A thermal straightening method for wavy defects in 355MPa grade steel plates

Abstract

This invention discloses a hot straightening method for wavy defects in 355MPa grade steel plates, belonging to the field of metal materials technology. The method employs an overall tilt adjustment of the upper row of straightening rollers on the straightening machine to determine the relative reduction of each roller; to determine the maximum plastic deformation rate and its corresponding roller number; and to determine a suitable plastic deformation rate and straightening parameter configuration scheme. Through calculation, the plastic deformation rate applied to the steel plate by each straightening roller is obtained, and the point of maximum deformation is taken as the evaluation index of the plate's straightening plastic deformation rate. The tilting value is adjusted to control the plate's straightening plastic deformation rate between 50% and 80%. This invention, starting from the hot straightening machine equipment, sets reasonable reduction and tilting amounts based on the steel plate size specifications, and sets a bending roller scheme according to the plate shape after final cooling. For medium-wavy plates, a negative bending roller process is applied, thereby effectively improving the hot straightening machine's ability to solve wavy problems, enhancing plate shape control, and reducing the probability of plate shape defects after hot straightening.

Description

Technical Field

[0001] This invention belongs to the field of metallic materials technology, and specifically relates to a hot straightening method for wavy defects in 355MPa grade steel plates. A nine-roll straightener is used, and straightening is performed by adjusting the overall tilt of only the upper row of straightening rolls. Background Technology

[0002] In the rolling process of medium and heavy plates, no tension is applied, and plate shape problems can be detected after finishing rolling. The strength and specifications of the plate have a significant impact on plate shape control. 355MPa grade steel plates are high-strength steels, and plate shape control is difficult and production volume is large. It is necessary to maintain stable quality control under high production efficiency, which requires high process stability and precision. As the width-to-thickness ratio increases, steel plates are more prone to waviness problems. Therefore, a hot straightening process is required to straighten the plate before it enters the cooling bed for cooling.

[0003] In practice, there is no clear configuration scheme for the coordinated use of straightening roller tilting, pressing, and bending roller amounts in hot straightening machines. For severely wavily shaped sheets, the straightening effect can be improved by increasing the pressing or bending roller amounts. However, excessive pressing can cause over-straightening of the sheet, resulting in significant residual stress after straightening, which can lead to sheet shape defects during cooling. At the same time, excessive bending roller amounts may also prevent the transverse fibers of the sheet from being sufficiently straightened. Improper coordination between bending roller amounts and pressing amounts can also lead to excessive plastic deformation of the sheet during straightening, failing to achieve the desired straightening effect. Summary of the Invention

[0004] The purpose of this invention is to provide a hot straightening method for wavy defects in steel plates with a pressure of 355MPa, so as to solve the problems in the prior art.

[0005] This invention is achieved through the following technical solution: a hot straightening method for wavy defects in 355MPa grade steel plates, characterized by: employing an overall tilt adjustment method for the upper row of straightening rollers on the straightening machine, including the following steps:

[0006] Step 1: Determine the relative reduction of each straightening roller:

[0007] First, determine the reduction amount of the inlet and outlet straightening rollers, and then calculate the relative reduction amount of each straightening roller according to formula (1):

[0008] Δh i =|(h i-1 +h i+1 )×0.5-h i | (1)

[0009] In the formula, Δh i The relative reduction of each roller is expressed in meters (m).

[0010] hi The straightening roller reduction is m; i = 1, 2, 3, ..., n;

[0011] Step 2: Determine the maximum plastic deformation rate and the corresponding roll number:

[0012] Given the relative reduction of each straightening roller, calculate the curvature of the sheet at each roller during the straightening process according to equation (2):

[0013] A i =16Δh / (l 2 +Δh i 2 (2)

[0014] In the formula, A i Let m be the curvature of the steel plate at each roller. -1 ;

[0015] l is the distance between the straightening rollers, in meters;

[0016] Based on the obtained bending curvature at each roller, the plastic deformation rate of the sheet at each roller is calculated according to equation (3):

