Efficient production method of dual-phase steel with tensile strength of 590 MPa

Abstract

The invention discloses an efficient production method of dual-phase steel with the tensile strength of 590 MPa. The efficient production method comprises the following steps of smelting, continuous casting, hot rolling, acid rolling and continuous annealing. Molten steel components obtained through smelting keep away from a sub-peritectic zone sensitive to cracks; during continuous casting, a straight crystallizer arc continuous casting machine is adopted, the arc alignment precision is controlled to be + / -0.5 mm, and the target temperature of the surface of a casting blank in a straightening area is larger than or equal to 900 DEG C; the casting blank is directly subjected to hot charging rolling without corner cleaning, the heating temperature is 1230 + / -40 DEG C, and a concave hammer head is adopted as a side press; different in-furnace running speeds are adopted according to different strip steel thicknesses, and after annealing, after slow cooling, rapid cooling I and rapid cooling II treatment, overaging treatment and flattening are carried out; the rapid cooler I and the rapid cooler II are adjacent front and back on the structural layout of the annealing furnace; through the integrated design of the chemical components, the continuous casting process, the hot rolling process and the continuous annealing process, efficient non-corner-cleaning treatment of the continuous casting process, efficient hot charging rolling of the hot rolling process and efficient speed-up production of the continuous annealing process are achieved.

Description

Technical Field

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

[0002] In the face of increasingly fierce competition, extreme efficiency is an important way to achieve extreme cost and extreme benefits. The process of automobile plates is long, and common processes include smelting, continuous casting, hot rolling, pickling, continuous annealing or coating. Adhering to extreme efficiency, improving process efficiency is the top priority. The specific measures currently taken by steel mills to improve production efficiency include: increasing the number of continuous casting furnaces, high-speed billet drawing, efficient rolling, and annealing speed increase.

[0003] Looking at the development of high-strength dual-phase steel for 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 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 CN115386693A further upgrades the graded rapid cooling innovation process, by reasonably 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, 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-up 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 and increase 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 speed while maintaining 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, the crack-sensitive sub-peritectic zone is effectively avoided, the cracks at the corners of the ingot are reduced and alleviated, efficient hot rolling is achieved, the correlation between process parameters and microstructure properties is reduced, the process window is expanded, the equipment bottleneck and process bottleneck are eliminated, and annealing speed is increased.

[0006] In order to achieve the above purpose, the specific technical scheme of the present invention is as follows:

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

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

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

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

[0011] The continuous annealing adopts a short slow cooling type continuous annealing unit with a slow cooling section cooling 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 adjacent to each other in the structural layout of the annealing furnace. The annealing 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 tensile strength 590 MPa grade dual-phase steel 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.

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

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

[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 one-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 dual-phase steel with a tensile strength of 590 MPa is ferrite+martensite+bainite, the elongation after fracture A80 is ≥24%, and the hole expansion rate is ≥40%.

[0023] Corner cracks in ingots are the most common and most numerous quality defects in ingots, and have long been a key and difficult issue that metallurgical workers have paid close attention to. In order to obtain the desired organizational structure and forming performance, the composition and process design of duplex steel is largely limited by the equipment configuration. Although the composition design in the prior art is very broad, there is currently no direct solution for how to accurately and reasonably design the composition of the continuous annealing unit using a high-speed jet rapid cooling process to avoid the crack-sensitive sub-peritectic zone. Further production technology, performance control and defect control are rarely addressed in the prior art.

