A method for increasing yield and efficiency of high-strength automobile sheet

Abstract

The application discloses a yield increasing and efficiency improving method of high-strength automobile plate, which comprises the following steps: smelting, continuous casting, hot rolling, pickling cold rolling and continuous annealing; the composition of molten steel obtained through smelting avoids the sub-peritectic zone sensitive to cracks; in the step of continuous casting, the corner of the casting blank is not cleaned after the casting blank is discharged; in the step of hot rolling, the casting blank is hot charged for rolling; in the step of continuous annealing, a long slow cooling type continuous annealing unit with a slow cooling section of 50-60 m and a fast cooling section of 14-18 m is adopted; the soaking temperature of the annealing is 790±20 DEG C, and the furnace steel speed, the slow cooling temperature, the fast cooling temperature, the overaging temperature and the flattening elongation rate are designed according to different steel thicknesses; through reasonable composition process design, the application not only effectively avoids the sub-peritectic zone sensitive to cracks and reduces the corner crack of the casting blank, but also breaks the speed-up bottleneck of slow cooling overcapacity + insufficient fast cooling capacity, so that the yield and the production can be increased.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of dual-phase steel, and particularly relates to a yield-increasing method for high-strength automobile plate. BACKGROUND

[0002] In the face of growing market demand and increasingly fierce competition, yield increase and efficiency increase are two extremely important issues. The automobile plate process flow is long, and common procedures include steelmaking, continuous casting, hot rolling, pickling, continuous annealing, and plating. Focusing on extreme efficiency and improving production efficiency is an important starting point for yield increase and efficiency increase. Specific measures of the steel plant include high-speed billet drawing, hot charging and rolling, efficient rolling, and annealing speed-up. There are many varieties of automobile plates, including mild steel (Mild), 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 forming steel (PHS), complex phase steel (CP), quenched ductile steel (QP), high plasticity dual-phase steel (DH), transformation induced plasticity steel (TRIP), and high plasticity complex phase steel (CH). Different varieties have different product properties; different strengths have different alloy types and contents. Therefore, the implementation of extreme efficiency is also different.

[0003] Compared with the continuous annealing unit adopting high-speed jet + roller cooling and high-hydrogen jet + water quenching rapid cooling process, the continuous annealing unit adopting high-speed jet rapid cooling process often has insufficient cooling capacity. For dual-phase steel / multi-phase steel with a tensile strength of 590 MPa, large steel plants have long used the composition design of hypoeutectic steel, and the C content in industrial practice generally falls within the range of 0.08-0.12%. The transformation of δ phase to γ phase during solidification of hypoeutectic steel is accompanied by a large volume shrinkage, thereby significantly reducing the high-temperature plasticity. Due to the high proportion of corner crack defects in the billet, the billet usually needs to be treated after being discharged, and cold charging is adopted for hot rolling. This not only increases the corner loss, but also increases the gas consumption. Further composition design, production technology, performance control, and defect control are not involved in the prior art.

[0004] For annealing speed-up, some yield-increasing and efficiency-increasing solutions for high-strength automobile plates have been disclosed in the prior art. The calculation model for the fast cooling outlet temperature and the overaging temperature based on the strip speed disclosed in Chinese patents CN116493421A and CN116334374A can simply and quickly achieve speed-up and yield 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 of restricting speed-up and yield increase for at least one of heating, slow cooling, and fast cooling. SUMMARY

[0005] To solve the above technical problems, the application provides a yield-increasing and benefit-increasing method for high-strength automobile plates, which effectively avoids the sub-peritectic zone sensitive to cracks, reduces the corner cracks of the casting blank, weakens the process sensitivity of the structure performance through process design, and improves the matching freedom of the process design-capacity performance.

[0006] To achieve the above object, the specific technical scheme of the application is as follows:

[0007] The yield-increasing and benefit-increasing method for the high-strength automobile plate includes the following steps: smelting, continuous casting, hot rolling, pickling cold rolling, and continuous annealing.