[0017] a i =(A i -A t ) / A i ×100%=(1-2q i / EHA i (3) × 100%

[0018] In the formula: a i Let A be the plastic deformation rate of the sheet at each roller. i t At that time, no plastic deformation occurred in the sheet material, and the plastic deformation rate was 0; A t Let m be the elastic limit curvature of the sheet metal. -1 ;q i E is the yield strength, Pa; E is the elastic modulus, Pa; H is the steel plate thickness, m;

[0019] Step 3: Determine the appropriate plastic deformation rate and straightening parameter configuration scheme:

[0020] Through the calculation in step 2, the plastic deformation rate applied to the steel plate by each straightening roller is obtained. The point with the maximum deformation is taken as the evaluation index of the plastic deformation rate of the plate. The tilting value is adjusted to control the plastic deformation rate of the plate between 50% and 80%.

[0021] Furthermore: the straightening machine has two rows of straightening rollers, with the lower row of straightening rollers fixed in place.

[0022] ​Further: The straightening machine has nine groups of straightening rolls. There are four groups in the upper row, namely the No. 2 straightening roll, No. 4 straightening roll, No. 6 straightening roll, and No. 8 straightening roll; there are five groups in the lower row, namely the No. 1 straightening roll, No. 3 straightening roll, No. 5 straightening roll, No. 7 straightening roll, and No. 9 straightening roll. Among them, the No. 2 straightening roll is the inlet straightening roll, and the No. 8 straightening roll is the outlet straightening roll.

[0023] Further: The reduction of the outlet straightening roll is 0.5 - 1 mm.

[0024] Further: The amount of bending roll is determined according to the waviness of the steel plate. Negative bending roll process is used for middle wave, and positive bending roll process is used for edge wave.

[0025] Further: When 8 < H ≤ 10, the amount of bending roll is -0.4 ~ -1.5 mm; when 10 < H ≤ 20, the amount of bending roll is -0.4 ~ -1.0 mm, where H is the thickness of the steel plate, in mm.

[0026] Further: When the maximum plastic deformation rate is lower than 50%, increase the tilting value; when the maximum plastic deformation rate is higher than 80%, decrease the tilting value.

[0027] Further: In step 3, adjust the tilting value to control the plastic deformation rate of the sheet straightening between 50% and 80%.

[0028] The advantages of the present invention are as follows: Starting from the equipment situation of the hot straightening machine, reasonable reduction and tilting amount are set in combination with the size specifications of the steel plate, and the bending roll scheme is set according to the shape of the plate after final cooling. Negative bending roll process is applied to the middle wave sheet, thereby effectively improving the ability of the hot straightening machine to solve the waviness problem, enhancing the shape control effect, and reducing the probability of shape defects after hot straightening. Brief Description of the Drawings

[0029] Figure 1 It is a schematic diagram of the straightening machine.

[0030] Figure 2 It is the layout diagram of the straightening machine.

[0031] Figure 3 It is the comparison diagram of the hot straightening effect. Detailed Embodiment

[0032] The present invention provides a hot straightening method for the waviness defect of 355 MPa grade steel plates, including the following steps.

[0033] Step 1: Determine the relative reduction of each straightening roll:

[0034] For the reduction adjustment scheme of the fully hydraulic roll straightening machine, the upper row of straightening rolls is adjusted as a whole by tilting, as Figure 1 shown in the nine-roll straightening machine; where 1 is the upper row of straightening rolls, 2 is the steel plate, and 3 is the lower row of straightening rolls.

[0035] According to commonly used straightening strategies, the pressing amount setting process is as follows: Determine the pressing amount of the inlet and outlet straightening rollers. The inlet straightening roller is straightening roller No. 2, represented by the symbol h2, and the outlet straightening roller is straightening roller No. 8, represented by the symbol h8.

[0036] After determining h2 and h8, the reduction of the remaining straightening rollers No. 4 and No. 6 in the upper row can be calculated sequentially according to the principle of linear distribution.

[0037] For the lower row of straightening rollers 1, 3, 5, 7, and 9, they are fixed in place and no reduction is provided.

[0038] Considering the positional relationship between each straightening roll and the front and rear rolls, the relative reduction of each straightening roll is obtained by equation (1): Δh i =|(h i-1 +h i+1 )×0.5-h i | (1)

[0039] In the formula, Δh i The relative reduction of each roller is expressed in meters (m).

