An efficient production method for duplex steel with a tensile strength of 590 MPa

Abstract

The present invention discloses an efficient production method for duplex steel with a tensile strength of 590 MPa, comprising the following steps: smelting, continuous casting, hot rolling, pickling and continuous annealing; the composition of the molten steel obtained by smelting avoids the crack-sensitive sub-peritectic zone; a straight crystallizer arc continuous casting machine is used during continuous casting, the arc accuracy is controlled within ±0.5 mm, and the target temperature of the surface of the ingot in the straightening zone is ≥900°C; the ingot is directly hot-rolled without clearing the corners, the heating temperature is 1230±40°C, and the side press adopts a concave hammer head; different furnace running speeds are adopted for different strip thicknesses, and after annealing, it is subjected to slow cooling, rapid cooling I and rapid cooling II, and then aged and leveled; rapid cooling I and rapid cooling II are adjacent to each other in the structural layout of the annealing furnace; the present invention realizes efficient corner clearing treatment in the continuous casting process, efficient hot charging and rolling in the hot rolling process, and efficient speed-up production in the continuous annealing process through the integrated design of chemical composition, continuous casting process, hot rolling process and continuous annealing process.

Description

Technical Field

[0001] The present invention belongs to the technical field of duplex steel, and in particular relates to a method for efficiently producing duplex steel with a tensile strength of 590 MPa. Background Art

[0002] In the face of increasingly fierce competition, extreme efficiency is crucial for achieving optimal costs and profitability. The automotive steel production process involves a lengthy process, with common steps including smelting, continuous casting, hot rolling, pickling, annealing, and coating. Improving process efficiency is paramount to achieving this goal. Steel mills are currently implementing specific measures to improve production efficiency, including increasing the number of continuous casting furnaces, high-speed billet drawing, efficient rolling, and increased annealing speeds.

[0003] Looking back at the development of high-strength dual-phase steel over more than half a century, the technology is relatively mature. From the composition process for the purpose of obtaining basic products to the composition process design for achieving performance improvement, the existing technology provides many solutions. Chinese patents CN109943778A and CN112760463A disclose a graded rapid cooling innovation process, which introduces an appropriate amount of bainite into the traditional ferrite + martensite dual-phase steel, thereby improving the flanging performance without compromising the drawing performance of the dual-phase steel. Moreover, Chinese patent CN115386693A 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 strength of the product and improving the plasticity of the product. However, there are few solutions aimed at efficient production.

[0004] In industrial practice, dual-phase steel with a tensile strength of 590MPa generally adopts the composition design of hypoeutectic steel. Due to the high proportion of corner cracks in the ingot, efficient hot rolling cannot be achieved economically. Due to the coupling influence of composition process-equipment capacity-organization performance, annealing speed increase is greatly restricted. The calculation model of rapid cooling outlet temperature and over-aging temperature based on strip speed disclosed in Chinese patents CN116493421A and CN116334374A can simply and quickly achieve quality assurance and increased production of cold-rolled dual-phase steel. However, the above solutions can only solve part of the problem. When the heating capacity and slow cooling capacity are insufficient, how to increase the speed with quality remains to be solved. Summary of the Invention

[0005] In order to solve the above technical problems, the present invention provides an efficient production method for dual-phase steel with a tensile strength of 590 MPa. Through reasonable composition design, it effectively avoids the crack-sensitive sub-peritectic zone, reduces and alleviates cracks in the corners of the ingot, realizes efficient hot rolling, reduces and weakens the correlation between process parameters and microstructure properties, expands the process window, breaks the equipment bottleneck and process bottleneck, and realizes annealing speed increase.

[0006] In order to achieve the above objectives, the specific technical solutions of the present invention are as follows:

[0007] A high-efficiency production method for dual-phase steel with a tensile strength of 590 MPa, comprising the following steps: smelting, continuous casting, hot rolling, pickling and annealing;

[0008] The composition of the molten steel obtained by smelting avoids the crack-sensitive hypoperitectic region;

[0009] During continuous casting, a straight mold arc continuous casting machine is used, with arc accuracy controlled within ±0.5mm, and the target temperature of the billet surface in the straightening area ≥900℃;

[0010] The ingot is directly hot-rolled without clearing the corners, the heating temperature is 1230±40℃, and the side pressing machine adopts a concave hammer head;

[0011] The continuous annealing adopts a short slow cooling type continuous annealing unit with a slow cooling section length of 20 to 25m. When the strip thickness is 0.5mm≤≤1.2mm, the strip speed in the furnace is 180±30m / min; when the strip thickness is 1.2mm<≤1.5mm, the strip speed in the furnace is 160±30m / min; when the strip thickness is 1.5mm<≤1.8mm, the strip speed in the furnace is 140±30m / min; when the strip thickness is 1.8mm<≤2.0mm, the strip speed in the furnace is 120±30m / min; when the strip thickness is 2.0mm<≤2.5mm, the strip speed in the furnace is 100±30m / min.

