Method for increasing yield and efficiency of high-strength automobile sheet

Abstract

The invention discloses a yield and efficiency increasing method for a high-strength automobile sheet. The method comprises the following steps of smelting, continuous casting, hot rolling, acid pickling and cold rolling and continuous annealing. Molten steel components obtained through smelting keep away from a sub-peritectic zone sensitive to cracks; in the continuous casting step, corners are not cleaned after casting blanks are offline; in the hot rolling step, a casting blank is subjected to hot charging rolling; in the continuous annealing step, a long slow cooling type continuous annealing unit with a slow cooling section of 50-60 m and a rapid cooling section of 14-18 m is adopted; the soaking temperature of annealing is 790 + / -20 DEG C, and the in-furnace strip steel speed, slow cooling temperature, rapid cooling temperature, overaging temperature and flattening elongation are designed according to different strip steel thicknesses; through reasonable component process design, a sub-peritectic region sensitive to cracks is effectively avoided, and corner cracks of a casting blank are reduced; and the speed-increasing bottleneck of excessive slow cooling capacity and insufficient rapid cooling capacity is broken, so that the efficiency can be improved, and the yield can be increased.

Description

Technical Field

[0001] The invention belongs to the technical field of dual-phase steel, and in particular relates to a method for increasing the production and efficiency of high-strength automobile plates. Background Art

[0002] In the face of growing market demand and increasingly fierce competition, increasing production and increasing efficiency are two extremely important issues. The process of automobile sheet is long, and common processes include steelmaking, continuous casting, hot rolling, pickling, continuous annealing, and coating. Adhering to extreme efficiency and improving production efficiency is an important means to increase production and efficiency. The specific measures of the steel plant are: high-speed billet drawing, hot rolling, high-efficiency rolling, and annealing speed increase. There are many varieties of automobile sheets, including mild steel (Miid), interstitial-free steel (IF), high-strength interstitial-free steel (HSSIF), bake hardening steel (BH), carbon manganese steel (CMn), high-strength low-alloy steel (HSLA), dual-phase steel (DP), martensitic steel (MART), hot-formed steel (PHS), complex phase steel (CP), quenched ductile steel (QP), high-plasticity dual-phase steel (DH), phase transformation induced plasticity steel (TRIP), and high-plasticity complex phase steel (CH). Different varieties have different product characteristics; different strengths, alloy types and contents are different. Therefore, the difficulty of implementing extreme efficiency is also different.

[0003] Compared with the continuous annealing units using high-speed jet + roller cooling, high-hydrogen jet + water quenching rapid cooling process, the cooling capacity of the continuous annealing units using high-speed jet rapid cooling process is often insufficient. For dual-phase steel / complex phase steel / multiphase steel with a tensile strength of 590MPa, major steel mills have long adopted the composition design of sub-peritectic steel, and the C content in industrial practice generally falls within the range of 0.08-0.12%. The transformation of sub-peritectic steel from δ phase to γ ​​phase during solidification is accompanied by a large volume shrinkage, which significantly reduces high-temperature plasticity. Due to the high proportion of crack defects in the corners of the ingot, the ingot usually needs to be cleared after it is off the line, and cold rolling is adopted for hot rolling. This not only increases the loss of clearing corners, but also increases gas consumption. Further composition design, production technology, performance control, and defect control are not involved in the existing technologies.

[0004] Regarding annealing speed-up, the prior art has disclosed some solutions for increasing production and efficiency of high-strength automotive plates. The calculation model of the rapid cooling outlet temperature and the over-aging temperature based on the strip speed disclosed in Chinese patents CN116493421A and CN116334374A can simply and quickly achieve the speed-up and production increase of cold-rolled dual-phase steel. In order to ensure the product performance of dual-phase steel after annealing speed-up, the above solutions still have great limitations in the case where at least one of heating, slow cooling and rapid cooling restricts the speed-up and production increase. Summary of the invention

[0005] In order to solve the above technical problems, the present invention provides a method for increasing production and efficiency of high-strength automobile plates. Through reasonable composition design, the crack-sensitive sub-peritectic zone is effectively avoided, and cracks at the corners of the ingot are reduced; the process sensitivity of the organizational properties is weakened through process design, and the matching freedom of process design and production capacity is improved. Finally, production and efficiency are increased.