[0008] The composition of the molten steel obtained by smelting avoids the sub-peritectic zone sensitive to cracks.

[0009] In the continuous casting step, the corner of the casting blank is not cleaned after being discharged.

[0010] In the hot rolling step, the casting blank is hot-rolled.

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

[0012] When the strip thickness is 0.5mm≤strip thickness≤1.0mm, the strip speed in the furnace is 250±50m / min, the annealed strip is slowly cooled to 640±20℃, then fast cooled to 315±15℃, then overaged at 310±20℃, and the leveling elongation is 0.8±0.2%.

[0013] When the strip thickness is 1.0mm<strip thickness≤1.5mm, the strip speed in the furnace is 200±50m / min, the annealed strip is slowly cooled to 620±20℃, then fast cooled to 305±15℃, then overaged at 300±20℃, and the leveling elongation is 0.7±0.2%.

[0014] When the strip thickness is 1.5mm<strip thickness≤2.0mm, the strip speed in the furnace is 160±40m / min, the annealed strip is slowly cooled to 600±20℃, then fast cooled to 295±15℃, then overaged at 290±20℃, and the leveling elongation is 0.6±0.2%.

[0015] When the strip thickness is 2.0mm<strip thickness≤2.5mm, the strip speed in the furnace is 130±30m / min, the annealed strip is slowly cooled to 580±20℃, then fast cooled to 285±15℃, then overaged at 275±20℃, and the leveling 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%, Al 0.010-0.080%, S≤0.015%, N≤0.0055%, Ca≤0.006%, and the balance of Fe and inevitable impurities.

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

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

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

[0020] In the continuous annealing step, the slow cooling section is designed with 3 passes and is equipped with 4 cooling fans.

[0021] In the continuous annealing step, the fast cooling section is designed with 1 pass and is equipped with 3 cooling fans.

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

[0023] The soaking temperature of the annealing is 780±10℃.

[0024] The high-strength automobile plate produced by the yield-increasing and benefit-increasing method of the high-strength automobile plate has a yield strength of 340-410MPa, a tensile strength of 590-670MPa, an elongation A 80 ≥24%, a hole expansion ratio≥40%, a strain hardening index≥0.14, and a yield strength ratio<0.65.

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

[0026] 1) By avoiding the composition design of the sub-peritectic zone, the corner crack of the casting blank is significantly reduced, and hot rolling without corner cleaning is realized. The acceleration of the casting blank turnover efficiency, the reduction of the heating furnace in-furnace time, the reduction of the corner cleaning iron loss, and the reduction of the heating furnace gas consumption are beneficial to cost reduction and benefit increase.

[0027] 2) By the design of reducing C, supplemented by the design of increasing Si, Mn, Cr, further limit 2.05%≤Mn+Cr≤2.25%, significantly improve the banded structure and its deterioration on the product plasticity, reduce the hardness difference of soft and hard phases to improve the product plasticity, reduce the influence of annealing process on the proportion of hard phase, and further significantly expand the design window of annealing process. For the continuous annealing unit with slow cooling strong + fast cooling weak, it is beneficial to reduce the slow cooling temperature, increase the fast cooling temperature, play the slow cooling capacity, and reduce the fast cooling pressure, so as to break the bottleneck of slow cooling capacity surplus + fast cooling capacity insufficient, realize the annealing speed-up, and realize the yield increase and benefit increase.

[0028] 3) According to the cooling characteristics of strip steel in different thickness intervals, the annealing process of strip steel speed in the furnace, slow cooling temperature, fast cooling temperature, overaging temperature, and flat extension rate is designed in thickness, the cooling load of slow cooling and fast cooling is reasonably distributed, the process boundary is expanded, and the process flexibility is improved. Not only can effectively avoid the 100% load of cooling fan for a long time, eliminate the deterioration influence of negative pressure leakage on the surface of strip steel, but also significantly improve the performance uniformity of different thickness, and the average difference of tensile strength is controlled within 25MPa. Under complex working conditions, the annealing speed-up is realized, and the high surface + high performance of the product is realized.

