A method of reducing head cupping in cold rolled dual phase steel blanks

Abstract

This invention relates to a method for reducing head warping in cold-rolled duplex steel, belonging to the field of hot-rolled strip steel production technology in the metallurgical industry. The technical solution is as follows: When rolling cold-rolled duplex steel, a cooling water delay program is initiated before the pinch rolls bite the steel; after the pinch rolls bite the steel, the cumulative contact length Lacc between the strip and the pinch rolls is obtained; simultaneously, the cumulative contact time Tacc between the strip and the pinch rolls is calculated, and the upper limit time Th of the pinch roll cooling water delay is calculated based on the coiling temperature setting. When either Lacc ≥ Lh or Tacc ≥ Th is satisfied, the pinch roll cooling water is turned on. The beneficial effects of this invention are: by dynamically controlling the opening and closing sequence of the coiler pinch roll cooling water, the strip head is placed in a relatively stable cooling environment, reducing interference from the cooling water, ensuring uniform temperature drop and phase transformation in the width direction of the strip head, reducing residual stress in the strip, maintaining a good strip shape at the strip head, and reducing the occurrence of head warping.

Description

Technical Field

[0001] This invention relates to a method for reducing head warping of cold-rolled duplex steel base material, belonging to the field of hot-rolled strip steel production technology in the metallurgical industry. Background Technology

[0002] The production of hot-rolled strip steel mainly includes processes such as heating, roughing, finishing, layer cooling and coiling. After the billet goes through processes such as heating, roughing, and finishing, it enters the layer cooling section for cooling, and then enters the coiling machine through pinch rolls to complete the coiling. To ensure the continuity of production, the coiling machine is generally set up with 2-3 units, working alternately, and each unit is equipped with pinch rolls.

[0003] In the production of cold-rolled duplex steel, due to its high alloy content and low coiling temperature, strip shape control is difficult. Analysis of the strip head microstructure revealed that the martensitic-bainitic phase transformation occurs earlier in the middle section of the strip's width direction than at the edges, indicating a faster cooling rate in the middle and slower cooling rate at the edges. This uneven temperature distribution leads to uneven phase transformation and stress unevenness, resulting in strip shape problems such as warping.

[0004] In actual production, it has been found that the cooling water of the pinch rolls, especially the cooling water of the upper pinch roll, is a significant influencing factor. The upper pinch roll cooling water is further divided into strong cooling and weak cooling. During normal production, only weak cooling is activated when the pinch roll is in the steel biting gap; both strong and weak cooling are activated simultaneously when the steel is biting. The upper pinch roll cooling water spray volume is large, and after being sprayed onto the roll surface, some of the water flows onto the strip surface and accumulates in the middle section. If a remote coiler is used, the weak cooling water from the near-end coiler pinch roll will also flow onto the strip surface, causing a cumulative effect. Summary of the Invention

[0005] The purpose of this invention is to provide a method for reducing head warping of cold-rolled duplex steel base material, which can keep the strip head in a relatively stable cooling environment, reduce the interference of cooling water, ensure uniform temperature drop and phase transformation in the width direction of the strip head, reduce residual stress in the strip, maintain good strip shape at the strip head, reduce the occurrence of head warping, and solve the problems existing in the background art.

[0006] The technical solution of this invention is:

[0007] A method for reducing head warping of cold-rolled duplex steel base material involves dynamically controlling the opening and closing sequence of the cooling water on the coiler pinch rolls to reduce head warping. The specific steps are as follows:

[0008] Step 1: When rolling cold-rolled duplex steel base material, start the cooling water delay program before the pinch rolls bite the steel;

[0009] Step 2: After the pinch rolls bite the steel, calculate the strip speed using the pinch roll diameter and rotation speed, and integrate the strip speed to obtain the cumulative contact length Lacc between the strip and the pinch rolls.

[0010] Step 3: After the pinch rolls bite the strip, simultaneously calculate the cumulative contact time Tacc between the strip and the pinch rolls, and calculate the upper limit of the cooling water delay time Th for the pinch rolls based on the coiling temperature setting.

[0011] Th = (1700 - 2 × CT) / 15, where: CT: winding temperature setpoint;

[0012] Step 4: When either Lacc≥Lh or Tacc≥Th is satisfied, turn on the cooling water of the pinch rolls, where Lh is the length of the strip head, that is, the length of the strip that causes the bend.

[0013] If there are multiple winding machines, when using the remote winding machine, the cooling water for the pinch rolls of the near winding machine should be turned on only after the cooling water for the remote pinch rolls is turned on; when using the near winding machine, the cooling water for the pinch rolls of the remote winding machine is turned on normally.

[0014] The far-end coiler is the coiler located far from the last stand of the finishing mill, while the near-end coiler is the coiler located close to the last stand of the finishing mill.

[0015] There are three coilers, named No. 1, No. 2, and No. 3 according to their distance from the last stand of the finishing mill. When No. 1 coiler is in use, the pinch roll cooling water of No. 2 and No. 3 coilers is turned on normally. When No. 2 coiler is in use, the pinch roll cooling water of No. 1 coiler is turned on after a delay, while the pinch roll cooling water of No. 3 coiler is turned on normally. When No. 3 coiler is in use, the pinch roll cooling water of No. 1 and No. 2 coilers is turned on after a delay.

[0016] The length of the strip head Lh is ≤120m and the strip thickness is ≤8.0mm.

[0017] The winding temperature setting CT is 550℃-700℃.