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

[0007] A method for increasing production and efficiency of a high-strength automobile sheet, wherein the tensile strength of the high-strength automobile sheet is 590 MPa; the method for increasing production and efficiency of the high-strength automobile sheet comprises the following steps: smelting, continuous casting, hot rolling, pickling and cold rolling, and continuous annealing;

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

[0009] In the continuous casting process, the corners of the ingot are not cleaned after it comes off the production line;

[0010] In the hot rolling step, the ingot is hot-rolled;

[0011] In the continuous annealing step, a long slow cooling type continuous annealing unit with a slow cooling section of 50 to 60 m and a fast cooling section of 14 to 18 m is used; after annealing, after slow cooling and fast cooling treatment, aging treatment and leveling are performed; the annealing soaking temperature is 790±20°C;

[0012] When the strip thickness is 0.5mm<≤1.0mm, the strip speed in the furnace is 250±50m / min, slowly cooled to 640±20℃ after annealing, then quickly cooled to 315±15℃, and then aged at 310±20℃, with a flat elongation of 0.8±0.2%;

[0013] When the strip thickness is 1.0mm<≤1.5mm, the strip speed in the furnace is 200±50m / min, slowly cooled to 620±20℃ after annealing, then quickly cooled to 305±15℃, and then aged at 300±20℃, with a flat elongation of 0.7±0.2%;

[0014] When the strip thickness is 1.5mm<≤2.0mm, the strip speed in the furnace is 160±40m / min, and after annealing, it is slowly cooled to 600±20℃, then quickly cooled to 295±15℃, and then aged at 290±20℃, with a flat elongation of 0.6±0.2%;

[0015] When the strip thickness is 2.0mm<≤2.5mm, the strip speed in the furnace is 130±30m / min. After annealing, it is slowly cooled to 580±20℃, then quickly cooled to 285±15℃, and then aged at 275±20℃. The flat elongation is 0.5±0.2%.

[0016] The chemical composition and weight percentage of the high-strength automobile plate are as follows: C 0.050-0.070%, Si 0.40-0.80%, Mn 1.75-2.00%, Cr 0.10-0.50%, 2.05%≤Mn+Cr≤2.25%, P≤0.040%, Als 0.010-0.080%, S≤0.015%, N≤0.0055%, Ca≤0.006%, and the balance is Fe and unavoidable impurities.

[0017] In the hot rolling step, the coiling temperature is 600-680°C.

[0018] In the hot rolling step, coiling is performed using an L-shaped or U-shaped cooling process.

[0019] In the pickling and cold rolling step, the pickling reduction rate is 40 to 70%.

[0020] In the continuous annealing step, the slow cooling section adopts a three-pass design and is equipped with four cooling fans.

[0021] In the continuous annealing step, the rapid cooling section adopts a one-pass design and is equipped with three cooling fans.

[0022] In the continuous annealing step, the rapid cooling stage adopts a high-hydrogen high-speed gas jet cooling process, and the volume fraction of hydrogen in the cooling medium is 4-38%.

[0023] The annealing soaking temperature is 780±10°C.

[0024] The high-strength automobile sheet produced by the method for increasing production and improving efficiency of the high-strength automobile sheet of the present invention has a yield strength of 340-410MPa, a tensile strength of 590-670MPa, and an elongation after fracture of A 80 ≥24%, hole expansion rate ≥40%, strain hardening index ≥0.14, yield strength ratio <0.65.

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

[0026] 1) By avoiding the composition design of the hypoeutectic zone, the cracks at the corners of the ingot are significantly reduced, and hot rolling without corner cleaning is achieved. The faster turnover efficiency of the ingot, the shorter the time in the heating furnace, the less iron loss during corner cleaning, and the lower gas consumption of the heating furnace are conducive to reducing costs and increasing efficiency.

[0027] 2) Through the reduction design of C, supplemented by the incremental design of Si, Mn, and Cr, 2.05%≤Mn+Cr≤2.25% is further limited. From the perspective of composition design, the banded structure and its degradation to the plasticity of the product are significantly improved, 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 continuous annealing unit with strong slow cooling and weak fast cooling, it is beneficial to reduce the slow cooling temperature, increase the fast cooling temperature, give full play to the slow cooling capacity, and reduce the fast cooling pressure, thereby breaking the bottleneck of excess slow cooling capacity and insufficient fast cooling capacity, realizing annealing speed-up, and achieving increased production and efficiency.