[0012] After annealing, the furnace is subjected to slow cooling, rapid cooling I and rapid cooling II, followed by aging treatment and leveling. Rapid cooling I and rapid cooling II are located adjacent to each other in the structural layout of the annealing furnace. The annealing soaking temperature is 790±20℃, and rapid cooling is performed after slow cooling to 710±20℃.

[0013] Furthermore, the chemical composition and mass fraction of the 590 MPa tensile strength grade dual-phase steel meet the following conditions in percentage: C: 0.050-0.075%, Si: 0.45-0.80%, Mn: 1.70-2.00%, Cr: 0.20-0.40%, P≤0.040%, Als: 0.010-0.060%, S≤0.015%, N≤0.0055%, Ca≤0.0060%, and the balance is Fe and unavoidable impurities.

[0014] In the continuous casting step, the casting speed is 1.1 to 1.8 m / min.

[0015] In the hot rolling step, the charging temperature is 400-900° C., the heating temperature is 1230±40° C., the finishing temperature is 890±40° C., the coiling temperature is 520-650° C., and the total width reduction is ≤200 mm.

[0016] In the continuous annealing step, the slow cooling rate is 4.5-16.5°C / s, the slow cooling section has a cooling length of 20-25m, and a single pass design is adopted.

[0017] In the continuous annealing step, the temperature of rapid cooling I is 520±40°C, and the cooling rate of rapid cooling I is 60-130°C / s, that is, rapid cooling to 520±40°C at a cooling rate of 60-130°C / s.

[0018] In the continuous annealing step, the temperature of rapid cooling II is 320±30°C, and the cooling rate of rapid cooling II is 20-50°C / s, that is, rapid cooling to 320±30°C at a cooling rate of 20-50°C / s.

[0019] Furthermore, slow cooling, rapid cooling I and rapid cooling II all adopt a high-speed gas jet cooling process.

[0020] The over-aging temperature is 310±30℃, and the time is controlled at 4.5~11.0min.

[0021] Flattening elongation 0.7±0.3%.

[0022] The metallographic structure of the 590MPa tensile strength dual-phase steel is ferrite+martensite+bainite, the elongation after fracture A80 is ≥24%, and the hole expansion rate is ≥40%.

[0023] Corner cracks in cast billets are the most common and numerous type of casting quality defect, and have long been a key and difficult issue of great concern to metallurgists. To achieve the desired microstructure and formability, the composition and process design of duplex steel is largely limited by equipment configuration. Although existing technologies offer a wide range of compositional designs, there is currently no direct solution for accurately and rationally designing the composition of continuous annealing units using high-speed jet rapid cooling to avoid the crack-sensitive sub-peritectic region. Further production technology, performance control, and defect control are rarely addressed in existing technologies.

[0024] Chemical composition is the foundation of steel plate materials, and composition design defects are often difficult to eliminate through subsequent processes. A C content of more than 0.075% will not only easily cause the carbon equivalent to fall in the sub-peritectic high-risk zone of 0.08-0.15%, but also cause a high proportion of corner crack defects in the ingot, requiring corner cleaning treatment and making hot rolling impossible. It will also increase the hardness difference between the soft and hard phases of the banded structure and the finished product, seriously affecting the forming performance of the duplex steel and narrowing the window for process design. A C content of less than 0.05% will not only increase the difficulty of controlling C in steelmaking, but also require more alloying elements to supplement the strength, which is uneconomical. In the present invention, the C content is 0.050-0.075%. Si and Cr are elements that reduce the carbon equivalent of continuous casting, while Mn increases it. A rational incremental design of Si, Mn, and Cr not only further ensures avoidance of the crack-sensitive, high-risk hypoperitectic zone, but also significantly reduces the sensitivity of the austenite to ferrite phase transformation to the cooling process (slow cooling + rapid cooling I) during the continuous annealing process, thereby reducing fluctuations in the hard phase ratio and expanding the process design window. Nitrogen content is a key control element. Exceeding 0.0055% significantly increases the sensitivity of the ingot to corner cracks, reducing the process tolerance of the continuous casting process. Therefore, the chemical composition and mass fraction of the 590 MPa tensile strength grade dual-phase steel in the present invention meet the following conditions in %: C: 0.050-0.075%, Si: 0.45-0.80%, Mn: 1.70-2.00%, Cr: 0.20-0.40%, P≤0.040%, Als: 0.010-0.060%, S≤0.015%, N≤0.0055%, Ca≤0.0060%, and the balance is Fe and unavoidable impurities.

