JP3114593B2 - Steel plate manufacturing method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION The present invention relates to a process for forcibly cooling a high-temperature steel sheet, performing hot straightening and then allowing it to cool naturally, such as direct quenching forced cooling of a hot-rolled steel sheet, Immediately after the straightening, the flat shape of the steel sheet after natural cooling is predicted, the flat shape of the product steel sheet is judged, and the flattened shape of the product steel sheet is determined. It is about.

[0002]

2. Description of the Related Art In recent years, in the process of manufacturing steel plates such as thick plates, a technique for obtaining high-strength, high-toughness steel plates by water-cooling hot steel plates after controlled rolling (hereinafter referred to as forced cooling) has been widely used. Is coming.

[0003] The above-mentioned method realizes the strengthening and toughness of a steel sheet, which has been conventionally performed by increasing the number of additional elements, by combining controlled rolling and forced cooling. Not only can the production cost be reduced significantly, but also a steel sheet excellent in weldability can be produced.

[0004] As described above, it is possible to produce steel sheets of excellent quality at low cost by forced cooling, but on the other hand, as the need for higher quality has been increasing more and more recently,
Several problems have arisen, of which an important one is the occurrence of flatness failure of the forcibly cooled steel plate.

That is, in the forced cooling, 200 ° C.
Generally, cooling water is injected from a cooling nozzle onto the surface of a steel sheet having a high temperature of about 900 ° C. to forcibly cool the steel sheet. At this time, a convective boiling heat transfer phenomenon occurs on the steel sheet surface. In forced cooling, this convection boiling heat transfer phenomenon can achieve a cooling rate several tens to hundreds times higher than natural cooling or the like, and a steel sheet having a finer crystal structure is obtained. A steel sheet having high toughness can be manufactured.

On the other hand, however, the convection boiling heat transfer phenomenon is very unstable because the heat transfer coefficient tends to increase as the steel sheet surface temperature decreases.

For example, if there is slight temperature unevenness on the steel sheet surface at the start of cooling, it is assumed that the water density of the cooling water, which is one of the factors affecting the heat transfer coefficient of the steel sheet surface, is controlled uniformly over the entire steel sheet surface. However, since cooling is further promoted in a region where the surface temperature is low, large temperature unevenness occurs after cooling is completed. The occurrence of these temperature irregularities not only causes a variation in the final mechanical property values, but also when the temperature irregularity exceeds a certain limit temperature difference, an ear wave or an ear wave is generated until the temperature reaches room temperature (room temperature). There is a problem that shape defects such as middle elongation occur.

The following two methods have been proposed as methods for preventing the occurrence of defective shape of the steel sheet in the forced cooling.

(1) The temperature distribution in the sheet width direction is measured before the start of the forced cooling, and based on the result calculated so as to obtain a uniform temperature distribution in the width direction after the cooling, the steel sheet width end is covered with the shielding gutter. To prevent the cooling water from directly hitting the edge of the steel plate width,
A method for preventing a temperature drop at a steel sheet width end (Japanese Patent Application Laid-Open No. 58-3
No. 2511).

(2) At least at the center of the steel plate in the vertical and width directions and at the width end, the temperatures immediately before the start of water cooling, during the water cooling, and after the end of the water cooling are detected, and the temperature between each temperature measuring point and the temperature between the temperature measuring points is detected. The temperature difference of each steel sheet is calculated, and the deformation of the steel sheet in the normal temperature range is predicted and calculated based on the relational expression determined in advance corresponding to the temperature difference between each temperature measurement point and the temperature difference between the temperature measurement points. Controlling the supply amount of the cooling water to the plurality of nozzles so as to fall within the allowable range (Japanese Patent Publication No. 63-47).
775).

In the former method (1), although it is not clear about the temperature difference between the center of the sheet width and the end of the sheet width at which flatness occurs, uniform cooling is performed in the width direction to prevent flatness. Therefore, it can be considered that the method is based on the premise that flatness failure occurs even with a slight temperature difference.

In the latter method (2), a limit value is provided for the temperature difference between the end portion of the sheet width and the center portion of the sheet width and the temperature difference between the upper and lower surfaces of the steel sheet, and these temperature differences exceed the respective limit values. It is assumed that a flat defect such as an ear wave or a medium wave or a flat defect due to a warp is generated when this occurs.

In addition, the residual stress of the steel sheet at normal temperature is calculated from the temperature distribution data obtained by two-dimensionally measuring the surface temperature of the steel sheet after the forced cooling, and the residual stress obtained by the calculation is calculated from the calculated residual stress. A method for estimating the amount of deformation of a part obtained by cutting a steel sheet at room temperature after cooling into a predetermined shape (Japanese Patent Laid-Open No. 62-23 / 1987)
No. 6617) has been proposed.

