Production method for improving large performance fluctuation of high-strength dual-phase steel

Abstract

The application provides a production method for improving the large performance fluctuation of high-strength dual-phase steel, which comprises smelting, continuous casting, heating, hot rolling, cooling, coiling, slow cooling and cold rolling in sequence; in the cooling stage, the temperature of the head section with the first preset length on the strip steel, the temperature of the tail section with the second preset length on the strip steel and the temperature of the remaining middle section on the strip steel are all greater than the temperature of the remaining middle section on the strip steel; in the coiling stage, when the last rolling mill bites the head end of the strip steel, the cooling water of all coiling machines and the cooling water of pinch rolls on the conveying path are turned off; when the corresponding coiling machine bites the head end of the strip steel and starts to coil in the first time window, the cooling water of the pinch rolls of the coiling machine is turned on; in the slow cooling stage, the temperature in the slow cooling furnace is uniformly distributed through the fan and the guide plate. Through the above method, the mechanical property uniformity of the head and tail is improved, the welding qualification rate is improved, and the production accident rate is reduced.

Description

Technical Field

[0001] This invention relates to the field of steel production technology, specifically to a production method for improving the large performance fluctuations of high-strength duplex steel. Background Technology

[0002] Duplex steel is widely used in the automotive industry, primarily for manufacturing structural components, safety components, and crash protection parts that require high mechanical properties. Its dual phases refer to ferrite and martensite in the matrix. The ferrite phase is relatively soft and has good ductility, while the martensite phase is harder and has high strength. Duplex steel combines the "hardness" of martensite with the "flexibility" of ferrite, possessing both high strength and good ductility and formability.

[0003] Currently, duplex steel is the most economical and widely used advanced high-strength steel; the International Steel Institute lists duplex steel as one of the Advanced High Strength Steels (AHSS). However, the performance fluctuations of ultra-high-strength steel are generally greater than those of ordinary steel, especially for high-grade duplex steels (HC420 / 780DP, HC550 / 980DP, HC650 / 980DP, HC820 / 1180DP), where the performance differences between the head / tail and middle sections are significant. This can lead to poor welding at the head and tail during subsequent pickling and rolling processes, and repeated re-welding can easily cause strip breakage. The uneven performance at the head and tail also affects the cold rolling process, as the set rolling force cannot provide timely feedback on the fluctuating performance. This results in large thickness fluctuations after pickling and rolling, exceeding customer tolerance requirements and greatly impacting order yield and fulfillment rates. At the same time, unstable production can easily lead to accidents such as high mill load causing power outages, steel pile-up, and strip breakage. Summary of the Invention

[0004] The purpose of this invention is to address the shortcomings of existing technologies by providing a production method for improving the large performance fluctuations of high-strength duplex steel. The method sequentially includes smelting, continuous casting, heating, hot rolling, cooling, coiling, slow cooling, and cold rolling. In the cooling stage, the temperatures of the head section of the strip at a first preset length and the tail section of the strip at a second preset length are controlled to be higher than the temperature of the remaining middle section of the strip, ensuring that the cooling rates of the head section of the first preset length, the tail section of the second preset length, and the remaining middle section of the strip are within a preset cooling rate range. In the coiling stage, when the last mill bites the head section of the strip… At the end, the cooling water of all coilers and the cooling water of the pinch rolls on the conveying path are turned off. When the coiler corresponding to the strip bites the end of the strip and starts coiling in the first time window, the cooling water of the pinch roll of the coiler is turned on. When the tail end of the strip leaves the last rolling mill, the cooling water of the pinch roll of the coiler is turned off. When the steel coil is wound and completely unloaded, the cooling water of the pinch roll of the coiler is turned on again. When the support arm of the auxiliary coiling roll is fully reset, the cooling water of the auxiliary coiling roll of the coiler is turned on and turned off after the second time window. In the slow cooling stage, the temperature distribution in the slow cooling chamber is controlled by the fan and the guide plate.