[0025] During continuous casting, the quality of the corners of the slab is affected by thermal stress, phase change stress, and mechanical stress. This invention controls the arc accuracy to ±0.5mm and sets the target temperature of the slab surface in the straightening zone to ≥900°C, significantly reducing the impact of mechanical stress.

[0026] During hot rolling, cracks in the corners of the ingot can genetically evolve into edge warping defects. Furthermore, improper hot rolling processes can cause the ingot to flatten and tilt the edges upwards under the alternating pressure of lateral and flattening, leading to edge cracks. The present invention utilizes a concave hammer head and controlled width reduction to increase edge temperature and reduce edge warping, thereby limiting edge defects to within 10 mm.

[0027] During continuous annealing, changing the strip speed in the furnace inevitably changes the duration of each annealing step, which will affect the structural ratio of the soft and hard phases and the hardness difference between the soft and hard phases. For continuous annealing units with short slow cooling lengths of 20-25m, the process is characterized by a high slow cooling rate and heavy slow cooling fan load. Compared with continuous annealing units with longer slow cooling lengths of 50-60m, the content of newly formed ferrite during slow cooling is low, and the overall plasticity of the product (drawability + flanging performance) is often worse. For traditional hypoperitectic zone composition design, to improve plasticity, the response measures are usually slow cooling low temperature design combined with low furnace speed design. However, the relatively low furnace strip speed not only affects the unit's production capacity release but also increases continuous annealing production costs. The relatively low slow cooling temperature increases the slow cooling fan load. Due to the insufficient slow cooling fan capacity, it is impossible to achieve faster production. At the same time, long-term high-load operation easily leads to negative pressure leakage and deterioration of surface quality. The present invention significantly expands the design window of the annealing process from the perspective of composition design by reducing the amount of C and increasing the amount of Si, Mn, and Cr. The present invention adopts a slow cooling temperature design of 710±20°C.

[0028] Compared with the prior art, the present invention has the following beneficial effects:

[0029] 1) By designing the composition to 0.050%≤C≤0.075%, Si≥0.45% and Cr≥0.20%, the crack-sensitive hypoperitectic region is avoided. Supplemented by the restrictions of N≤0.0055%, 0.010%≤Als≤0.060%, the tendency of corner cracking in the casting is eliminated from the source of the composition design.

[0030] 2) By rationally designing the target surface temperature of the ingot in the straightening zone of the continuous casting process, the arc accuracy, and the hammer head shape, side pressure reduction, and heating temperature of the hot rolling process, edge defects (edge ​​warping + edge cracks) can be reduced, alleviated, or even eliminated from the source of process design. The edge defects can be controlled within 10mm of the edge, making it easier to remove them in subsequent processes.

[0031] 3) Through a carbon reduction design, supplemented by incremental Si, Mn, and Cr additions, the design significantly improves banded structure and its impact on product plasticity, reduces the hardness difference between the soft and hard phases, thereby improving product plasticity, and reduces the impact of the annealing process on the hard phase ratio, significantly expanding the annealing process design window. For short-slow-cooling continuous annealing units with a slow-cooling section length of 20 to 25 meters, this breaks the traditional bottleneck of slow-cooling low-temperature design combined with low furnace speed design, achieving a speed-up to ensure quality.

[0032] 4) By designing a wide range of strip speed in the furnace, the maneuverability of on-site control is improved, which can avoid long-term high load of the cooling fan and eliminate the deterioration effect of negative pressure leakage on the strip surface, thereby achieving both speed increase and high surface quality of the product.

[0033] In summary, the present invention adopts an integrated design of chemical composition, continuous casting process, hot rolling process, and continuous annealing process, which realizes efficient corner elimination in the continuous casting process, efficient hot rolling in the hot rolling process, and efficient speed-up production in the continuous annealing process. BRIEF DESCRIPTION OF THE DRAWINGS

[0034] Figure 1 This is the hole expansion morphology of the 590 MPa tensile strength dual-phase steel in Example 3;

[0035] Figure 2 This is a physical picture of the concave hammer head adopted in the present invention. DETAILED DESCRIPTION

[0036] The present invention is described in detail below with reference to the embodiments.