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

[0030] Figure 1 The microstructure of the core of the automobile plate in Example 1 in hot rolling state is shown in the figure, which is ferrite + pearlite;

[0031] Figure 2 The thickness fluctuation of the tail of the automobile plate in Comparative Example 3 after cold rolling is shown in the figure. DETAILED DESCRIPTION

[0032] The slab corner crack is the most common and largest number of slab quality defects, has long been a very concerned metallurgical workers difficult problem. High strength dual phase steel technology is relatively mature, the existing technology has involved a large number of component process design, from the component design point of view, C content includes 0.01-0.35%; from the strength grade point of view, the tensile strength covers 500-1600MP. In order to obtain the required microstructure and forming performance, the component process design of dual phase steel is largely limited by the equipment configuration. Although the component design in the prior art is very wide, how to accurately and reasonably design the component for the continuous annealing unit using high speed air jet rapid cooling process to avoid the subperitectic zone sensitive to cracks, there is no direct reference solution at present. Further production technology, performance control and defect control, the prior art is rarely involved.

[0033] The chemical composition is the root of the steel plate material, and the component design defect is often difficult to eliminate through subsequent processes. C content more than 0.07%, not only easy to lead to carbon equivalent fall in 0.08-0.15% of the subperitectic high risk area, and increase the coiling temperature, soaking temperature, banded structure, the correlation degree of forming performance. C content less than 0.05%, not only increase the difficulty of C control in steelmaking, need more alloying elements to make up the strength, not economical. Si and Cr are the elements for reducing the carbon equivalent of continuous casting, Mn is the element for increasing the carbon equivalent of continuous casting, supplemented by reasonable increment design of Si, Mn and Cr, which can not only further ensure to avoid the subperitectic high risk area sensitive to cracks, but also can weaken the correlation degree of slow cooling temperature, rapid cooling temperature, soft and hard phase composition and mechanical properties. The content of Mn+Cr is too high, the hardenability of supercooled austenite is too large, which is not conducive to obtain the microstructure with multiple gradient distribution of hardness, and the proportion of martensite in 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. N content is a key control element, when it is more than 0.0055%, the sensitivity of slab corner crack is significantly improved, and the process tolerance of continuous casting process is reduced. The chemical composition and mass fraction of high strength automobile plate in the present application satisfy the following conditions: 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%, the balance being Fe and inevitable impurities.

[0034] The process flow of continuous annealing includes heating, soaking, slow cooling, rapid cooling and overaging. During the slow cooling, the austenite is transformed into ferrite, during the rapid cooling, the austenite is transformed into martensite or bainite, during the overaging, tempering and further transformation of residual austenite occur. Changing the strip speed in the furnace will inevitably change the time of each process of the continuous annealing, which will affect the structure ratio and hardness difference of the soft and hard phases. Although the process sensitivity of the structure performance can be weakened by reasonable composition design, it cannot be completely eliminated. The cooling difficulty of the strip in different thickness ranges is different. For thin gauge 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 gauge strips, the slow cooling temperature is too high, the proportion of bainite in the hard phase is large, the yield ratio is easy to exceed 0.65, and the strain hardening index is easy to be substandard, and it is not conducive to the slow cooling capacity and the improvement of the annealing speed. Under the condition of extreme high-speed annealing, the slow cooling fan and the rapid cooling fan cannot be operated at 100% full load for a long period, which is easy to cause negative pressure leakage, surface quality deterioration, and even degradation. Expanding the process limit and improving the process flexibility are beneficial to the on-site production.