[0005] Furthermore, during the cooling stage, the cooling curve of the strip along its length is controlled to be a U-shaped cooling curve. Even further, during the cooling stage, the second preset length = the first preset length ≤ 100m.

[0006] Furthermore, laminar flow cooling is employed during the cooling stage.

[0007] Furthermore, during the curling stage, the first time window is 8~12s.

[0008] Furthermore, during the curling stage, the second time window is 5-10 seconds.

[0009] Furthermore, during the slow cooling phase, the fan speed is controlled at 1~6m / s, and the temperature difference inside the slow cooling chamber is controlled at ≤20℃.

[0010] Furthermore, during the slow cooling stage, the walls of the slow cooling storage are constructed using rock wool sandwich panels, and the roof is constructed using double-layer polyurethane sandwich panels. Even further, during the slow cooling stage, the thickness of the rock wool sandwich panels is 100-140 cm.

[0011] Furthermore, during the slow cooling phase, the average temperature inside the slow cooling chamber is controlled at 40°C.

[0012] The beneficial effects of this invention are as follows: 1. By controlling the cooling curve using a U-shaped profile, the difference in martensite ratio between the head, tail, and middle sections of the strip is reduced, thus minimizing fluctuations in mechanical properties (strength, elongation). Improved uniformity of mechanical properties at both ends increases the welding pass rate and reduces the incidence of production accidents.

[0013] 2. By precisely compensating for cooling losses at the beginning and end, the length of the defective area is shortened, increasing the yield from 80% to over 95%. This improved uniformity of performance reduces the thickness tolerance of the cold-rolled material.

[0014] 3. By controlling the supply time window of the coiler cooling water, the possibility of the coiler cooling water being sprayed onto the strip steel is eliminated, preventing strip steel cooling outside the production plan. Combined with the slow cooling chamber, the temperature drop rate of the strip steel is strictly controlled, thereby improving the yield. Attached Figure Description

[0015] Figure 1 This is a U-shaped cooling curve diagram. Detailed Implementation

[0016] Unless otherwise specified, all raw materials used below are commercially available products, and all methods used below are conventional methods in this field.

[0017] In pickling and rolling lines, the beginning and end of adjacent steel coils need to be welded. If the hardness of the beginning and end is too high (too much martensite) or too low (too much ferrite), the weld strength will be insufficient, leading to the risk of strip breakage. In cold rolling, the rolling force is set based on the performance of the middle section; uneven performance at the beginning and end will result in thickness deviations (e.g., 1.2mm in the middle section, 1.15mm or 1.3mm at the beginning and end). Based on these situations, the unqualified portions at the beginning and end must be removed, leading to a decrease in yield.

[0018] For example, the ideal cooling rate for HC420 / 780DP is 15~30℃ / s. When the cooling rate is <15℃ / s, too much ferrite + pearlite will be produced, resulting in insufficient strength; when the cooling rate is 15~30℃ / s, ferrite + a suitable amount of martensite will be produced, which is an ideal dual-phase structure with a relatively balanced strength and elongation; when the cooling rate is >30℃ / s, too much martensite will be produced, along with residual austenite, leading to a decrease in elongation, an increase in residual stress, and increased brittleness.

[0019] Based on the above reasons, the present invention provides a production method for improving the large performance fluctuations of high-strength duplex steel, which includes smelting, continuous casting, heating, hot rolling, cooling, coiling, slow cooling and cold rolling in sequence.