[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. In addition, the hardness difference between the banded structure and the soft and hard phases of the finished product will be increased, seriously affecting the forming performance of the dual-phase 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 not economical. 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, and Mn is an element that increases the carbon equivalent of continuous casting. With the reasonable incremental design of Si, Mn, and Cr, it can not only further ensure that the crack-sensitive sub-peritectic high-risk area is avoided, but also significantly reduce the sensitivity of the austenite → ferrite phase transformation process to the cooling process of the continuous annealing process (slow cooling + fast cooling I), thereby reducing the fluctuation of the hard phase ratio and expanding the process design window. N content is a key control element. When it exceeds 0.0055%, the sensitivity of the corner crack of the ingot is significantly increased, reducing the process tolerance of the continuous casting process. Therefore, the chemical composition and mass fraction of the dual-phase steel with a tensile strength of 590 MPa in the present invention meet the following conditions in terms of %: 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 ingot will be affected by thermal stress, phase change stress and mechanical stress. The present invention controls the arc accuracy to ±0.5mm, and the target temperature of the ingot surface in the straightening zone is ≥900℃, which can significantly reduce the impact of mechanical stress.

[0026] During hot rolling production, cracks at the corners of the ingot are inherited and evolved into edge warping defects. In addition, if the hot rolling process is not appropriate, the ingot will be subjected to alternating side pressure and flat pressure, and the metal at the edge will be turned up and flattened, which will cause edge cracks. The present invention adopts a concave hammer head and controls the width reduction to increase the edge temperature and reduce the metal turning up at the edge, so that the edge defects are controlled within 10mm of the edge.

[0027] During continuous annealing production, if the speed of the strip in the furnace is changed, the time of each continuous annealing process will inevitably change, which will affect the structural ratio of the soft and hard phases and the hardness difference of the soft and hard phases. For the short slow cooling continuous annealing unit with an effective cooling length of 20 to 25m, the process shows the characteristics of large slow cooling rate and large slow cooling fan load. Compared with the continuous annealing unit with a longer slow cooling length of 50 to 60m, the content of newly formed ferrite during slow cooling is low, and the comprehensive plasticity (drawing performance + flanging performance) of the product is often worse. For the traditional sub-eutectic zone composition design, in order to improve plasticity, the countermeasures are usually slow cooling low temperature design + low speed design in the furnace. However, the relatively low speed of the strip in the furnace not only affects the capacity release of the unit, but also increases the production cost of continuous annealing. The relatively low slow cooling temperature will increase the load of the slow cooling fan. Due to the insufficient capacity of the slow cooling fan, it is impossible to achieve speed-up production. At the same time, long-term high-load operation is prone to negative pressure leakage and surface quality degradation. The present invention significantly expands the design window of the annealing process from the perspective of component 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 of 0.050%≤C≤0.075%, Si≥0.45% and Cr≥0.20%, the crack-sensitive sub-eutectic region is avoided, and the tendency of corner cracking of the ingot is eliminated from the source of the composition design by limiting N≤0.0055%, 0.010%≤Als≤0.060%.

[0030] 2) By rationally designing the target surface temperature of the ingot in the straightening zone of the continuous casting process, the arc accuracy, the hammer shape of the side press in the hot rolling process, the side press width reduction, and the heating temperature, the 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, which is convenient for removal in subsequent processes.

[0031] 3) Through the reduction design of C, supplemented by the incremental design of Si, Mn, and Cr, the banded structure and its degradation on the plasticity of the product are significantly improved from the perspective of composition design, the hardness difference between the soft and hard phases is reduced to improve the plasticity of the product, and the influence of the annealing process on the hard phase ratio is reduced, thereby significantly expanding the design window of the annealing process. For the short slow cooling type continuous annealing unit with a slow cooling section cooling length of 20 to 25m, the traditional bottleneck of slow cooling low temperature design + low speed design in the furnace is broken to achieve speed-up for quality assurance.

[0032] 4) By designing a wide range of strip speed in the furnace, the mobility 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 treatment 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 The hole expansion morphology of the dual-phase steel with a tensile strength of 590 MPa in Example 3;

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

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

[0037] Examples 1 to 6 and Comparative Examples 1 to 5 are dual-phase steels with a tensile strength of 590 MPa, and their chemical compositions are shown in Table 1. The thickness of the ingot is 230 mm, the reduction rate of the acid rolling is 45-70%, and the heat treatment is carried out in a short slow cooling continuous annealing unit with an effective cooling length of 20-25 m. The high-speed gas jet cooling process is adopted, and the rapid cooling I and the rapid cooling II are 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 dual-phase steel are shown in Table 2.

