can reach more than 90%, the time in the heating furnace can be reduced by more than 20 minutes compared with cold rolling, and the hot rolling yield rate is increased by more than 0.3%. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Figure 1 A physical picture of the concave hammer head used in the hot-loading rolling method without cleaning of the present invention;

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

[0032] Figure 3 This is the hot-rolled edge defect diagram of Comparative Example 4;

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

[0034] Figure 5 This is the metallographic structure diagram of Example 1. DETAILED DESCRIPTION

[0035] In order to better explain the present invention, the main contents of the present invention are further explained below in conjunction with the drawings and specific embodiments, but the contents of the present invention are not limited to the following embodiments.

[0036] The high strength automobile plates in Examples 1 to 4 (S1 to S4) and Comparative Examples 1 to 4 (D1 to D4) are dual phase steels, and their chemical compositions are shown in Table 1. The target thickness of the ingot is 230 mm, and the shape of the ingot corner is a right angle.

[0037] Table 1: Main chemical composition (wt%, the balance is 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 automobile plates in Examples 1 to 4 are prepared by the method of the present invention. The difference between Comparative Examples 1 to 4 and Examples 1 to 4 lies in the differences in 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 program effects

[0045]

[0046]

[0047] Note: The hot coil yield rate refers to the proportion of steel coils with edge defects within 10mm of the hot coil edge.

[0048] The corners of the ingots of Examples 1 to 4 produced by the method of the present invention have good quality, and no cracks are found in the corners after low-magnification pickling. Figure 2 As shown, the surface quality is good. And after heat treatment, Examples 1 to 4 present the structural characteristics of dual-phase steel, which is ferrite + martensite + bainite, and the area ratio of martensite is greater than that of bainite. Figure 5 As shown, the tensile strength also reaches 590MPa.

[0049] The effects of the solutions of Examples 1 to 4 and Comparative Examples 1 to 4 are shown in Table 4. The C content of Comparative Examples 1 to 2 is relatively high, and the carbon equivalent CE>0.07%. The hypoeutectic zone is not avoided. Despite the continuous casting process of the present invention, the ingot still has serious corner cracks. When the corner cracks of the ingot are serious, no matter what hot rolling process is used, the yield rate of edge defects within 10mm of the hot rolled edge is very low. Comparative Examples 3 to 4 have the same chemical composition as Example 1, but the continuous casting process of Comparative Example 3 is different from that of Example 1. The arc accuracy of Comparative Example 3 is lower, and the ingot pulling speed is also lower, which directly affects the surface quality of the ingot. The hot rolling process of Comparative Example 4 is different from that of Example 1. A concave hammer head is not used, resulting in edge defects in the ingot, such as Figure 3 As shown in the figure, the yield rate of edge defects within 10mm of the hot rolled edge is very low, and there are still edge defects that affect the use after trimming in the subsequent production process, such as Figure 4 As shown. The hot-rolled coil yield rate of each embodiment of the present invention is above 90%, the hot-rolled product rate is not less than 90%, and the furnace time is less than 180 minutes. Compared with the comparative example 2 of cold rolling, the hot-rolled product rate is increased by 0.5%, and the furnace time is reduced by more than 20 minutes.

[0050] In summary, the method of the present invention adopts an integrated design of chemical composition, continuous casting process, and hot rolling process, which can realize the industrialized hot rolling of high-strength dual-phase steel without corner cleaning, and has the advantages of low cost, good product surface quality, and simple process flow, and reduces corner cleaning iron loss and oxidation burning loss.

Claims

1. A method for hot-rolling without corner clearing of high-strength automobile plates, characterized in that: The following steps are involved: 1) Smelting: Smelting is performed according to the chemical composition and mass percentage of the high-strength automobile sheet, wherein the chemical composition of the high-strength automobile sheet is 0.050%≤C≤0.075%, and N≤0.0060%; the carbon equivalent of the high-strength automobile sheet is controlled to be less than 0.07%; wherein the calculation method of the carbon equivalent CE is as follows: CE = 0.1C / (0.1+0.08Si-0.02Mn-0.04P-0.25S+0.03Cr+0.03Ti), the unit is %, the element symbol in the formula represents the mass percentage of the corresponding element; 2) Continuous casting: Use straight mold arc continuous casting machine, the arc accuracy is controlled at ±0.5mm, the target temperature of the billet surface in the straightening zone is ≥900℃, and the billet pulling speed is 1.1~1.8m / min; 3) Hot rolling: The ingot is not cleared, the charging temperature is 400-900℃, the heating temperature is 1230±40℃, the total width reduction is ≤200mm, the side press uses a concave hammer head, and the concave depth is 10-40mm.

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

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

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

5. The corner-clearing-free hot-rolling method according to claim 1, characterized in that: In the step 3), the cast billet is not a head billet, and the final rolling temperature is 890±40°C.

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

7. The corner-clearing-free hot-rolling method according to any one of claims 1 to 6, characterized in that: In the step 3), the depth of the concave shape is 25 to 40 mm; and the total width reduction is ≤150 mm.

8. A high-strength automobile sheet produced by the hot-rolling method without cleaning as claimed in claim 1, characterized in that: The chemical composition and mass percentage of the high-strength automobile 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%, and the balance is Fe and unavoidable residual elements; the carbon equivalent CE of the high-strength automobile sheet is <0.07%; wherein, the calculation method of the carbon equivalent CE is 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 symbol in the formula represents the mass percentage of the corresponding element.

9. The high-strength automobile 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 automobile sheet according to claim 8 or 9, characterized in that: The high-strength automobile plate is a dual-phase steel with a tensile strength of ≥590MPa.

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