JP2807194B2 - Hot rolled steel sheet manufacturing method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] The present invention relates to a method for producing a hot-rolled steel sheet.

[0002]

2. Description of the Related Art Generally, a hot-rolled steel sheet is formed into a desired steel sheet size by hot rolling, and then is subjected to a water-cooled TMCP (Thermo Mesh).
(chanical Control Process) The steel sheet is subjected to accelerated cooling and then manufactured through online hot leveler straightening. At that time, from the viewpoint of the shape of the steel sheet, if it is flat after hot straightening, it is shipped as it is, and if the shape defect remains, it is shipped after being corrected and flattened by a roller leveler etc. in the cold It is common.

[0003] However, when the steel plate is flat at the time of shipment, a defective shape such as a warp or a wave may occur when the placement of the steel plate is changed thereafter. It is known that this is a phenomenon caused by buckling deformation due to residual stress inside the steel sheet. In other words, the steel plate whose residual stress level is near the critical buckling stress was apparently flat on the table roller due to the weight of the steel plate and the restraint by the table roller. Buckling deformation occurs due to a change in the stress state, and a shape defect occurs.

[0004] In particular, in an accelerated cooling steel sheet, uneven residual stress is likely to be generated inside the steel sheet due to a temperature deviation due to non-uniform cooling in the sheet surface during accelerated cooling. In many cases, the residual stress state is near the critical buckling stress, and the buckling deformation as described above is likely to occur. In order to prevent such buckling deformation, it is necessary to identify a steel sheet in which the residual stress exceeds the critical buckling stress even when apparently flat, that is, a steel sheet having a so-called unstable shape, in shipping judgment. is there.

Conventionally, in order to identify such a steel sheet having an unstable shape, a flatness is visually checked by using a square bar or a state of being suspended by a crane.

[0006]

However, in the above-described visual check, there are cases where the shape changes depending on, for example, how to place it on a square bar, and cases where the shape does not change. It is difficult to do. Further, in the shape check when suspended by a crane, a droop due to its own weight occurs, so that a wave due to buckling cannot be identified. Therefore, such a method has a problem that it is difficult to perform reliable identification. In addition, in these methods, it is necessary to install a steel plate on a square bar or to lift the steel plate with a crane at the time of shipment determination, so that there is a problem that productivity is reduced.

[0007] On the other hand, it is known that when correction is performed by a roller leveler in a cold state, the residual stress can be reduced depending on the correction conditions. Therefore, it is conceivable that a steel sheet having an unstable shape as described above is corrected by a roller leveler in a cold state, thereby reducing its residual stress before shipping. However, it is unclear how much the residual stress should be reduced by the straightening at this time so as not to be lower than the critical buckling stress so that buckling does not occur. Therefore, it is not possible to set appropriate straightening processing conditions. As a result, in actual production of a steel sheet, it is difficult to identify a steel sheet having an unstable shape and prevent buckling from occurring.

SUMMARY OF THE INVENTION The present invention has been made in view of the above-mentioned conventional problems, and is intended to provide a hot-rolled steel sheet which can prevent buckling due to residual stress of the steel sheet without lowering productivity. It is intended to provide a manufacturing method.

[0009]

In order to achieve the above object, a method for producing a hot-rolled steel sheet according to claim 1 of the present invention comprises:
Provide a temperature measuring means for measuring the surface temperature distribution of the steel sheet at the outlet side of the accelerated cooling equipment or hot straightening machine, and a cold leveler for straightening the steel sheet to control the residual stress of the steel sheet. A first step of calculating the residual stress of the steel sheet based on the sheet surface temperature distribution, a second step of determining whether the steel sheet buckles based on the residual stress, and for the steel sheet determined to buckle, A third step of calculating the residual stress after correction under each processing condition in the cold leveler and determining whether or not the steel plate buckles based on each residual stress, and processing in which buckling of the steel plate does not occur. And setting a condition to a cold leveler to perform a straightening process on the steel sheet.

As described above, according to the above-described method, the residual stress generated after air cooling substantially coincides with the thermal stress caused by uneven temperature at the exit of the hot straightening machine at the time of stopping the accelerated cooling or following the accelerated cooling. Therefore, first, the sheet surface temperature distribution is measured, and from the result, the residual stress generated in the steel sheet after air cooling is calculated. And, based on the above residual stress, for example,
By comparison with the buckling critical stress calculated as described below,
Determine whether buckling occurs, and further, for steel sheets that are determined to cause buckling, compare the leveler processing conditions where buckling of the steel sheet does not occur, for example, with the critical buckling stress as described above. Is applied to reduce the residual stress.