[0020] During the cooling stage, laminar flow cooling is employed. The cooling equipment's built-in program dynamically adjusts the water spray pattern in the laminar flow cooling zone, achieving zoned dynamic temperature control (e.g., closing 50% of the nozzles at the beginning and end). This reduces the cooling intensity at the beginning and end of the strip, ensuring that the temperature of the first 100m and the last 100m of the strip is 50°C higher than the remaining middle section. This results in a U-shaped cooling curve along the strip's length. Figure 1As shown in the figure, the horizontal axis represents the length of the strip in meters, with values ​​increasing from left to right, indicating the length extension of the strip from head to tail. 0 represents the starting position of the strip head, and the values ​​increase sequentially to represent the head, middle, and tail sections. The vertical axis represents the coiling temperature of the strip in degrees Celsius, increasing from bottom to top, representing the actual coiling temperature at the corresponding length position. This is the core temperature control indicator during the cooling stage. The target value of 550 indicates that the process control target value for the coiling temperature during the strip cooling stage is 550℃. This is the core temperature benchmark to ensure the formation of the ideal ferrite-martensite dual-phase structure in high-strength dual-phase steel. The middle section of the strip needs to be stably controlled near this temperature. The positive tolerance of 20 indicates that the upper limit of the allowable deviation for the coiling temperature is +20℃, meaning the actual coiling temperature can be controlled up to 550 + 20 = 570℃. Exceeding this value is considered a temperature deviation and can easily lead to abnormal phase transformation in the strip structure. The negative tolerance of -20 indicates that the lower limit of the allowable deviation of the coiling temperature is -20℃. This means the actual coiling temperature of the strip can be controlled to a minimum of 550-20=530℃. Temperatures below this value are considered out of tolerance, which can cause problems such as excessive martensite content and decreased elongation. Simultaneously, it ensures that the cooling rates of the first preset length section, the second preset length section, and the remaining middle section of the strip are within a preset cooling rate range, such as 15~30℃ for HC420 / 780DP. Compared to traditional spray cooling, laminar flow cooling offers better uniformity (temperature difference ≤10°C) and avoids localized overcooling. When the strip leaves the finishing mill, the head section is exposed to air, and the tail section is similarly exposed when leaving the coiler. The heat dissipation rate is much faster than that of the coiled middle section (which has better interlayer insulation after being coiled). The natural cooling rate of the head and tail sections may be 20-30°C / s higher than that of the middle section, leading to an abnormal martensite ratio in both sections (e.g., 15% martensite in the middle section, but only 8% at the head and tail). Furthermore, the head and tail sections pass through pinch rolls, auxiliary rolls, and other equipment, further exacerbating the local temperature drop due to the heat conduction of the metal rolls. Temperature compensation at the head and tail sections offsets the natural heat loss, ensuring that the actual phase transformation temperature matches that of the middle section and maintaining a stable martensite ratio.

[0021] During the coiling stage, when the last rolling mill bites the head end of the strip, the cooling water of all coilers and the pinch rolls on the conveying path is shut off to ensure that the strip is not splashed by water from the coilers during conveying. When the corresponding coiler bites the head end of the strip and begins the first time window of coiling (e.g., 10 seconds, the principle of setting the first time window is to avoid the head section of the strip being sprayed), it is considered that the head section of the strip is safely wound on the drum and is no longer exposed to the outside, to avoid water splashing causing abnormal martensite formation in the head section. Then the cooling water of the pinch rolls of that coiler is turned on, while the cooling water of other coilers remains closed. This step is to ensure that the head section of the strip is safe before turning on the cooling water to cool the pinch rolls that are working at high speed, to prevent the rolls from being damaged by high temperature. The delay in the first time window (e.g., 10 seconds) is to ensure that the head section of the strip has fully entered the coiling state and to avoid being splashed by water. When the tail end of the strip leaves the last rolling mill, shut off the pinch roll cooling water of the coiler to prevent cooling water from spraying onto the tail end of the strip, which is another performance-sensitive area and needs to be protected like the head end to avoid overcooling. When the coil is wound and completely unloaded, reopen the pinch roll cooling water of the coiler to prepare for the next work cycle and cool the idle pinch roll. When the support arm of the help roll is fully reset, turn on the help roll cooling water of the coiler and continue for a second time window (e.g., 7 seconds, the principle of setting the second time window is to ensure that no water remains or splashes onto the head end of the next coil) and then turn it off. The help roll comes into contact with the hot strip at the beginning of coiling and is very hot itself. The brief second time window of water spraying (e.g., 7 seconds) is sufficient to cool it down quickly and prevent it from thermal deformation or damage. At the same time, the water volume and time are extremely strictly controlled to ensure that no water remains or splashes onto the head end of the next coil. The above operations are specifically achieved by setting the timing control information for the cooling water of the auxiliary winding roll and the cooling water of the pinch roll in the secondary model.

