Galvanized bake-hardened steel cross grain defect eliminating method

Abstract

The invention discloses a method for eliminating cross grain defects of galvanized bake-hardened steel, and belongs to the technical field of galvanized plates. The method comprises the steps that S1, main parameters include the zinc pot temperature t, the tower top roller temperature T, the tower top roller height H, the production line speed V and the cooling tower strip steel cooling speed T, and T = (t-T) / (H / V) = (t-T) * V / H can be known through measurement and calculation; s2, the generation of the cross grain defect is mainly caused by too fast cooling of the plated strip steel, and the cross grain defect can be controlled only by controlling T in a proper range; s3, according to the strip steel surface cross grain state, the tower top roller temperature T is continuously adjusted, the minimum value T2 is obtained, and the T range can be solved; and S4, according to the formula T = (t-T) * V / H, the production line process zinc pot temperature t, the tower top roller temperature T and the production line speed V can be adjusted, and the surface quality of the bake-hardened steel is improved. The cross grain defect can be eliminated by adjusting the production line process, and the product quality is improved.

Description

Technical Field

[0001] The invention relates to the technical field of galvanized sheet materials, in particular to a method for eliminating transverse grain defects of galvanized bake-hardened steel. Background Art

[0002] Bake-hardening steel, abbreviated as BH steel, is made on the basis of low-carbon or ultra-low-carbon steel through the addition of trace elements and corresponding process systems. A small amount of interstitial atoms C and N are dissolved during the production process, resulting in a relatively low yield strength of the steel plate in the delivered state and easy forming. The dislocation density of the steel base increases during the stamping process, and the interstitial atoms C and N diffuse toward the dislocations during the painting and baking process, thereby pinning the dislocations. The yield strength of the steel is increased through strain aging, ultimately making the produced automotive parts have a high resistance to dents.

[0003] With the development of the automotive industry, demand for stamping steel is rapidly increasing. Galvanized sheet, in particular, is rapidly replacing cold-rolled sheet due to its superior corrosion resistance. Bake-hardened steel, on the other hand, has a low yield strength before forming. After stamping, painting, and baking, the strength of the steel sheet is enhanced, resulting in excellent deep-drawing and high-strength properties. The properties of galvanized bake-hardened steel align with the weight reduction, energy conservation, and corrosion resistance trends of automotive steel sheet, leading to its widespread use in high-end automotive manufacturers.

[0004] Due to its unique structural composition, galvanized bake-hardened steel is prone to developing transverse streaks during the production process. Analysis shows that these defects primarily form in the cooling tower area after galvanizing. While this defect does not affect customer use in standard automotive structural panels, it can severely impact high-end automotive panels. Summary of the Invention

[0005] The purpose of the present invention is to provide a method for eliminating transverse streak defects in galvanized bake-hardened steel, which is simple to operate and can achieve transverse streak defects elimination by adjusting the production line process, thereby improving product quality, reducing internal quality rejudgment losses and customer complaints, and solving the problems raised in the above-mentioned background technology.

[0006] To achieve the above object, the present invention provides the following technical solution: a method for eliminating transverse streak defects in galvanized bake hardening steel, comprising the following steps:

[0007] S1. Main parameters: zinc pot temperature t, tower top roller temperature T, tower top roller height H, production line speed V, cooling tower strip cooling rate △T. Through measurement, we know that: △T = (tT) / (H / V) = (tT) * V / H; among which, the tower top roller temperature T is divided into the appropriate maximum temperature T1 of the tower top roller and the appropriate minimum temperature T2 of the tower top roller;

[0008] S2. The generation of cross striation defects is mainly caused by the too rapid cooling of the strip after plating. To control cross striation defects, it is only necessary to control △T within an appropriate range. Since the working conditions of each galvanizing production line are different, including the diameter of the top roll, the diameter of the immersion roll, and the position of the cooling fan after plating, etc., the △T of each production line for producing bake hardening steel is inconsistent. However, for each production line, △T is a constant value that conforms to the tooling configuration of its own production line. Generally, the top roll of the production line is treated with wrapped cloth, and the appropriate maximum temperature T1 of the top roll is the maximum temperature at which the wrapped cloth works normally.

[0009] S3. Determine the appropriate minimum temperature T2 of the top roll: In the formula △T = (t - T) * V / H, the height H of the top roll is a fixed value for each production line. During the production of bake hardening steel, determine the zinc pot temperature t value and the production line speed V value. According to the cross striation state on the strip surface, continuously adjust the temperature T of the top roll to obtain the minimum value T2, and then the △T range can be calculated.