Therefore, the steel sheet manufactured through such steps is shipped with the residual stress being smaller than the buckling critical stress, so that the steel sheet caused by the residual stress is not affected even if the method is changed thereafter. Buckling does not occur, and the occurrence of shape defects can be reliably prevented. In addition, according to the above method, it is not necessary to perform a special operation for determining flatness such as hanging a crane only by measuring the plate surface temperature distribution, so that productivity can be improved.

In the method for producing a hot-rolled steel sheet according to a second aspect, when the buckling is determined in the second step, the residual stress distribution calculated in the first step is defined as σ _act (x, y). the average surface residual stress is divided into the a region Omega ₁ and the other region Omega ₂ and compression, area in the seat屈臨field state Omega ₁
The average stress σ _cr ^(-) at

[0013]

(Equation 2)

Here, σ ₀ is a critical buckling stress when the residual stress distribution is approximated by a rectangle based on a constant determined according to the steel sheet size, buckling mode, and how to divide the region. F (x _i, y _j ): steel sheet size, buckling mode, the function depends on the dividing way of the region ^{_{Δσ act (-) (x i}} , y j) = σ act -σ act (-) Δσ act (+) (x i, y j) = σ act - ^{_{σ act (+) σ act (}} -): average sigma _act in the region Ω ₁ σ _act ^(+): average sigma _act in the region Ω ₂ N _1: point region _{Ω 1 (x i, y j} ) whole N ₂ : Calculated from the set of all points (x _i, y _j ) in the region Ω ₂ , and it is determined that buckling occurs when σ _act ⁽⁻⁾ / σ _cr ⁽⁻⁾ ≧ 1 holds. When the condition is not satisfied, it is determined that buckling does not occur.

That is, the reason why the above-mentioned shape unstable state occurs is that, despite the residual stress exceeding the buckling critical stress, the deformation of the steel sheet is restrained by the influence of its own weight. Therefore, by predicting the critical buckling stress, by determining whether the residual stress exceeds the critical buckling stress,
Even if it is flat, it is possible to identify a steel sheet having an unstable shape.

[0016] Therefore, divided into a region Omega ₁ and other areas Omega ₂ the average of the residual plate surface stress is compressive, the average stress in the region Omega ₁ in seat屈臨field state σ _cr ^(-) a seat When calculating the critical stress as a buckling critical stress, as shown in the above equation (a), the residual stress distribution is further considered to the buckling critical stress σ ₀ corresponding to the rectangular approximation of the residual stress distribution ((a ) ⁾ , The calculation of σ _cr ⁽⁻⁾ more accurately corresponding to the actual residual stress distribution becomes possible. Thereby, highly accurate buckling determination can be easily performed.

In the method of manufacturing a hot-rolled steel sheet according to a third aspect, when determining buckling in the third step, the corrected residual stress distribution under each processing condition in the cold leveler is calculated, and the sheet is calculated. The average σ _act ^{af (-)} of the residual stress in the region Ω ₁ ′ where the residual stress after correction on the surface is compressed and the average σ _cr ^{af (-)} of the stress in the region Ω ₁ ′ in the buckling critical state. Calculated, it is determined that buckling occurs when σ _act ^{af (−)} / σ _cr ^{af (−)} ≧ 1 holds, and it is determined that buckling does not occur when it does not hold.

That is, buckling occurs in the second step.
Then, for the steel sheet determined to be
We perform straightening processing to reduce, but this leveler processing condition
When setting the parameters, firstly,
Based on the degree of residual stress reduction
Calculate the force distribution. And the residual stress distribution after this correction
Buckling critical stress σ based on _cr ^{af (-)}Is calculated
Out, and after making a correction by comparing
It is determined whether buckling occurs. Based on this judgment result
And no buckling occurs in the steel plate in the fourth step.
The conditions are set for cold levelers and the steel plate is straightened.
Will be.

Therefore, in the above description, it is possible to set an appropriate leveler condition that does not cause buckling simply by obtaining the degree of reduction of the residual stress for each leveling condition.

[0020]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments of the present invention will be described below with reference to a TMCP (Thermo Mechanical Control Process) steel plate as an example. TMCP steel sheet is manufactured by accelerated cooling while maintaining its shape after controlled rolling, but the cooling rate during water cooling is greatly affected by rolling conditions and surface properties, etc. Tends to be. Then, in the steel sheet when the accelerated cooling is stopped, the stress caused by the non-uniform cooling in the water cooling process during the accelerated cooling is superimposed on the stress generated in the heating / rolling process, and the stress is applied to the accelerated cooling. Reduced by subsequent hot straightening. Further, in the subsequent air cooling process, thermal stress due to temperature non-uniformity at the time of stopping the accelerated cooling is applied, and a residual stress after the air cooling is formed.