[0018] The beneficial effects of this invention are: by dynamically controlling the opening and closing sequence of the cooling water of the coiler pinch roll, the strip head is in a relatively stable cooling environment, reducing the interference of the cooling water, ensuring uniform temperature drop and phase transformation in the width direction of the strip head, reducing the residual stress of the strip, maintaining a good strip shape at the strip head, and reducing the occurrence of head warping. Attached Figure Description

[0019] Figure 1 This is a process layout diagram for the present invention;

[0020] In the diagram: 1. Finishing mill last stand; 2. Layer cooling system; 3. No. 1 coiler; 4. No. 2 coiler; 5. No. 3 coiler. Detailed Implementation

[0021] See attached document Figure 1 A method for reducing head warping of cold-rolled duplex steel base material involves dynamically controlling the opening and closing sequence of the cooling water on the coiler pinch rolls to reduce head warping. The specific steps are as follows:

[0022] Step 1: When rolling cold-rolled duplex steel base material, start the cooling water delay program before the pinch rolls bite the steel;

[0023] Step 2: After the pinch rolls bite the steel, calculate the strip speed using the pinch roll diameter and rotation speed, and integrate the strip speed to obtain the cumulative contact length Lacc between the strip and the pinch rolls.

[0024] Step 3: After the pinch rolls bite the strip, simultaneously calculate the cumulative contact time Tacc between the strip and the pinch rolls, and calculate the upper limit of the cooling water delay time Th for the pinch rolls based on the coiling temperature setting.

[0025] Th = (1700 - 2 × CT) / 15, where: CT: winding temperature setpoint;

[0026] Step 4: When either Lacc≥Lh or Tacc≥Th is satisfied, turn on the cooling water of the pinch rolls, where Lh is the length of the strip head, that is, the length of the strip that causes the bend.

[0027] In this embodiment, refer to the appendix Figure 1 There are three coilers, named No. 1, No. 2, and No. 3 coilers in order of distance from the last stand of the finishing mill. When No. 1 coiler is in use, the pinch roll cooling water of No. 2 and No. 3 coilers is turned on normally. When No. 2 coiler is in use, the pinch roll cooling water of No. 1 coiler is turned on after a delay, while the pinch roll cooling water of No. 3 coiler is turned on normally. When No. 3 coiler is in use, the pinch roll cooling water of No. 1 and No. 2 coilers is turned on after a delay.

[0028] The strip head length Lh is the length at which the strip is prone to warping; Lh ≤ 120m, and the strip thickness ≤ 8.0mm. The cooling water delay time Th varies with the target coiling temperature (CT), which ranges from 550℃ to 700℃, and the maximum value of Th generally does not exceed 40 seconds.

[0029] Five strip steel coils were produced according to the above implementation method, as shown in Table 1. Among them, the first condition is to calculate the upper limit time Th of the cooling water delay of the pinch roll based on the different coiling set temperatures, and to determine whether the cumulative time meets the upper limit time Th; the second condition is to calculate the cumulative length of the strip head in contact with the pinch roll based on the actual speed, and to determine whether it meets the upper limit length Lh. The actual speed is not constant, but is dynamically adjusted according to the set temperature of the finishing mill exit.

[0030] The cooling water status of the pinch rolls refers to the delayed or cooled status of the cooling water for the respective pinch rolls of the three coilers on the line. Specifically, for strips 1-4, the cooling water is turned on after condition two (cumulative contact length) is met first, while for strip 5, the cooling water is turned on after condition one (cumulative contact time) is met first.

[0031] .

Claims

1. A method for reducing head warping in cold-rolled duplex steel base material, characterized in that: By dynamically controlling the opening and closing sequence of the cooling water on the coiler pinch rolls, the head warping of cold-rolled duplex steel base material can be reduced. The specific steps are as follows: Step 1: When rolling cold-rolled duplex steel base material, start the cooling water delay program before the pinch rolls bite the steel; Step 2: After the pinch rolls bite the steel, calculate the strip speed using the pinch roll diameter and rotation speed, and integrate the strip speed to obtain the cumulative contact length Lacc between the strip and the pinch rolls. Step 3: After the pinch rolls bite the strip, simultaneously calculate the cumulative contact time Tacc between the strip and the pinch rolls, and calculate the upper limit of the cooling water delay time Th for the pinch rolls based on the coiling temperature setting. Th = (1700 - 2 × CT) / 15, where: CT: winding temperature setpoint; Step 4: When either Lacc≥Lh or Tacc≥Th is satisfied, turn on the cooling water of the pinch rolls, where Lh is the length of the strip head, i.e. the length of the strip that causes the bend. If there are multiple winders, when using the remote winder, the cooling water for the pinch rolls of the near winder should be turned on after a delay until the cooling water for the pinch rolls of the remote winder is turned on; when using the near winder, the cooling water for the pinch rolls of the remote winder is turned on normally. The length of the strip head Lh is ≤120m, and the strip thickness is ≤8.0mm; The winding temperature setting CT is 550℃-700℃.

2. The method for reducing head warping of cold-rolled duplex steel base material according to claim 1, characterized in that: The far-end coiler is the coiler located far from the last stand of the finishing mill, while the near-end coiler is the coiler located close to the last stand of the finishing mill.

3. The method for reducing head warping of cold-rolled duplex steel base material according to claim 2, characterized in that: There are three coilers, named No. 1, No. 2, and No. 3 according to their distance from the last stand of the finishing mill. When No. 1 coiler is in use, the pinch roll cooling water of No. 2 and No. 3 coilers is turned on normally. When No. 2 coiler is in use, the pinch roll cooling water of No. 1 coiler is turned on after a delay, while the pinch roll cooling water of No. 3 coiler is turned on normally. When No. 3 coiler is in use, the pinch roll cooling water of No. 1 and No. 2 coilers is turned on after a delay.

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