[0010] S4. According to the formula △T = (t - T) * V / H, the zinc pot temperature t, the temperature T of the top roll, and the production line speed V of the production line process can be adjusted to improve the surface quality of the bake hardening steel. Through this formula, the reasonable control range of the strip speed under different processes can be explored. On the premise of no cross striations, the maximum production line speed V = △T * H / (t - T1).

[0011] Preferably, in S3, the △T range is △Tmin~△Tmax, and in S4, the maximum production line speed Vmax = △Tmax * H / (t - T1).

[0012] Preferably, by replacing the wrapped cloth with better quality, the appropriate maximum temperature T1 of the top roll can be increased, thereby increasing the maximum production line speed Vmax in S4.

[0013] Compared with the prior art, the beneficial effects of the present invention are:

[0014] The present invention can eliminate cross striation defects by adjusting the production line process and improve product quality. According to the characteristics of the galvanizing production line, a simple model formula for bake hardening steel without generating cross striation defects is established. This formula can effectively guide the production of the production line and is very popularizable. Specific Embodiments

[0015] The technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all of the embodiments. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts shall fall within the protection scope of the present invention.

[0016] The bake hardening steel has a strong aging tendency before skin pass rolling. In actual production, the power of the fan after plating is too large, resulting in the BH steel cooling too quickly in the upward section. Especially, the cooling rate at the edge is much faster than that in the middle. The internal stress generated by the uneven cooling rate between the edge and the middle "tears" the strip steel, resulting in drawing marks, and it will gradually worsen with the storage time. Through targeted investigation and analysis, the following technical solutions are summarized: A method for eliminating crosswise grain defects of galvanized bake hardening steel, including the following steps:

[0017] S1. Main parameters: The zinc pot temperature is t, the top roll temperature is T, the height of the top roll is H, the production line speed is V, and the strip steel cooling rate in the cooling tower is △T. Through calculation, it can be known that: △T = (t - T) / (H / V) = (t - T)*V / H; among them, the top roll temperature T is divided into the appropriate maximum temperature T1 of the top roll and the appropriate minimum temperature T2 of the top roll;

[0018] S2. The generation of crosswise grain defects is mainly caused by the too fast cooling of the strip steel after plating. To control the crosswise grain defects, it is only necessary to control △T within an appropriate range; since the working conditions of each galvanizing production line are different, including the diameter of the top roll, the diameter of the immersion roll, and the position of the fan for cooling after plating, etc., the △T of each production line for producing bake hardening steel is inconsistent. However, for each production line, △T is a constant value that conforms to the tooling configuration of its own production line; generally, the top roll of the production line is treated with wrapping cloth, and the appropriate maximum temperature T1 of the top roll is the maximum temperature at which the wrapping cloth works normally;

[0019] S3. Determine the appropriate minimum temperature T2 of the top roll: In the formula △T = (t - T)*V / H, the height H of the top roll is a fixed value for each production line. During the production of bake hardening steel, determine the values of the zinc pot temperature t and the production line speed V, and continuously adjust the top roll temperature T according to the crosswise grain state on the strip steel surface to obtain the minimum value T2, and then the △T range can be calculated;

[0020] S4. According to the formula △T = (t - T)*V / H, the zinc pot temperature t, the top roll temperature T, and the production line speed V of the production line process can be adjusted to improve the surface quality of the bake hardening steel; through this formula, the reasonable control range of the strip steel speed under different processes can be explored. Without the appearance of crosswise grains, the maximum production line speed V = △T*H / (t - T1).

[0021] Among them, the △T range in S3 is △Tmin~△Tmax, and the maximum production line speed Vmax in S4 = △Tmax*H / (t - T1); by replacing the wrapping cloth with better quality, the appropriate maximum temperature T1 of the top roll can be increased, so as to increase the maximum production line speed Vmax in S4.

[0022] Taking a galvanizing line of Magang as an example, this production line produces bake hardening steel automotive sheets. The height of the top roll is H = 66m, the temperature of the zinc pot is t = 460°C. The top roll of this galvanizing line is treated with wrapping cloth, and the maximum normal working temperature of the wrapping cloth is 250°C, that is, the appropriate maximum temperature of the top roll is T1 = 250°C.

[0023] 1) Set the speed V = 1.5m / s, △Tmin = (t - T1) * V / H = (460 - 250) * 1.5 / 66 = 4.77°C / s.