Incidentally, the residual stress (average value in the thickness direction) of the accelerated cooling steel sheet substantially coincides with the thermal stress caused by the temperature non-uniformity when the acceleration cooling is stopped or after the hot leveler following the acceleration cooling (Kobe Steel Technical Report) Vol.41, No.4; P52-P55).
Therefore, the residual stress can be evaluated by the temperature distribution at the time of stopping the accelerated cooling or after the hot leveler following the accelerated cooling.

Therefore, in this embodiment, a sheet surface temperature profiler for measuring the sheet surface temperature distribution of the steel sheet is installed as a temperature measuring means on the exit side of the accelerated cooling equipment or the hot straightening machine, and this sheet surface temperature probe is provided. The surface temperature distribution of the steel sheet is measured by a feeler meter. Then, from the measurement result, a residual stress distribution σ _act generated in the steel sheet after air cooling is calculated. Note that a method for calculating such residual stress due to thermal stress is disclosed, for example, in Japanese Patent Publication No. 4-8128.

Next, the residual stress distribution .sigma.
_{From the act} , it is determined whether a buckling wave is generated when the way of placing the steel plate is changed or the like. This is possible by determining whether the residual stress of the steel sheet exceeds the critical buckling stress. The buckling critical stress changes according to the distribution pattern of the residual stress σ _act (x, y), as described later. Therefore, in the present embodiment, a simple prediction formula of buckling critical stress corresponding to the distribution pattern of the residual stress σ _act (x, y) is constructed,
The buckling is determined by comparing the result of the calculation with the prediction formula. Hereinafter, the process of deriving such a prediction formula will be described.

First, as shown in FIG. 4, for a steel plate having a plate length L and a plate width b, a coordinate system in which the plate length direction is x and the width direction is y is set. According to the buckling theory, σ _act (x, y) acting on the central plane in the thickness direction of the steel sheet when the bending w occurs
The work ΔT and the bending strain energy ΔV are

[0025]

(Equation 3)

Where E: Young's modulus, Δ: Poisson's ratio,
t: Expressed as a plate thickness. Here, if ΔT is larger than the bending strain energy ΔV in an arbitrary bent shape, buckling deformation occurs. That is, buckling deformation occurs when ΔT / ΔV ≧ 1.

Prediction of critical buckling stress based on the above concept
In constructing the equation, first, the area of the plate surface is
_actThe average of (x, y) is the compression area Ω₁And other areas Ω
_TwoAnd split into Then, the residual stress component in the buckling critical state
Cloth σ_cr(x, y) = ησ_act(x, y)
And the area Ω₁Σ at_crbuckling critical stress σ _cr
^(-)And At the critical point of buckling,

[0028]

(Equation 4)

Here, Δσ _act ⁽⁻⁾ = σ _act −σ _act ⁽⁻⁾ Δσ _act ⁽⁺⁾ = σ _act −σ _act ⁽⁺⁾ σ _act ⁽⁻⁾ : average of σ _act in the region Ω ₁ σ _act ^{( +)} : Average of σ _act in region Ω ₂ η: An unknown constant representing the ratio between the residual stress of the steel sheet and the critical buckling residual stress is established. Therefore, the following equation (4) is obtained from the above equations (1) and (2). Derived.

[0030]

(Equation 5)

[0031] However, sigma _0: steel size, buckling mode, and the seat屈臨field stress further discretization when the residual stress distribution in constant determined in accordance with the divided how region is rectangular approximation, sigma _cr ^- a, ⁽⁾

[0032]

(Equation 6)

However, it is determined by F (x, y) = η (∂w / ∂x) ² . Further, the deflection w is represented by a polynomial expression relating to x and y,

[0034]

(Equation 7)

Then, σ _cr ⁽⁻⁾ becomes

[0036]

(Equation 8)

[0037] d _i, e _n: unknown constants Δx determined by buckling mode _ω, Δy: _x, calculated as the spacing y directions discretization. The unknown constant of the equation (7) can be determined for each buckling mode by comparing with the buckling analysis result by FEM or the like. In the magnitude relationship between σ _cr ⁽⁻⁾ and σ _act ⁽⁻⁾ , σ _{If act} ^(-) ≧ σ _cr ^(-) holds, it is determined that buckling occurs.