[0022] During the slow cooling stage, the temperature distribution within the slow cooling chamber is controlled by fans and deflectors. In practice, slow cooling chambers are built near the hot rolling production line (to reduce the transport distance of steel coils and minimize temperature drop). These chambers are divided into an entrance area (for receiving, inspecting, and preparing for stacking steel coils), a slow cooling area (multiple slow cooling areas arranged in parallel, equipped with roller conveyors and elevators), and an exit area (for steel coils leaving the chamber and being transferred to the cold rolling or coating production line). The walls of the slow cooling chambers are constructed with 100-140cm thick rock wool sandwich panels, and the roof is made of double-layer polyurethane sandwich panels to enhance insulation performance, control the rate of temperature drop, and prevent the formation of bainite during the slow cooling process. Steel coils are transported via conveyor chain to the entrance area of ​​the slow cooling silo. After receiving and inspection, they are moved to the slow cooling zone and then transported by roller conveyor to designated pits. Once the first pit is full, the second, third, and fourth pits are filled using elevators, and then covered with insulation. Fans and baffles are adjusted to ensure temperature uniformity within the silo. After the set slow cooling time, the steel coils are transported via conveyor chain to the exit area for unloading and transfer to the cold rolling or coating production line. The fan speed is controlled at 1~6 m / s, the temperature difference within the slow cooling silo is controlled to ≤20℃, and the average temperature within the silo is controlled to 40℃. This optimizes the martensitic tempering effect, forces air cooling to homogenize the core and surface temperature of the steel coils, reduces residual stress, and compared to natural cooling, slow cooling treatment can reduce the breakage rate during pickling and rolling.

[0023] Actual measurements showed that the large fluctuations in performance at the beginning and end of the roll were significantly improved. The loss caused by the uneven performance at the beginning and end of the roll was reduced, and the yield increased from 80% to over 95%. Examples and comparative examples are provided below for detailed explanation.

[0024] Example 1 Example 1 describes the preparation of HC420 / 780DP high-strength duplex steel using the above-mentioned production method for improving the large performance fluctuations of high-strength duplex steel. The specific process parameters are as follows: Cooling stage: Laminar flow cooling method is adopted, and the preset lengths of the head and tail sections of the strip are controlled to be 100m. The temperature of the head and tail sections is 50℃ higher than that of the middle section, forming a U-shaped cooling curve. The overall cooling rate is controlled to be 15~30℃ / s, and the laminar flow cooling temperature difference is ≤10℃.

[0025] Coiling stage: When the last mill bites the end of the strip, shut off the cooling water of all coilers and pinch rolls; after the corresponding coiler bites the end of the strip and coils for 10 seconds, turn on the cooling water of the pinch roll of that coiler; when the tail end of the strip leaves the last mill, turn off the cooling water of the pinch roll of that coil; after the coil is completely unloaded, turn on the cooling water of the pinch roll again; after the auxiliary coiling roll support arm is reset, turn on the cooling water of the auxiliary coiling roll and continue for 7 seconds before turning it off.

[0026] Slow cooling stage: The walls of the slow cooling warehouse are made of 120cm thick rock wool sandwich panels, and the top is made of double-layer polyurethane sandwich panels. The fan speed is controlled at 3m / s, the average temperature inside the warehouse is 40℃, and the temperature difference inside the warehouse is ≤15℃. The temperature distribution inside the warehouse is ensured by the fan and the baffle plate.