[0028] 3) According to the cooling characteristics of strip steel in different thickness ranges, the annealing process including the speed of strip steel in the furnace, slow cooling temperature, fast cooling temperature, over-aging temperature, and flat elongation is designed according to thickness, and the cooling load of slow cooling and fast cooling is reasonably allocated to expand the process limit and improve process flexibility. It not only effectively avoids the long-term 100% load of the cooling fan and eliminates the deterioration effect of negative pressure leakage on the strip surface, but also significantly improves the performance uniformity of different thicknesses, and the mean difference in tensile strength is controlled within 25MPa. Under complex working conditions, it can not only achieve annealing speed-up, but also achieve high surface + high performance of the product.

[0029] 4) Through the matching design of hot rolling process and annealing process, the performance problems of high yield strength ratio, low elongation after fracture and low strain hardening index of dual-phase steel with tensile strength of 590MPa under the component system of the present invention are solved. 80 ≥24%, average 26.0%; strain hardening index ≥0.14, average 0.16; yield strength ratio <0.65, average 0.58. The product not only has good drawing performance, but also better flanging performance, which can meet the requirements of forming complex automotive parts. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Figure 1 This is the metallographic structure diagram of the hot-rolled core of the automobile sheet in Example 1, and the structure is ferrite + pearlite;

[0031] Figure 2 This is the fluctuation of the thickness of the tail of the automobile plate after acid rolling in Comparative Example 3. DETAILED DESCRIPTION

[0032] 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. The technology of high-strength dual-phase steel is relatively mature, and the existing technology has involved a large number of component process designs. From the perspective of component design, the C content includes 0.01-0.35%; from the perspective of strength grade, the tensile strength covers 500-1600MP. In order to obtain the required organizational structure and forming performance, the component process design of dual-phase steel is largely limited by the equipment configuration. Although the component design in the existing technology 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 involved in the existing technology.

[0033] Chemical composition is the foundation of steel plate materials, and composition design defects are often difficult to eliminate through subsequent processes. When the C content exceeds 0.07%, it is not only easy to cause the carbon equivalent to fall into the sub-peritectic high-risk zone of 0.08-0.15%, but also increases the correlation between coiling temperature, soaking temperature, banded structure, and forming performance. When the C content is less than 0.05%, it not only increases the difficulty of controlling C in steelmaking, but also requires more alloying elements to supplement the strength, which is not economical. 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 zone is avoided, but also weaken the correlation between slow cooling temperature, rapid cooling temperature, soft and hard phase composition, and mechanical properties. When the content of Mn+Cr is too high, the hardenability of supercooled austenite is too large, which is not conducive to obtaining a multi-gradient hardness distribution structure. The proportion of martensite in the hard phase is too large, which reduces the hole expansion rate; the content of Mn+Cr is less than 2.05%, which increases the process sensitivity. The N content is a key control element. When the N content exceeds 0.0055%, the sensitivity of the corner crack of the casting billet is significantly increased, which reduces the process tolerance of the continuous casting process. The chemical composition and mass fraction of the high-strength automobile plate of the present invention meet the following conditions in terms of %: C 0.050-0.070%, Si 0.40-0.80%, Mn 1.75-2.00%, Cr 0.10-0.50%, 2.05%≤Mn+Cr≤2.25%, P≤0.040%, Als 0.010-0.080%, S≤0.015%, N≤0.0055%, Ca≤0.006%, and the balance is Fe and unavoidable impurities.

[0034] The process flow of continuous annealing includes: heating, soaking, slow cooling, rapid cooling and over-aging. During the slow cooling period, austenite transforms into ferrite, during the rapid cooling period, austenite transforms into martensite or bainite, and during the over-aging period, tempering and further transformation of residual austenite occur. Changing the speed of the strip in the furnace will inevitably change the time of each continuous annealing process, which will affect the structural ratio of the soft and hard phases and the hardness difference between the soft and hard phases. Although the process sensitivity of the microstructure and performance can be weakened through reasonable composition design, it cannot be completely eliminated. The difficulty of cooling strips in different thickness ranges is different. For thin strips, the slow cooling temperature is too low, the proportion of martensite in the hard phase is too large, and the hole expansion rate is reduced; for thick strips, the slow cooling temperature is too high, the proportion of bainite in the hard phase is large, the yield strength ratio is easy to exceed 0.65, the strain hardening index is easy to fail to meet the standard, and it is not conducive to exerting the slow cooling ability and increasing the annealing speed. Under extreme high-speed annealing conditions, slow cooling fans and fast cooling fans cannot operate at 100% full load for a long period of time, which can easily lead to negative pressure leakage, surface quality degradation, and even degradation. Expanding process boundaries and improving process flexibility are conducive to flexible adjustment during on-site production.