The accuracy of the simple buckling prediction formula constructed as described above is shown below. Here, w = (a ₀ + a ₁ x + a ₂ x ² ) (b ₀ + b ₁ y + b ₂ y
² ) However, a ₀ , a ₁ , a ₂ , b ₀ , b ₁ , b ₂ are constants. For steel sheets having various residual stress distributions σ _act (x, y), buckling critical stress was calculated by the above-mentioned prediction formula (7) and FEM analysis. FIG. 5 shows the comparison result. By using the above simple buckling prediction formula, the critical buckling stress of a steel sheet having an arbitrary stress distribution can be predicted with high accuracy.

Conventionally, there has been known a method of calculating a buckling critical stress by approximating a residual stress distribution in a rectangular shape in a compression region and treating the residual stress as uniform compression of a flat plate capable of calculating an analytical solution. In order to verify the accuracy of this simple prediction method, the residual stress distribution pattern was variously changed and compared with the critical buckling stress obtained using FEM. FIG. 6 shows the result. The buckling critical stress σ _cr ^(-) , which is calculated by FEM, changes greatly according to the residual stress pattern in the compression region at the plate edge, whereas the residual stress in the compression region is only approximated by a rectangle. In the simple method described above, the distribution pattern does not reflect the difference and becomes constant, and does not have sufficient accuracy for application to the actual machine. On the other hand, in order to determine buckling with high accuracy, FEM analysis using a large-sized computer is necessary, and it is difficult to apply the method online.

On the other hand, in the present embodiment, when the residual stress distribution is approximated by a rectangle over the entire compression region Ω ₁ and the other intercalation Ω ₂ as shown in the above equation (5), for example. Buckling critical stress σ ₀ corresponding to the following equation, and a correction term according to the residual stress distribution state (the second and third expressions on the right side of the equation (5)).
), The calculation of σ _cr ⁽⁻⁾ more accurately corresponding to the actual residual stress distribution is possible. As a result, the critical buckling stress can be easily and accurately predicted.

By the above method, buckling can be easily determined for any steel plate having any residual stress distribution in the plate surface. The steel sheet determined to have buckled is corrected by a cold roller leveler. At this time, the residual stress σ _act ^af after correction by the roller leveler can be calculated by σ _act ^af = λ _f · σ _act where λ _f : reduction coefficient of residual stress under leveler processing condition Λ _f. For σ _act ^af , as described above, the average of the residual stress after correction on the plate surface is divided into a region Ω ₁ ′ where the average is compressed and the other region Ω ₂ ′, and the buckling critical defined as above. Calculate the stress σ _cr ^{af (-)} .

Then, for each leveler processing condition Λ _f , buckling occurs when σ _act ^{af (−)} / σ _cr ^{af (−)} ≧ 1 is satisfied, and no buckling occurs when not satisfied. Is determined. These determination results, leveler processing conditions buckling does not occur, ^{_{i.e., σ act af (-) /}} σ cr af (-) <1 selects the satisfactory leveler processing conditions lambda _f a, this condition lambda _f
By applying the roller leveler straightening, it is possible to ship as a steel sheet that does not generate buckling waves even if the way of placement is variously changed.

[0043]

EXAMPLE A steel sheet having a size of 20 mm ^t × 3600 mm ^W × 15000 mm ^L , a yield point of 36 kgf / mm ² class, and a temperature distribution at the time of stopping accelerated cooling shown in FIG. 2 was produced by the method of the present invention. Compressive residual stress σ _act predicted after air cooling of steel sheet
^(-) Is, as shown in FIG. 3, 89 N / mm ² , whereas buckling critical stress σ calculated based on the above equation (7)
_cr ^{(- )} is as shown in the figure for each of the first, second and third buckling modes. For this steel sheet, the occurrence of buckling waves below the second mode was predicted (in the first mode). Buckling critical stress σ _cr ^(-) = 55 N / mm ² ).

Based on the result of the prediction, roller leveler correction conditions for preventing generation of buckling waves were obtained. FIG.
FIG. 7 shows the compressive residual stress σ _act ^{af (−)} and the buckling critical stress σ _cr ^{af (−)} after correction under each leveling condition Λ _f (f = 1 to 3). From the results shown in the figure, σ _act ^{af (-)} becomes σ _cr
^The condition was selected to be smaller than ^{af (-)} , and the correction was performed under these conditions. As a result, after straightening, buckling waves did not occur even when the end region of the steel sheet was free.

The present invention is not limited to the above-described embodiments and examples, and various modifications can be made within the scope of the present invention. For example, in the above, in determining the buckling, the buckling critical stress σ _cr ⁽⁻⁾ · σ _cr ^{af (−)} is obtained and compared with these. For example, in the scope of claim 1, for example, Δ calculated based on equations (1) and (2)
The occurrence of buckling may be determined based on the magnitude relationship between T and ΔV.