[0027] The remaining smelting, continuous casting, heating, hot rolling, and cold rolling processes were all carried out using conventional process parameters in this field. Performance testing and production yield statistics were performed on the prepared HC420 / 780DP high-strength duplex steel. The difference in martensite ratio between the head and tail sections and the middle section of the strip was controlled within 3%, and the fluctuation range of mechanical properties (strength, elongation) was ≤5%. The head and tail welding qualification rate in the pickling and rolling process was increased to over 98%, and no production accidents such as mill power outages, steel piling, or strip breakage occurred. Shear losses during pickling and rolling due to uneven head and tail performance were reduced, and the yield reached 96.5%.

[0028] Comparative Example 1 Comparative Example 1 used conventional production methods to prepare the same type of HC420 / 780DP high-strength duplex steel as Example 1. The smelting, continuous casting, heating, hot rolling, and cold rolling processes were consistent with those of Example 1, except that: Cooling stage: The traditional spray cooling method is adopted, and the same cooling intensity is used throughout the entire strip. There are no temperature compensation measures at the beginning and end. The local temperature difference can reach 25℃ during the cooling process, and the overall cooling rate fluctuates between 10~38℃ / s.

[0029] During the coiling stage: the cooling water for the coiler, pinch rolls, and auxiliary coiling rolls is running continuously without timing control, and the strip is easily sprayed with cooling water during the conveying and coiling process.

[0030] Slow cooling stage: There is no dedicated slow cooling silo. The strip steel is cooled naturally. During the cooling process, the temperature difference between the core and the surface of the steel coil can reach more than 40°C. There are no temperature uniformity control measures.

[0031] Performance testing and production yield statistics were conducted on the HC420 / 780DP high-strength duplex steel prepared in Comparative Example 1. The difference in martensite ratio between the head and tail sections and the middle section of the strip was over 10%, with the head and tail sections having a martensite ratio of only about 8% and the middle section exceeding 15%. The mechanical properties (strength and elongation) fluctuated by ≥15%. During the pickling and rolling process, the uneven performance at the head and tail sections resulted in a welding qualification rate of only 70%, and multiple re-welding incidents led to strip breakage. During the cold rolling process, the rolling force fluctuated greatly, and the strip thickness exceeded the tolerance (1.2 mm in the middle section, and deviations of 1.15 mm or 1.3 mm at the head and tail sections). There were also multiple instances of high-load power outages and steel piling. The pickling and rolling shear losses were large due to the unqualified performance at the head and tail sections, resulting in a yield of only 78.2%.

[0032] Comparative Example 2 Comparative Example 2 prepared the same type of HC420 / 780DP high-strength duplex steel. The smelting, continuous casting, heating, hot rolling, and cold rolling processes were the same as in Example 1, except that the cooling stage used the same cooling process as in Example 1. The coiling and slow cooling were the same as in Comparative Example 1. The specific differences are as follows: Cooling phase: exactly the same as in Example 1.

[0033] During the coiling stage: Similar to Comparative Example 1, the cooling water is on throughout the entire process without timing control, and the head and tail of the strip are easily sprayed with cooling water, causing additional temperature drops.

[0034] Slow cooling stage: Similar to Comparative Example 1, natural cooling, no dedicated slow cooling chamber, large temperature difference on the surface of the steel coil core, no temperature uniformity control.

[0035] Performance testing and production yield statistics were conducted on the HC420 / 780DP high-strength duplex steel prepared in Comparative Example 2. The difference in martensite ratio between the head and tail sections and the middle section of the strip was 6%~8%, and the mechanical properties (strength and elongation) fluctuated by 8%~12%. The welding qualification rate in the pickling and rolling process was about 85%, but there were still a few cases of re-welding. The rolling force fluctuated slightly during the cold rolling process, the strip thickness exceeded the tolerance in some areas, and there were occasional minor steel piling problems. The pickling and rolling shear loss due to the uneven performance at the head and tail was reduced compared with Comparative Example 1, but there was still a significant loss, and the yield was only 86.3%.