[0037] Examples 1-6 and Comparative Examples 1-5 all comprised duplex steels with a tensile strength of 590 MPa, and their chemical compositions are shown in Table 1. The ingots were 230 mm thick, with a pickling reduction of 45-70%. Heat treatment was performed in a short-slow annealing line with an effective cooling length of 20-25 m, using a high-speed gas jet cooling process, with rapid cooling lines I and II positioned adjacent to each other on the structure.

[0038] Table 1 Main chemical components (wt%)

[0039]

[0040]

[0041] The continuous casting process parameters and ingot quality of each duplex steel are shown in Table 2.

[0042] Table 2 Main continuous casting processes and billet quality

[0043] serial number Arc accuracy / mm Target temperature of the billet surface in the straightening zone / ℃ Billet drawing speed / (m / min) Corner cracks in castings Example 1 ±0.4 925 1.2 none Example 2 ±0.4 900 1.4 none Example 3 ±0.5 920 1.6 none Example 4 ±0.4 925 1.2 none Example 5 ±0.4 925 1.2 none Example 6 ±0.3 920 1.4 none Comparative Example 1 ±0.4 900 1.4 none Comparative Example 2 ±0.4 850 1.4 have Comparative Example 3 ±0.4 925 1.2 have Comparative Example 4 ±0.4 925 1.2 have Comparative Example 5 ±0.4 925 1.2 have

[0044] The hot rolling process parameters and hot coil quality of each dual-phase steel are shown in Table 3.

[0045] Table 3 Main hot rolling processes and hot coil quality

[0046]

[0047]

[0048] The continuous annealing process parameters of each dual-phase steel are shown in Table 4.

[0049] Table 4 Main continuous annealing processes

[0050]

[0051]

[0052] The comparison of the finished surface and properties of each duplex steel is shown in Table 5.

[0053] Table 5 Comparison of finished product surface and performance

[0054]

[0055] Note: The mechanical properties are determined according to the national standard GB / T 228.1-2021, the specimen type is P6, the specimen direction is longitudinal, and the hole expansion rate is determined according to the national standard GB / T 15825.4-2008, using punching and tapered punches.

[0056] Comparative Examples 1 and 2 had the same chemical composition as Example 2, but the hot rolling process in Comparative Example 1 differed from that in Example 2. Comparative Example 1 used a flat hammer, resulting in a total width reduction greater than 200 mm. This resulted in edge cracking defects at a 30 mm radius along the hot rolled edge, preventing clean removal in subsequent steps and resulting in substandard surface quality in the finished product. Comparative Example 2 did not utilize the continuous casting process of the present invention, with a target surface temperature of 850°C in the straightening zone. This resulted in corner cracking defects in the cast slab, and during hot rolling without clearing the corners, edge warping defects at a 15 mm radius along the hot rolled edge, resulting in substandard surface quality in the finished product.

[0057] Comparative Examples 3 to 5 have the same chemical composition, but a higher C content, and the hypoeutectic zone is not avoided. Despite the continuous casting process of the present invention, the ingots still have serious corner cracks. For Comparative Examples 3 to 4 in which the ingots are not subjected to corner cleaning treatment, the surface quality of the final products is also unqualified. Even if Comparative Example 5 is subjected to corner cleaning treatment, the surface quality of the final product is still unqualified due to the use of a flat hammer head.

[0058] It is not difficult to see from Table 5 that for Example 1, Example 4, and Example 5, although the strip speed and slow cooling temperature in the furnace are different, the difference in product performance is small, and the two major plasticity indicators, elongation after fracture and hole expansion rate, are very excellent. A wide range of speed control in the furnace can be achieved, and long-term high load of the cooling fan can be avoided, and the deterioration effect of negative pressure leakage on the strip surface is eliminated.

[0059] The finished product thicknesses of Comparative Examples 3 to 5 are the same as those of Example 1, but only Comparative Example 4, which has a lower strip speed in the furnace, has relatively excellent plasticity indicators, namely, elongation after fracture and hole expansion rate. As long as the speed in the furnace is increased, no matter how other annealing processes are adjusted, the two major plasticity indicators, namely, elongation after fracture and hole expansion rate, are very poor and cannot meet the forming requirements of complex parts. Speeding up production is restricted by the bottleneck of product plasticity.