[0046]

As described above, according to the first aspect of the present invention,
According to the method for manufacturing a hot-rolled steel sheet, it is determined whether or not buckling due to residual stress occurs. Further, for the steel sheet determined to cause buckling, the buckling is performed using the residual stress after straightening. Since the steel sheet is subjected to the straightening process in order to determine the condition under which the bending does not occur, the steel sheet is shipped with the residual stress surely smaller than the critical buckling stress. As a result, buckling due to residual stress does not occur even when the placement is changed thereafter, and the occurrence of a shape defect can be more reliably prevented. Moreover, according to the above-described method, only the plate surface temperature distribution is measured, and there is no need to perform a special operation for determining flatness such as hanging a crane, thereby improving productivity.

According to the manufacturing method of the second aspect, when obtaining the critical buckling stress, the calculation is performed in consideration of the residual stress distribution in addition to the stress obtained by rectangular approximation of the residual stress distribution. The buckling critical stress can be accurately calculated by the residual stress distribution described above, and as a result, buckling determination with high accuracy can be easily performed. According to the manufacturing method of the third aspect, for example, an appropriate leveler condition that does not cause buckling can be set by merely obtaining the degree of reduction of the residual stress in the cold leveler for each leveler processing condition.

[Brief description of the drawings]

FIG. 1 is an explanatory diagram showing calculation results of a residual stress and a buckling critical stress in a steel sheet manufactured according to the method of the present invention before and after correction by a roller leveler.

FIG. 2 is a graph showing a measurement result of a temperature distribution in a width direction of the steel sheet when acceleration cooling is stopped.

FIG. 3 is an explanatory diagram showing calculation results of a residual stress corresponding to the temperature distribution in the plate width direction and a critical buckling stress in each of the first to third buckling modes.

FIG. 4 is an explanatory diagram of a coordinate system in a process of deriving a simple buckling prediction formula in the method of the present invention.

FIG. 5 is a graph for explaining an evaluation result of accuracy of the simple buckling prediction formula.

FIG. 6 is an explanatory diagram of an evaluation result for accuracy in a conventional simple buckling prediction formula.

──────────────────────────────────────────────────続 き Continued on front page (58) Field surveyed (Int.Cl. ⁶ , DB name) B21D 1/05

Claims (3)

(57) [Claims] 1. A temperature measuring means for measuring a surface temperature distribution of a steel sheet at an outlet of an accelerated cooling facility or a hot straightening machine, and a cold leveler for performing a straightening process on the steel sheet to control a residual stress of the steel sheet. A first step of calculating a residual stress of a steel sheet generated after air cooling based on the sheet surface temperature distribution; a second step of determining whether or not the steel sheet buckles based on the residual stress; A third step of calculating the corrected residual stresses in the cold leveler under the respective processing conditions and determining whether or not the steel sheet buckles based on the residual stresses; A fourth step of performing a straightening process on the steel sheet by setting a processing condition in which bending does not occur to a cold leveler. 2. The residual stress calculated in the first step
Distribution_actWhen (x, y), in the second step
The area where the average of the residual stress is compressed ₁And its
Other area Ω_TwoAnd Ω in the buckling critical state₁To
Average stress_cr ^(-)Is given byWhere σ₀: Adapts to steel plate size, buckling mode, area division
The constant F (x_i, y_j): Steel plate size, buckling mode, how to divide the area
Function Δσ_act ^(-)(x_i, y_j) = Σ_act−σ_act ^(-) Δσ_act ⁽⁺⁾(x_i, y_j) = Σ_act−σ_act ⁽⁺⁾ σ_act ^(-): Area Ω₁Σ at_actThe average σ of_act ⁽⁺⁾: Area Ω_TwoΣ at_actThe average of N₁, N_Two: Each area Ω₁, Ω_TwoPoint (x_i, y_j)_act ^(-)/ Σ_cr ^(-)It is determined that buckling occurs when ≧ 1 is satisfied, and is not satisfied
In this case, it is determined that buckling does not occur.
A method for producing a hot-rolled steel sheet according to claim 1. 3. In the third step, the residual stress distribution after correction under each processing condition in the cold leveler is calculated, and the residual stress in the plate surface in the region Ω ₁ ′ where the corrected residual stress is compressed is obtained. The average stress σ _act ^{af (-)} and the average stress σ _cr ^{af (-)} in the region Ω ₁ ′ in the buckling critical state are calculated, and σ _act ^{af (−)} / σ _cr ^{af (−)} ≧ 3. The method for manufacturing a hot-rolled steel sheet according to claim 1, wherein it is determined that buckling occurs when 1 is satisfied, and it is determined that buckling does not occur when it is not satisfied.