[0040] h i Let m be the amount of pressure applied by the straightening roller; i = 1, 2, 3, ..., n.

[0041] Step 2: Determine the maximum plastic deformation rate and the corresponding roll number:

[0042] Given the relative reduction of each straightening roller, the bending curvature of the sheet at each roller during the straightening process can be further obtained by analyzing the relationship between the relative reduction, deflection, and curvature. The specific formula is shown in equation (2).

[0043] Ai=16Δh / (l 2 +Δh i 2 (2)

[0044] Based on the obtained bending curvature at each roller, the plastic deformation rate of the sheet at each roller can be calculated, which can be expressed by equation (3):

[0045] a i =(A i -A t ) / A i ×100%=(1-2q i / EHA i (3) × 100%

[0046] In the formula: a i Let A be the plastic deformation rate of the sheet at each roller. i t ​At that time, no plastic deformation occurred in the sheet material, and the plastic deformation rate was 0; A t Let m be the elastic limit curvature of the sheet metal. -1 ;q i The yield strength is given in Pa. E H is the elastic modulus, Pa; H is the steel plate thickness, m;

[0047] Based on this type of straightening machine, the plastic deformation rate of the sheet material at each straightening roller is compared to determine the roller number where the maximum plastic deformation rate is located.

[0048] Step 3: Determine the appropriate plastic deformation rate and straightening parameter configuration scheme:

[0049] The second step of the calculation can obtain the plastic deformation rate applied to the steel plate by each straightening roller. Since the upper roller adopts a linear decompression strategy, the degree of plastic deformation applied by each straightening roller is different. Usually, the point with the largest degree of deformation is taken as the evaluation index of the plastic deformation rate of the plate. The adjustment of the straightening process parameters should be made to control the maximum plastic deformation rate between 50% and 80%.

[0050] For a linear decreasing straightening strategy, the adjustable reduction process parameters mainly include two parts: the tail roll reduction and the tilting amount, where the tilting amount is the difference between the head and tail roll reductions. During the straightening process, the bending amount generally decreases from large to small, that is, the reduction of each working roll gradually decreases from the inlet to the outlet. Through repeated bending, the steel plate successively achieves plastic deformation and elastic deformation. Therefore, at the outlet, the main task is to flatten the plate, and the tail roll reduction is less than 1 mm.

[0051] Given a fixed tail roll reduction, the head roll reduction is adjusted by changing the tilting amount, thereby altering the maximum plastic deformation rate during steel plate straightening. When the maximum plastic deformation rate is below 50%, the tilting value is increased; when the maximum plastic deformation rate is above 80%, the tilting value is decreased, ensuring the maximum plastic deformation rate during steel plate straightening reaches the recommended range of 50%~80%, thus obtaining a suitable tilting amount.

[0052] The amount of bending rolls is usually determined based on the wavy defects of the steel plate before straightening. For wavy defects in the middle, a negative bending roll straightening process is used, while for wavy defects in the edge, a positive bending roll straightening process is used. The more severe the wavy defects, the greater the amount of bending rolls.

[0053] The following examples further illustrate this solution.

[0054] Example: The technical solution of the present invention will be further explained in conjunction with the equipment parameters and specific implementation of a nine-roll hot straightening machine. First, according to the plate specifications and the current hot straightening process parameters, the relative reduction of each straightening roll is determined by formula (1); Second, given the relative reduction, the straightening roll number applied to the maximum bending curvature and maximum plastic deformation rate under the parameters is determined by formulas (2) to (3); Third, it is determined whether the maximum plastic deformation rate under the current parameters is appropriate. If it is less than 50%, the tilting amount needs to be increased; if it is higher than 80%, the tilting amount needs to be reduced to obtain a suitable tilting amount. Combined with the plate shape after hot straightening and cooling, the bending amount of the roll is determined to form a hot straightening process configuration scheme for this machine type and plate specifications.