[0035] Based on the above findings, the present invention provides a method for increasing the production and efficiency of high-strength automobile plates. The production steps include smelting, continuous casting, hot rolling, pickling and cold rolling, and continuous annealing.

[0036] 1) Smelting: Control the above composition and use LF single process for refining;

[0037] 2) Continuous casting, no corner cleaning after the billet comes off the line;

[0038] 3) Hot rolling, the coiling temperature of the ingot is 600-680°C;

[0039] 4) Acid rolling, with a reduction rate of 40-70%;

[0040] 5) Continuous annealing, the annealing temperature is 790±20℃;

[0041] When the strip thickness is 0.5mm<≤1.0mm, the strip speed in the furnace is 250±50m / min, slowly cooled to 640±20℃ after annealing, then quickly cooled to 315±15℃, and then aged at 310±20℃, with a flat elongation of 0.8±0.2%;

[0042] When the strip thickness is 1.0mm<≤1.5mm, the strip speed in the furnace is 200±50m / min, slowly cooled to 620±20℃ after annealing, then quickly cooled to 305±15℃, and then aged at 300±20℃, with a flat elongation of 0.7±0.2%;

[0043] When the strip thickness is 1.5mm<≤2.0mm, the strip speed in the furnace is 160±40m / min, and after annealing, it is slowly cooled to 600±20℃, then quickly cooled to 295±15℃, and then aged at 290±20℃, with a flat elongation of 0.6±0.2%;

[0044] When the strip thickness is 2.0mm<≤2.5mm, the strip speed in the furnace is 130±30m / min. After annealing, it is slowly cooled to 580±20℃, then quickly cooled to 285±15℃, and then aged at 275±20℃. The flat elongation is 0.5±0.2%.

[0045] The cooling length of the slow cooling section of continuous annealing is 50-60m, with a 3-pass design and equipped with 4 cooling fans; the cooling length of the fast cooling section is 14-18m, with a 1-pass design and equipped with 3 cooling fans. The fast cooling section adopts a high-hydrogen high-speed gas jet cooling process, and the volume fraction of hydrogen in the cooling medium is 4-38%.

[0046] The high-strength automotive sheet obtained by the above method has a yield strength of 340-410 MPa and a tensile strength of 590-670 MPa. 80 ≥24%, average 26.0%; strain hardening index ≥0.14, average 0.16; yield strength ratio <0.65, average 0.58. The product can meet the requirements of forming complex automotive parts.

[0047] The present invention is described in detail below with reference to the embodiments.

[0048] The difference between Comparative Example 1 and Example 1 of the same finished product thickness is that: in terms of composition design, the C content is higher and the Mn+Cr content is lower; in terms of process design, the slow cooling temperature is high, and even if the rapid cooling fan is running at full load, the annealing speed is not high. Comparative Example 1 has obvious cracks at the corners of the ingot, and there are edge warping defects when the ingot is hot rolled without clear corners. Although the mechanical properties are not much different, the hole expansion rate is smaller. Comparative Example 2 and Example 3 have the same chemical composition, but the coiling temperature is lower and the slow cooling temperature is higher. Not only does the thickness fluctuation of the acid rolling tail increase, but there are also problems such as higher yield strength, lower elongation after fracture, and substandard strain hardening index.

[0049] The chemical composition and weight percentage of the high-strength automobile sheet in each embodiment and comparative example are shown in Table 1, and the balance is Fe and unavoidable impurities.

[0050] Table 1 Main chemical components (wt%)

[0051]

[0052] The main production process parameters of the high-strength automobile plates in each embodiment and comparative example are shown in Table 2. During hot rolling, the heating temperature is 1230±20°C, and the final rolling temperature is 880±20°C; during continuous annealing, the length of the slow cooling section, the length of the fast cooling section, and the length of the over-aging section are 55m, 16m, and 952m, respectively.

[0053] Table 2 Main process parameters

[0054]

[0055]

[0056] The properties of the high-strength automobile panels produced in the embodiments and comparative examples are shown in Table 3.

[0057] Table 3 Product performance comparison

[0058]

[0059] 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.