[0014]

In order to prevent flatness failure due to temperature unevenness caused by forced cooling, the steel plate is completely and uniformly cooled so that temperature unevenness does not occur or the flatness is limited. Either the temperature unevenness is clarified and the cooling is controlled so as to suppress the temperature unevenness below the limit. The two conventional techniques proposed for preventing flatness failure, namely,
JP-A-58-32511 and JP-B-63-4777.
The methods disclosed in Japanese Patent Publication No. 5 correspond to these, respectively.

JP-A-58-32511 corresponding to the former
The method proposed in the above publication aims to prevent the ear wave of the steel sheet by preventing the width end portion of the steel sheet from being supercooled, but in practice, the surface properties of the steel sheet are not uniform. Due to cooling fluctuations and the like, supercooling occurs in portions other than the width end portion of the steel sheet, which often causes poor flatness. Therefore, according to the method disclosed in Japanese Patent Application Laid-Open No. 58-32511, it is impossible to predict the occurrence of poor flatness when the part other than the width end is supercooled.

Further, in the method proposed in Japanese Patent Application Laid-Open No. 58-32511, since the requirements to be met for the temperature non-uniformity of the steel sheet required to prevent the flatness failure are not clarified, the prediction of the flatness failure is not possible. Prevention is ad hoc and poorly reliable.

Japanese Patent Publication No. 63-77775 corresponding to the latter
In the method proposed in the publication, the deformation amount of the steel sheet in a normal temperature range is predicted based on a relational expression predetermined according to the temperature at each temperature measuring point and the temperature difference between the temperature measuring points. There is a disadvantage that it is not possible to accurately predict the amount of deformation only by the temperature difference between the hot spots and the temperature difference between the hot spots.

Further, the method disclosed in Japanese Patent Application Laid-Open No. 62-236617 discloses a method in which the temperature distribution of a steel sheet is measured to predict the residual stress of the steel sheet at normal temperature, and the result is used to cut the steel sheet. Since the present invention is applied to the prediction of the bending deformation of the strip, and the stripping deformation amount and the flatness defect do not correspond to each other, it is impossible to predict the ear wave or the middle elongation flatness defect of the steel sheet.

As described above, in order to prevent a flat defect, it is necessary to correctly predict the occurrence of the flat defect and to clarify which part of the temperature unevenness of the steel sheet starts from the flat defect. . However, in the conventional method, it is difficult to correctly predict the occurrence of flatness failure. Therefore, although the flatness failure of the steel sheet may be accidentally prevented, it cannot be surely prevented. For this reason, it is necessary to check for the occurrence of flat defects after allowing the steel sheet after forced cooling to cool to room temperature, and the steel sheet cannot be immediately conveyed to the next refining process, which delays the steel plate cooling manufacturing process. Cause.

If the occurrence of poor flatness at normal temperature can be predicted immediately after cooling, the steel sheet determined to be flat can be immediately sent to the refining process. On the other hand, by re-correcting a steel sheet determined to be poor in flatness while the temperature of the steel sheet is within an appropriate range, it is possible to prevent the occurrence of a poorly flat steel sheet unsuitable as a product.

The present invention has been made in view of the above-mentioned problems of the prior art, and in the production of a forced cooling steel sheet, the occurrence of flat defects immediately after the completion of forced cooling until the temperature reaches room temperature is ensured. The purpose of the present invention is to provide a manufacturing method for predicting and determining the quality of a product steel sheet, and in particular, when the occurrence of flat defects is predicted, by performing re-correction, thereby preventing the occurrence of flat defects after re-correction. I do.

[0022]

SUMMARY OF THE INVENTION The present invention relates to a method for producing a steel sheet in which a hot steel sheet is forcibly cooled, hot-straightened, and then naturally cooled to room temperature. The gist is to be

The temperature distribution on the entire surface of the steel sheet after the hot straightening is measured using a surface thermometer or the like.

On this measured temperature distribution, one half of the steel sheet width
Is formed while moving a partial region (block) r having a length twice as long as the width of the steel sheet in the length direction of the steel sheet.

The average temperature TBr in the partial region r is compared with the temperature of each part of the steel plate.