[0055] (1) Determine the relative reduction of each straightening roller:

[0056] The layout of a certain nine-roll cold straightener is as follows: Figure 2 As shown. The pressure adjustment strategy is a linear decreasing pressure bending scheme for the upper row of straightening rollers. Here, a 355MPa grade steel plate with a thickness of 16mm is taken, and the process parameters are as follows: the first roller pressure h2=2.5mm, the tail roller pressure h8=0.5mm, and the lower roller pressure h=0. Since the upper row of straightening rollers adopts a linear decreasing arrangement, we can obtain h4=1.83mm and h6=1.17mm; according to formula (1), the relative pressure of each roller is:

[0057] Δh2=|(h1+h3)×0.5-h2|=|(0+0)×0.5-2.5|=2.5mm;

[0058] Δh3=|(h2+h4)×0.5-h3|=|(2.5+1.83)×0.5-0|=2.165mm;

[0059] Δh4=|(h3+h5)×0.5-h4|=|(0+0)×0.5-1.83|=1.83mm;

[0060] Δh5=|(h4+h6)×0.5-h5|=|(1.83+1.17)×0.5-0|=1.5mm;

[0061] Δh6=|(h5+h7)×0.5-h6|=|(0+0)×0.5-1.17|=1.17mm;

[0062] Δh7=|(h6+h8)×0.5-h7|=|(1.17+0)×0.5-0|=0.585mm;

[0063] Δh8=|(h7+h9)×0.5-h2|=|(0+0)×0.5-0.5|=0.5mm;

[0064] (2) Determine the maximum plastic deformation rate and the roller number at which it occurs:

[0065] Given the relative reduction of each straightening roller, refer to equations (2) to (3) to calculate the location where the maximum bending curvature and plastic deformation rate occur during the straightening process of the plate.

[0066] Calculations show that a1 = 20.60%; a2 = 42.54%; a3 = 32.02%; a4 = 42.54%; a5 = 17.06%.

[0067] a8 <a7<a6<0。

[0068] By calculating the plastic deformation rate, it can be seen that the straightener of this type of equipment experiences the maximum plastic deformation at the No. 3 straightening roller.

[0069] (3) Determine a suitable plastic deformation rate and straightening parameter configuration scheme:

[0070] The calculation in the second step shows that the maximum plastic deformation occurs at the No. 3 straightening roller during the straightening process. The tilting amount is then adjusted to make the plastic deformation rate of the steel plate at the No. 3 roller between 50% and 80%. If the plastic deformation rate is lower than 50%, the tilting amount is increased; if the plastic deformation rate is higher than 80%, the tilting amount is decreased.

[0071] Table 1. Solution for suitable tilting amount

[0072]

[0073] Since the steel plate temperature is generally above 600℃ during the straightening process, it is considered hot straightening. Combined with the appearance of mid-wave defects in the steel plate after final cooling, negative bending rollers are used. Taking a 355MPa (yield strength at room temperature) grade steel plate as an example, the configuration scheme for the hot straightening process parameters for mid-wave defects after final cooling is as follows.

[0074] Table 2. Configuration Scheme for Hot Straightening Process Parameters for Final Cooling Defects in 355MPa Grade Steel Plates

[0075]

[0076] This approach was used to configure the hot straightening parameters. The rolled 355 MPa grade ship plates were straightened, resulting in good overall plate shape and a reduced probability of shape defects after hot straightening.

[0077] Table 3. Examples of hot straightening process parameters for 355MPa grade steel plates at room temperature

[0078]

[0079] After using the above-mentioned hot straightening process parameter configuration scheme, the plate straightening capability of the hot straightening machine for 355MPa grade steel plates can be significantly improved, and the probability of plate shape defects after hot straightening can be reduced. The proportion of wavy defects (cold straightening rate) after final cooling of steel plates in this thickness range is lower than the initial target. The initial cold straightening rate was 24.8%, and the cold straightening rates of the two batches after the test decreased to 14.3% and 10.3%, respectively. Figure 3 As shown, the wave shape control effect is obvious.