[0060] The chemical composition of Example 6 is the same as that of Example 3, and the ingots do not have corner crack defects. Although Example 6 has edge crack defects at the 8mm edge during hot rolling, the subsequent pickling and annealing production can remove them cleanly, and the final product surface quality is acceptable.

[0061] In summary, the present invention adopts an integrated design of chemical composition, continuous casting process, hot rolling process, and continuous annealing process, which realizes efficient corner elimination in the continuous casting process, efficient hot rolling in the hot rolling process, and efficient speed-up production in the continuous annealing process.

[0062] The above-mentioned detailed description of an efficient production method for a dual-phase steel with a tensile strength of 590 MPa with reference to the embodiment is illustrative rather than restrictive, and several embodiments can be listed according to the limited scope. Therefore, changes and modifications without departing from the overall concept of the present invention should be within the scope of protection of the present invention.

Claims

1. An efficient production method for dual-phase steel with a tensile strength of 590 MPa, characterized in that: The efficient production method comprises the following steps: smelting, continuous casting, hot rolling, pickling and continuous annealing; The composition of the molten steel obtained by smelting avoids the crack-sensitive hypoperitectic region; During continuous casting, a straight mold arc continuous casting machine is used, with arc accuracy controlled within ±0.5mm, and the target temperature of the billet surface in the straightening area ≥900℃; The ingot is directly hot-rolled without clearing the corners, the heating temperature is 1230±40℃, and the side pressing machine adopts a concave hammer head; The continuous annealing adopts a short slow cooling type continuous annealing unit with a slow cooling section length of 20~25m. When the strip thickness is 0.5mm≤≤1.2mm, the strip speed in the furnace is 180±30m / min; when the strip thickness is 1.2mm<≤1.5mm, the strip speed in the furnace is 160±30m / min; when the strip thickness is 1.5mm<≤1.8mm, the strip speed in the furnace is 140±30m / min; when the strip thickness is 1.8mm<≤2.0mm, the strip speed in the furnace is 120±30m / min; when the strip thickness is 2.0mm<≤2.5mm, the strip speed in the furnace is 100±30m / min. After annealing, it is subjected to slow cooling, rapid cooling I and rapid cooling II, followed by aging treatment and leveling. Rapid cooling I and rapid cooling II are located adjacent to each other in the annealing furnace structure. The annealing temperature is 790±20℃, and after slow cooling to 710±20℃, rapid cooling is performed. The chemical composition and mass fraction of the 590 MPa tensile strength duplex steel meet the following conditions in percentage: C: 0.050-0.075%, Si: 0.45-0.80%, Mn: 1.70-2.00%, Cr: 0.20-0.40%, P≤0.040%, Als: 0.010-0.060%, S≤0.015%, N≤0.0055%, Ca≤0.0060%, and the balance is Fe and unavoidable impurities.

2. The efficient production method of dual-phase steel with a tensile strength of 590 MPa according to claim 1, characterized in that: In the continuous casting step, the casting speed is 1.1-1.8 m / min.

3. The efficient production method of dual-phase steel with a tensile strength of 590 MPa according to claim 1, characterized in that: In the hot rolling step, the charging temperature is 400-900° C., the heating temperature is 1230±40° C., the finishing temperature is 890±40° C., the coiling temperature is 520-650° C., and the total width reduction is ≤200 mm.

4. The efficient production method of dual-phase steel with a tensile strength of 590 MPa according to claim 1, characterized in that: In the continuous annealing step, the slow cooling rate is 4.5-16.5°C / s.

5. The efficient production method of dual-phase steel with a tensile strength of 590 MPa according to claim 1, characterized in that: In the continuous annealing step, the temperature of rapid cooling I is 520±40°C, and the cooling rate of rapid cooling I is 60-130°C / s.

6. The efficient production method of dual-phase steel with a tensile strength of 590 MPa according to claim 1, characterized in that: In the continuous annealing step, the temperature of rapid cooling II is 320±30°C, and the cooling rate of rapid cooling II is 20-50°C / s.

7. The efficient production method of dual-phase steel with a tensile strength of 590 MPa according to claim 1, characterized in that: The over-aging temperature is 310±30℃, and the time is controlled within 4.5~11.0min.

8. The efficient production method of dual-phase steel with a tensile strength of 590 MPa according to claim 1, characterized in that: Flattening elongation 0.7±0.3%.

9. The efficient production method of dual-phase steel with a tensile strength of 590 MPa according to claim 1, characterized in that: The tensile strength 590 MPa grade dual-phase steel has an elongation after fracture A80 of ≥24% and a hole expansion rate of ≥40%.

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