[0035] Based on the above findings, the application provides a high-strength automobile plate yield-increasing method. The production steps include smelting, continuous casting, hot rolling, pickling cold rolling and continuous annealing.

[0036] 1) Smelting, controlled according to the above composition, and LF single process is used for refining;

[0037] 2) Continuous casting, no corner cleaning after the billet is discharged;

[0038] 3) Hot rolling, the coiling temperature of the billet is 600-680℃;

[0039] 4) Acid rolling, the reduction rate of acid rolling is 40-70%;

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

[0041] When the strip thickness is 0.5mm≤strip thickness≤1.0mm, the strip speed in the furnace is 250±50m / min, the slow cooling temperature after annealing is 640±20℃, the rapid cooling temperature is 315±15℃, then the overaging treatment is carried out at 310±20℃, and the flat elongation is 0.8±0.2%;

[0042] When the strip thickness is 1.0mm<strip thickness≤1.5mm, the strip speed in the furnace is 200±50m / min, the slow cooling temperature after annealing is 620±20℃, the rapid cooling temperature is 305±15℃, then the overaging treatment is carried out at 300±20℃, and the flat elongation is 0.7±0.2%;

[0043] When 1.5 mm < strip thickness < 2.0 mm, the strip speed in the furnace is 160 ± 40 m / min, the annealed strip is slowly cooled to 600 ± 20 °C, then rapidly cooled to 295 ± 15 °C, and then over-aged at 290 ± 20 °C, and the leveling elongation is 0.6 ± 0.2 %.

[0044] When 2.0 mm < strip thickness < 2.5 mm, the strip speed in the furnace is 130 ± 30 m / min, the annealed strip is slowly cooled to 580 ± 20 °C, then rapidly cooled to 285 ± 15 °C, and then over-aged at 275 ± 20 °C, and the leveling elongation is 0.5 ± 0.2 %.

[0045] The slow cooling section of the continuous annealing has a cooling length of 50-60 m, is designed with 3 passes, and is equipped with 4 cooling fans; the rapid cooling section has a cooling length of 14-18 m, is designed with 1 pass, and is equipped with 3 cooling fans. The rapid cooling section uses 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 automobile sheet obtained by the above method has a yield strength of 340-410 MPa, a tensile strength of 590-670 MPa, an elongation A 80 ≥ 24 %, an average of 26.0 %; a strain hardening index ≥ 0.14, an average of 0.16; and a yield strength ratio < 0.65, an average of 0.58. The product can meet the requirements of forming complex automobile parts.

[0047] The application will be described in detail below in conjunction with examples.

[0048] The difference between Comparative Example 1 and Example 1 of the same finished product thickness lies in that the C content is higher and the Mn+Cr content is smaller in the component design, and the slow cooling temperature is higher in the process design, so that even if the rapid cooling fan is full-load running, the annealing speed is not large. The slab corner cracks of Comparative Example 1 are obvious, and there are edge buckling defects when the slab is hot-rolled without clearing the corner. 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, so that not only the thickness fluctuation of the pickling tail is large, but also the yield strength is higher, the elongation after fracture is reduced, and the strain hardening index is not up to standard.

[0049] The chemical composition and weight percentage of the high-strength automobile sheet in each example 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 sheet in each embodiment and the comparative example are shown in Table 2. In hot rolling, the heating temperature is 1230±20℃, and the final rolling temperature is 880±20℃; in continuous annealing, the slow cooling section length, the fast cooling section length, and the overaging section length are 55m, 16m, and 952m, respectively.

[0053] Table 2 Main process parameters

[0054]

[0055]

[0056] The properties of the high-strength automobile sheet produced in each embodiment and the comparative example are shown in Table 3.

[0057] Table 3 Product performance comparison

[0058]

[0059] Note: The mechanical property measurement method adopts the national standard GB / T 228.1-2021, the sample type is P6, the sample direction is longitudinal, and the hole expansion rate measurement method adopts the national standard GB / T 15825.4-2008, and punching and conical punch are selected.