[0060] The above-mentioned detailed description of a method for increasing production and efficiency of a high-strength automobile plate 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 fall within the scope of protection of the present invention.

Claims

1. A method for increasing production and efficiency of high-strength automotive plates, characterized in that: The tensile strength of the high-strength automobile sheet is 590MPa; the method for increasing production and efficiency of the high-strength automobile sheet comprises the following steps: smelting, continuous casting, hot rolling, pickling and cold rolling, and continuous annealing; The composition of the molten steel obtained by smelting avoids the crack-sensitive sub-peritectic region; In the continuous casting process, the corners of the ingot are not cleaned after it comes off the production line; In the hot rolling step, the ingot is hot-rolled; In the continuous annealing step, a long slow cooling type continuous annealing unit with a slow cooling section of 50 to 60 m and a fast cooling section of 14 to 18 m is used; after annealing, after slow cooling and fast cooling treatment, aging treatment and leveling are performed; the annealing soaking temperature is 790±20°C; When the strip thickness is 0.5mm<≤1.0mm, the strip speed in the furnace is 250±50m / min, slowly cooled to 640±20℃ after annealing, then quickly cooled to 315±15℃, and then aged at 310±20℃, with a flat elongation of 0.8±0.2%; When the strip thickness is 1.0mm<≤1.5mm, the strip speed in the furnace is 200±50m / min, slowly cooled to 620±20℃ after annealing, then quickly cooled to 305±15℃, and then aged at 300±20℃, with a flat elongation of 0.7±0.2%; When the strip thickness is 1.5mm<≤2.0mm, the strip speed in the furnace is 160±40m / min, and after annealing, it is slowly cooled to 600±20℃, then quickly cooled to 295±15℃, and then aged at 290±20℃, with a flat elongation of 0.6±0.2%; When the strip thickness is 2.0mm<≤2.5mm, the strip speed in the furnace is 130±30m / min. After annealing, it is slowly cooled to 580±20℃, then quickly cooled to 285±15℃, and then aged at 275±20℃. The flat elongation is 0.5±0.2%.

2. The method for increasing production and efficiency of high-strength automobile sheets according to claim 1, characterized in that: The chemical composition and weight percentage of the high-strength automobile plate are as follows: C 0.050-0.070%, Si 0.40-0.80%, Mn 1.75-2.00%, Cr 0.10-0.50%, 2.05%≤Mn+Cr≤2.25%, P≤0.040%, Als0.010-0.080%, S≤0.015%, N≤0.0055%, Ca≤0.006%, and the balance is Fe and unavoidable impurities.

3. The method for increasing production and efficiency of high-strength automobile sheets according to claim 1 or 2, characterized in that: In the hot rolling step, the coiling temperature is 600-680°C.

4. The method for increasing production and efficiency of high-strength automobile sheets according to claim 1 or 2, characterized in that: In the hot rolling step, coiling is performed using an L-shaped or U-shaped cooling process.

5. The method for increasing production and efficiency of high-strength automobile sheets according to claim 1 or 2, characterized in that: In the pickling and cold rolling step, the pickling reduction rate is 40 to 70%.

6. The method for increasing production and efficiency of high-strength automobile sheets according to claim 1 or 2, characterized in that: In the continuous annealing step, the slow cooling section adopts a three-pass design and is equipped with four cooling fans.

7. The method for increasing production and efficiency of high-strength automobile sheets according to claim 1 or 2, characterized in that: In the continuous annealing step, the rapid cooling section adopts a one-pass design and is equipped with three cooling fans.

8. The method for increasing production and efficiency of high-strength automobile sheets according to claim 1 or 2, characterized in that: In the continuous annealing step, the rapid cooling stage adopts a high-hydrogen high-speed gas jet cooling process, and the volume fraction of hydrogen in the cooling medium is 4-38%.

9. The method for increasing production and efficiency of high-strength automobile sheets according to claim 1 or 2, characterized in that: The annealing soaking temperature is 780±10°C.

10. The high-strength automobile sheet produced by the method for increasing production and efficiency of high-strength automobile sheet according to any one of claims 1 to 9, characterized in that: The high-strength automobile sheet has a yield strength of 340-410 MPa, a tensile strength of 590-670 MPa, and an elongation after fracture of A 80 ≥24%, hole expansion rate ≥40%, strain hardening index ≥0.14, yield strength ratio <0.65.

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