In all the partial regions, a continuous low-temperature region p in which the temperature of the steel sheet is continuously lower than the average temperature TBr in the length direction of the steel sheet is detected. Here, “continuously low” means that
In the sub-domain, "Half or more of the temperature-side fixed points in the length direction
Lower than TBr. "

The buckling limit temperature difference (ΔTa) cr defined from the width of the continuous low temperature region and the position in the width direction, and the average value (ΔTa) of the difference between the steel plate temperature and the partial region average temperature TBr in the continuous low temperature region p ) P and are calculated.

Based on the following equation (A), it is predicted that buckling will occur in the process of naturally cooling the steel sheet,
Judge the flatness of the product steel plate.

(ΔTa) p> (ΔTa) cr (A) When the above-mentioned equation (A) is satisfied, it is desirable to continue the following procedure.

The continuous low temperature range p in the natural cooling process
The average temperature difference (ΔTa) p ′ at the above is calculated.

The correction is performed again while the value of (ΔTa) p ′ is within the range shown by the following equation (B).

(ΔTa) p− (ΔTa) cr <(ΔTa) p ′ <(ΔTa) cr (B)

[0033]

BEST MODE FOR CARRYING OUT THE INVENTION Usually, a steel sheet which has been forcibly cooled is subjected to flat cooling by a hot straightening machine after forced cooling, and then naturally cooled to room temperature. In such a manufacturing process, the unevenness in temperature of the steel sheet caused by the forced cooling reaches a uniform room temperature (room temperature) by cooling, causing unevenness in heat shrinkage to generate internal stress in the steel sheet. Flatness that occurs during the natural cooling process is buckling that occurs due to the internal stress (residual stress). Therefore, if the residual stress falls within the buckling occurrence limit range, flatness failure does not occur. Conversely, if the buckling limit is exceeded, flatness failure occurs.

However, it is not easy to quantify the buckling limit due to thermal stress due to temperature unevenness generated by forced cooling. The temperature unevenness formed on the steel sheet by normal forced cooling is not a simple one in which a part of the steel sheet in the width direction is supercooled, and there is a temperature distribution with differences at multiple locations in the width direction and the length direction. Often become. For this reason,
As in the method proposed in Japanese Unexamined Patent Publication No. 3-47775, the temperature at at least two points at the width end and the center of the width is measured, and buckling occurs from the temperature at each temperature measurement point and the temperature difference between the temperature measurement points. , It is difficult to accurately predict buckling.

FIG. 3 is a view showing the correspondence between the maximum temperature difference between the width end portion and the width center portion of the steel sheet measured before the hot straightening machine after forcibly cooling the hot steel sheet and the occurrence of flat defects. is there.

FIG. 3 shows a width of 2200 m after hot rolling.
m, steel plate thickness 30.7 mm, length 19000 mm steel plate 16
Sheet, width 3180mm, thickness 30.5mm, length 270
A laminar water film that allows the upper surface of a steel sheet to flow down from a plurality of slit nozzles while conveying 20 sheets of 00 mm steel from 766 ° C. to 786 ° C. at a speed of 38 m / min to 56 m / min. 452 ° C-5 with water spray
The temperature of the entire upper surface of the steel sheet is measured with a surface thermometer in front of the hot straightening machine following the forced cooling device, and the temperature difference between the width end and the width center is measured. The distribution in the length direction is obtained, and the maximum value is shown for each steel sheet. The white mark indicates that the steel sheet was flat while being naturally cooled to room temperature, and the black mark indicates that flatness occurred during the natural cooling process.

From the results shown in FIG. 3, it can be seen that it is impossible to derive a limit value at which a flat defect occurs with respect to the temperature difference between the width end and the width center.

FIG. 4 is a diagram showing the relationship between the average of the temperature difference between the width end portion and the width center portion in the length direction of the steel sheet and the actual occurrence of flat defects in the temperature measurement data of the steel sheet shown in FIG. It is. In the case of FIG. 4, as in FIG. 3, the limit value of the flat defect cannot be determined.

The reason for this is that the supercooled portion due to the forced cooling does not occur at a fixed position at, for example, a width end portion of the steel sheet, but has a fixed width, as shown in FIG. This is because they are dispersed in the length direction. FIG. 10 is a diagram illustrating a temperature distribution of a steel sheet generated by forced cooling.

Further, it cannot be discussed whether or not flatness occurs in the steel sheet only by the magnitude of the temperature difference in the width direction of the steel sheet.

FIG. 5 shows the result of obtaining the maximum temperature difference in the width direction at each position in the length direction of the steel sheet, and further obtaining and displaying the maximum value within the length of the steel sheet. From FIG. 5, it can be seen that it is difficult to determine the generation limit of the flat defect only by the magnitude of the temperature difference.