[0024] 2) By continuously adjusting the temperature T of the top roll, it is obtained that on the premise of V = 1.5m / s, the lowest temperature T2 at which the strip does not produce crosswise stripes is 180°C, and △Tmax = (460 - 180) * 1.5 / 66 = 6.36°C / s.

[0025] 3) During the production of bake hardening steel on this production line, the cooling rate of the post - plating cooling tower should be controlled between 4.77°C / s and 6.36°C / s. In the actual mass production process, △Tmin has little guiding significance for production. Each production line is pursuing high efficiency, the speed is continuously increased, and △Tmin can be further reduced by replacing the type of wrapping cloth to increase T1. The practical significance of the △Tmax value is greater. If high - efficiency production and increased speed are to be achieved, it is necessary to continue to increase the temperature of the top roll and reduce the temperature of the zinc pot. In fact, since the temperature of the zinc pot has a great influence on coating defects such as zinc ash and zinc slag, the lowest temperature of the zinc pot for producing automotive sheets is generally 455°C. Therefore, during the production of bake hardening steel, on the premise of ensuring no crosswise stripe defects, high - efficiency production can be achieved by increasing the temperature of the top roll.

[0026] 4) When this production line produces bake hardening steel, with the zinc pot temperature t = 460°C and the temperature of the top roll set at 220°C, without crosswise stripes, the maximum speed Vmax = △Tmax * H / (t - T1) = 6.36 * 66 / (460 - 220) = 1.75m / s. That is, when the temperature of the top roll is 220°C, the maximum speed of the production line can reach 1.75m / s while ensuring no crosswise stripe defects.

[0027] It should be noted that in this article, relational terms such as first and second are only used to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply any actual relationship or order between these entities or operations. Moreover, the term "including", "comprising" or any other variant thereof is intended to cover non - exclusive inclusion, so that a process, method, article or device including a series of elements not only includes those elements, but also includes other elements not expressly listed, or also includes elements inherent to such process, method, article or device.

[0028] Although embodiments of the present invention have been shown and described, it will be understood by those of ordinary skill in the art that various changes, modifications, substitutions and variations can be made to these embodiments without departing from the principles and spirit of the present invention, and the scope of the present invention is defined by the appended claims and their equivalents.

Claims

1. A method for eliminating cross - grain defects of galvanized bake - hardening steel, characterized in that: It includes the following steps: S1. Main parameters: the temperature t of the zinc pot, the temperature T of the top roll, the height H of the top roll, the production line speed V, and the strip cooling rate △T of the cooling tower. Through measurement, it can be known that: △T = (t - T) / (H / V) = (t - T)*V / H; among them, the temperature T of the top roll is divided into the appropriate maximum temperature T1 and the appropriate minimum temperature T2 of the top roll. S2. The generation of transverse stripes defects is mainly caused by the too fast cooling of the strip after plating. To control the transverse stripes defects, it is only necessary to control △T within an appropriate range; due to the differences in the working conditions of each galvanizing production line, including the diameter of the top roll, the diameter of the immersion roll, and the position of the cooling fan after plating, etc., the △T of each production line for producing bake hardening steel is inconsistent. However, for each production line, △T is a constant value that conforms to the tooling configuration of its own production line; generally, the top rolls of the production line are all treated with wrapping cloth, and the appropriate maximum temperature T1 of the top roll is the maximum temperature at which the wrapping cloth works normally. S3. Determine the appropriate minimum temperature T2 of the top roll: In the formula △T = (t - T)*V / H, the height H of the top roll is a fixed value for each production line. During the production of bake hardening steel, determine the value of the zinc pot temperature t and the value of the production line speed V, and continuously adjust the temperature T of the top roll according to the transverse stripe state of the strip surface to obtain the minimum value T2, and the △T range can be calculated. S4. According to the formula △T = (t - T)*V / H, the zinc pot temperature t, the temperature T of the top roll, and the production line speed V of the production line process can be adjusted to improve the surface quality of the bake hardening steel; through this formula, the reasonable control range of the strip speed under different processes can be explored. Without the appearance of transverse stripes, the maximum production line speed V = △T*H / (t - T1).

2. A method for eliminating transverse defects of galvanized bake hardening steel according to claim 1, characterized in that: In S3, the △T range is △Tmin~△Tmax, and in S4, the maximum production line speed Vmax = △Tmax*H / (t - T1).

3. A method for eliminating cross-grain defects of galvanized bake-hardening steel according to claim 2, characterized in that: By replacing the wrapping cloth with better quality, the appropriate maximum temperature T1 of the top roll can be increased, thereby increasing the maximum production line speed Vmax in S4.