[0060] The above detailed description of the reference embodiment for the yield-increasing and efficiency-improving method of the high-strength automobile sheet is illustrative rather than limiting, and a number of embodiments can be listed within the limited range, so that changes and modifications within the overall concept of the present application shall be within the protection scope of the present application.

Claims

1. A method for increasing productivity and efficiency of high strength automotive sheet characterized in that, The tensile strength of the high-strength automobile plate is 590 MPa; the yield-increasing and benefit-increasing method of the high-strength automobile plate comprises the following steps: smelting, continuous casting, hot rolling, pickling cold rolling and continuous annealing; The composition of the molten steel obtained by smelting avoids the sub-peritectic zone sensitive to cracks; the mass percentages of C, Mn and Cr in the composition of the molten steel satisfy: C 0.050~0.070%, 2.05%≤Mn+Cr≤2.25%; In the continuous casting step, the slab is not cornered after being discharged; In the hot rolling step, the slab is hot charged for 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 fast cooling section of 14~18 m is used; after annealing, slow cooling, fast cooling, overaging treatment and leveling are performed; the soaking temperature of the annealing is 790±20℃; When the strip thickness is 0.5mm<strip thickness≤1.0mm, the strip speed in the furnace is 250±50m / min, after annealing, slow cooling to 640±20℃, then fast cooling to 315±15℃, then overaging treatment at 310±20℃, and the leveling elongation is 0.8±0.2%; When the strip thickness is 1.0mm<strip thickness≤1.5mm, the strip speed in the furnace is 200±50m / min, after annealing, slow cooling to 620±20℃, then fast cooling to 305±15℃, then overaging treatment at 300±20℃, and the leveling elongation is 0.7±0.2%; When the strip thickness is 1.5mm<strip thickness≤2.0mm, the strip speed in the furnace is 160±40m / min, after annealing, slow cooling to 600±20℃, then fast cooling to 295±15℃, then overaging treatment at 290±20℃, and the leveling elongation is 0.6±0.2%; When the strip thickness is 2.0mm<strip thickness≤2.5mm, the strip speed in the furnace is 130±30m / min, after annealing, slow cooling to 580±20℃, then fast cooling to 285±15℃, then overaging treatment at 275±20℃, and the leveling elongation is 0.5±0.2%.

2. The method for increasing productivity and efficiency of high strength automotive sheet as claimed in claim 1, wherein, 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 inevitable impurities.

3. The method for increasing productivity and efficiency of high-strength automotive sheets according to claim 1 or 2, characterized in that, In the hot rolling step, the coiling temperature is 600~680℃.

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

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

6. The method for increasing productivity and efficiency of high-strength automotive sheets according to claim 1 or 2, characterized in that, In the continuous annealing step, the slow cooling section is designed with 3 passes and is equipped with 4 cooling fans.

7. The method for increasing productivity and efficiency of high-strength automotive sheets according to claim 1 or 2, characterized in that, In the continuous annealing step, the fast cooling section is designed with 1 pass and is equipped with 3 cooling fans.

8. The method for increasing productivity and efficiency of high-strength automotive sheets according to claim 1 or 2, characterized in that, In the continuous annealing step, a high-hydrogen high-speed gas jet cooling process is used in the fast cooling section, and the volume fraction of hydrogen in the cooling medium is 4~38%.

9. The method for increasing productivity and efficiency of high-strength automotive sheets according to claim 1 or 2, characterized in that, The soaking temperature of the annealing is 780±10℃.

10. High-strength automotive sheet according to any one of claims 1 to 9, characterized in that, The high-strength automobile plate has yield strength of 340-410 MPa, tensile strength of 590-670 MPa, and elongation after fracture A 80 ≥ 24%, hole expansion rate ≥ 40%, strain hardening index ≥ 0.14, and yield strength ratio < 0.65.

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