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

[0043] serial number Arc accuracy / mm Target temperature of the billet surface in the straightening zone / ℃ Billet drawing speed / (m / min) Corner crack of ingot 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 conical punches.

[0056] Comparative Examples 1 to 2 have the same chemical composition as Example 2, but the hot rolling process of Comparative Example 1 is different from that of Example 2. Comparative Example 1 uses a flat hammer head, and the total width reduction is greater than 200 mm, resulting in edge crack defects at 30 mm of the hot rolled edge, which cannot be cleanly removed in the subsequent process, and the surface quality of the final product is unqualified. Comparative Example 2 does not use the continuous casting process of the present invention, and the target temperature of the billet surface in the straightening zone is 850°C, resulting in corner crack defects in the billet, and edge warping defects at 15 mm of the hot rolled edge during hot rolling without clearing the corners, and the surface quality of the final product is also unqualified.

[0057] Comparative Examples 3 to 5 have the same chemical composition, but a high C content, and the hypoeutectic zone is not avoided. Although the continuous casting process of the present invention is adopted, 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 speed of the strip steel in the furnace and the slow cooling temperature 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. The speed in the furnace can be controlled in a wide range, and the long-term high load of the cooling fan can be avoided, and the deterioration effect of negative pressure leakage on the strip steel surface can be 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 have no corner crack defects. Although there are edge crack defects at 8 mm of the edge during hot rolling in Example 6, the subsequent acid rolling and continuous annealing can remove them cleanly, and the surface quality of the final product is qualified.

[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 treatment 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 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 fall 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, acid rolling and continuous annealing; The composition of the molten steel obtained by smelting avoids the crack-sensitive sub-peritectic region; During continuous casting, a straight mold arc continuous casting machine is used, the arc accuracy is controlled at ±0.5mm, and the target temperature of the billet surface in the straightening zone is ≥900℃; The ingot is directly hot-rolled without clearing the corners, the heating temperature is 1230±40℃, and the side press adopts a concave hammer head; The continuous annealing adopts a short slow cooling type continuous annealing unit with a slow cooling section cooling 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; 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 adjacent to each other in the structural layout of the annealing furnace. The annealing temperature is 790±20℃, and rapid cooling is performed after slow cooling to 710±20℃.

2. The efficient production method of dual-phase steel with a tensile strength of 590 MPa according to claim 1, characterized in that: The chemical composition and mass fraction of the dual-phase steel with a tensile strength of 590 MPa 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 remainder is Fe and unavoidable impurities.

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

4. The efficient production method of dual-phase steel with a tensile strength of 590 MPa according to claim 1 or 2, characterized in that: In the hot rolling step, the charging temperature is 400-900°C, the heating temperature is 1230±40°C, the final rolling temperature is 890±40°C, the coiling temperature is 520-650°C, and the total width reduction is ≤200mm.

5. The efficient production method of dual-phase steel with a tensile strength of 590 MPa according to claim 1 or 2, characterized in that: In the continuous annealing step, the slow cooling rate is 4.5-16.5°C / s.

6. The efficient production method of dual-phase steel with a tensile strength of 590 MPa according to claim 1 or 2, 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.

7. The efficient production method of dual-phase steel with a tensile strength of 590 MPa according to claim 1 or 2, 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.

8. The efficient production method of dual-phase steel with a tensile strength of 590 MPa according to claim 1 or 2, characterized in that: The over-aging temperature is 310±30℃, and the time is controlled at 4.5~11.0min.

9. The efficient production method of dual-phase steel with a tensile strength of 590 MPa according to claim 1 or 2, characterized in that: Flattening elongation 0.7±0.3%.

10. The efficient production method of dual-phase steel with a tensile strength of 590 MPa according to claim 1 or 2, characterized in that: The tensile strength 590MPa grade dual-phase steel has an elongation after fracture A80≥24% and a hole expansion rate≥40%.

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