[0036] Comparative Example 3 Comparative Example 3 prepared the same type of HC420 / 780DP high-strength duplex steel. The cooling and rolling stages were completely the same as in Example 1, except that the parameters of the slow cooling stage were adjusted: the fan speed was controlled at 8 m / s (deviating from the preset range of 1~6 m / s), and the rest of the slow cooling operation was the same as in Example 1.

[0037] Excessive fan speed caused the surface temperature of the steel coil to drop too quickly, with a temperature difference of 25°C between the core and the surface, resulting in the formation of a small amount of bainite. The mechanical properties of the head, tail, and middle sections fluctuated by 7% to 9%. The qualified rate of pickling and rolling welding was 88%, and the rolling force of cold rolling fluctuated significantly. The yield was 88.6%.

[0038] The above-described embodiments are only used to illustrate the technical solutions of this application, and are not intended to limit them. Although this application has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to the intermediate technical features. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of this application, and should all be included within the protection scope of this application.

Claims

1. A production method for improving the large performance fluctuations of high-strength duplex steel, comprising, in sequence, smelting, continuous casting, heating, hot rolling, cooling, coiling, slow cooling, and cold rolling, characterized in that: During the cooling stage, the temperature of the first preset length head section and the second preset length tail section of the strip are both controlled to be greater than the temperature of the remaining middle section of the strip, so that the cooling rate of the first preset length head section, the second preset length tail section of the strip and the temperature of the remaining middle section of the strip are within the preset cooling rate range. During the coiling stage, when the last mill bites the head end of the strip, the cooling water of all coilers and the cooling water of the pinch rolls on the conveying path are turned off. When the corresponding coiler bites the head end of the strip and begins the first time window of coiling, the cooling water of the pinch roll of the coiler is turned on. When the tail end of the strip leaves the last mill, the cooling water of the pinch roll of the coiler is turned off. When the steel coil is coiled and completely unloaded, the cooling water of the pinch roll of the coiler is turned on again. When the support arm of the auxiliary coiling roll is fully reset, the cooling water of the auxiliary coiling roll of the coiler is turned on and continues for the second time window before being turned off. During the slow cooling phase, the temperature distribution inside the slow cooling chamber is controlled by fans and deflectors to ensure uniformity.

2. The production method for improving the large performance fluctuations of high-strength duplex steel according to claim 1, characterized in that: During the cooling stage, the cooling curve of the strip along its length is controlled to be a U-shaped cooling curve.

3. The production method for improving the large performance fluctuations of high-strength duplex steel according to claim 2, characterized in that: During the cooling phase, the second preset length = the first preset length ≤ 100m.

4. The production method for improving the large performance fluctuations of high-strength duplex steel according to claim 1, characterized in that: During the cooling stage, laminar flow cooling is employed.

5. The production method for improving the large performance fluctuations of high-strength duplex steel according to claim 1, characterized in that: During the curling stage, the first time window is 8~12s.

6. The production method for improving the large performance fluctuations of high-strength duplex steel according to claim 1, characterized in that: During the curling stage, the second time window is 5~10s.

7. The production method for improving the large performance fluctuations of high-strength duplex steel according to claim 1, characterized in that: During the slow cooling phase, the fan speed is controlled at 1~6m / s, and the temperature difference inside the slow cooling chamber is controlled at ≤20℃.

8. The production method for improving the large performance fluctuations of high-strength duplex steel according to claim 1, characterized in that: During the slow cooling phase, the walls of the slow cooling storage are made of rock wool sandwich panels, and the top cover is made of double-layer polyurethane sandwich panels.

9. The production method for improving the large performance fluctuations of high-strength duplex steel according to claim 7, characterized in that: During the slow cooling stage, the thickness of the rock wool sandwich panel is 100~140cm.

10. The production method for improving the large performance fluctuations of high-strength duplex steel according to claim 1, characterized in that: During the slow cooling phase, the average temperature inside the slow cooling chamber is controlled at 40℃.

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