[0080] To verify the effectiveness of this scheme, calculations were performed according to the scheme, and the parameters in Table 4 were applied. The original parameters (original cold straightening rate) were used for one batch of testing, while the parameters corresponding to this scheme were used for two batches of testing (referred to as Experiment 1 and Experiment 2). Each batch of production covered steel plates of different thickness ranges, with a total of no less than 300 sheets per batch. After the tests, the proportion of waviness defects occurring during final cooling (cold straightening rate) was lower than the initial target. The initial cold straightening rate was 24.8%, and after the tests, the cold straightening rates decreased to 14.3% and 10.3%, respectively. Figure 3 As shown, the wave shape control effect is obvious.

[0081] Table 4. Test parameters for hot straightening process of 355MPa grade steel plates

[0082]

[0083] In summary, since existing hot straighteners do not provide a clear configuration scheme for the coordinated use of straightening roller tilting, pressing, and bending rollers, this invention provides a method for configuring straightening process parameters, which can improve the straightening rate of the sheet material to a certain extent, improve the sheet shape after straightening, reduce the cold straightening rate, improve on-site production efficiency, and reduce production costs.

Claims

1. A hot straightening method for a 355 MPa grade steel sheet wave defect, characterized in that: Adopting the overall inclination adjustment mode of the straightening roll in the upper row of the straightening machine, comprising the following steps: Step 1: determining the relative reduction of each straightening roll: First, the reduction of the inlet and outlet straightening rolls is determined, and then the relative reduction of each straightening roll is calculated according to formula (1): Δh i = | (h i-1 +h i+1 ) x 0.5 - h i | (1) where Δh i is the relative reduction of each roll, m; h i is the reduction of the straightening roll; i = 1, 2, 3,..., n; Step 2: determining the maximum plastic deformation rate and the roll number where it occurs: In the case of knowing the relative reduction of each straightening roll, the bending curvature of the plate at each roll in the straightening process is calculated according to formula (2): A i = 16Δh / (l 2 + Δh i 2 ) (2) In the formula, A i is the bending curvature of the steel plate at each roller, m -1 ; l is the roll spacing of the straightening roll, m; On the basis of obtaining the bending curvature at each roll, the plastic deformation rate of the plate at each roll is calculated according to formula (3): a i = (A i -A t / A i ) x 100% = (1 - 2q i / EHA i ) x 100% (3) wherein: a i is the plastic deformation rate of the sheet at each roll, when A i <A t , the sheet does not undergo plastic deformation and the plastic deformation rate is 0; A t is the elastic limit curvature of the sheet, m -1 ; q i is the yield strength, Pa; E is the elastic modulus, Pa; H is the thickness of the steel sheet, m; Step 3: determining the appropriate plastic deformation rate and the straightening parameter configuration scheme: Through the calculation of the second step, the plastic deformation rate of each straightening roll on the steel plate can be obtained, and the maximum deformation degree is taken as the evaluation index of the plate straightening plastic deformation rate, and the inclination value is adjusted to control the plate straightening plastic deformation rate between 50% and 80%; When 8<H≤10, the bending amount is-0.4~-1.5mm; when 10<H≤20, the bending amount is-0.4~-1.0mm, wherein H is the thickness of the steel plate, mm; When the maximum plastic deformation rate is less than 50%, increase the inclination value, and when the maximum plastic deformation rate is higher than 80%, reduce the inclination value.

2. The method for hot straightening of the wave defect of the 355 MPa grade steel plate according to claim 1, characterized in that: The straightening machine has two rows of straightening rolls, and the lower row of straightening rolls is fixed.

3. The method for hot straightening of the wave defect of the 355 MPa grade steel plate according to claim 1, characterized in that: The straightening machine has nine groups of straightening rolls, and the upper row has four groups of 2, 4, 6 and 8 straightening rolls, respectively; the lower row has five groups of 1, 3, 5, 7 and 9 straightening rolls, respectively; wherein the 2 straightening roll is the inlet straightening roll, and the 8 straightening roll is the outlet straightening roll.

4. The method for hot straightening of the wave defect of the 355 MPa grade steel plate according to claim 1, characterized in that: The reduction of the outlet straightening roll is 0.5-1mm.

5. The method for hot straightening of the wave defect of the 355 MPa grade steel plate according to claim 1, characterized in that: The bending amount is determined according to the wave shape of the steel plate, and the negative bending roll process is adopted for the middle wave and the positive bending roll process is adopted for the side wave.

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