As described above, when the cause of the problem of the conventional method and the measures for more accurately predicting the flat defect were examined by theoretical analysis and experiments, the following was found.

The flatness failure is buckling caused by thermal stress generated in the cooling process, and the occurrence of the buckling is dominated by the longitudinal compressive stress of the steel sheet.

In the temperature distribution of the steel sheet immediately before or immediately after hot straightening following forced cooling, the above thermal stress is compression at a relatively low temperature portion and tension at a relatively high temperature portion.

The above-mentioned relatively low temperature portion does not always occur at a fixed portion of the steel sheet. Therefore, the flatness failure of the steel sheet cannot be predicted using the magnitude of the temperature difference measured at several fixed points such as the width end and the width center of the steel sheet.

Even if a low-temperature portion having a relatively large temperature difference exists in the width direction and the length direction of the steel sheet, when the low-temperature portion is local, poor flatness hardly occurs.

Therefore, when discussing the occurrence of a flat defect from the above, it is necessary to detect a relatively low-temperature portion and consider its continuity in the width and length directions.

Since the average temperature in the width direction of the steel sheet differs in the length direction of the steel sheet, the above-mentioned portion where the compressive stress is generated does not coincide with the portion where the temperature is lower than the average temperature of the entire surface of the steel sheet. The average temperature (hereinafter, referred to as "partial region average temperature") in a certain fixed section (hereinafter, referred to as "partial region") continuous in the length direction of the steel sheet corresponds well to a portion having a low temperature.

The occurrence of flatness failure is determined by the width of the low-temperature portion, the position in the width direction of the low-temperature region, the average temperature of the low-temperature region, etc., of the low-temperature portions determined above that are continuous to some extent in the steel sheet length direction. Prediction is possible depending on the size of the parameter to be defined.

Based on the above findings, the necessity of measuring the temperature of at least the entire upper surface of the steel sheet will be discussed first.

FIG. 2 is a structural view of a forced cooling steel plate manufacturing apparatus in which the method of the present invention has been carried out, including a rolling mill.

Here, reference numeral 1 denotes a steel plate, 2 denotes a rolling mill, 3 denotes a forced cooling device, 4 denotes a hot straightening machine, and 5 and 7 denote infrared radiation surface thermometers (hereinafter abbreviated as “surface thermometers”). , 6 and 8 denote a temperature measurement control device for controlling the timing of measurement sampling of the surface thermometer 5, and 9 denotes a correction control device. In addition, U after a code | symbol shows for steel plate upper surfaces, and L shows for steel plate lower surfaces.

The flatness defect that occurs when a hot plate material is forcibly cooled is caused by unevenness in the temperature of the plate material caused by the forced cooling, and in the plate length direction generated between the time when the plate material reaches room temperature after the forced cooling. Buckling deformation due to thermal stress of This thermal stress is determined by the temperature of the steel sheet immediately after the completion of the straightening, because when the straightening is performed by the hot leveler after the forced cooling, the stress generated so far is released by the straightening. Therefore, measuring the steel sheet temperature distribution immediately after hot straightening has the same meaning as measuring the residual stress at room temperature, and basically predicting the occurrence of flat defects from the temperature distribution measurement results. It is possible.

However, as described above, simply measuring the temperature of a part of the steel sheet in a spot manner is not sufficient for predicting flatness failure, and it is necessary to measure the temperature at least over the entire upper surface of the steel sheet. is there. The steel sheet immediately after the forced cooling usually has a temperature distribution also in the sheet thickness direction, but in the conveyance to the hot straightening machine following the forced cooling and the hot straightening,
Since the temperature distribution in the thickness direction is averaged on the upper and lower surfaces, if the temperature is measured at the hot straightening machine position (before or after the straightening machine), if it is measured over the entire upper surface of the steel sheet, It is often sufficient to evaluate temperature unevenness in the plate length direction.

When measuring the upper surface temperature of the steel sheet,
It is advisable to use a radiation surface thermometer such as an infrared ray. With a surface thermometer, the temperature of the entire upper surface can be measured instantaneously, but with a thermometer that scans a spot thermometer in the width direction, the temperature distribution of the conveyed steel sheet in the width direction is sufficiently accurate. And it is difficult to measure accurately. The accuracy of the surface thermometer is determined by the number of pixel points of the surface thermometer in the width direction and the length direction of the steel plate. When putting the whole steel plate in the field of view of the surface thermometer, the width is 2 to 4.5 m and the length is 2
It is possible to take about 50-130 pixels in the width direction and about 300-400 pixels in the length direction with a steel plate of 0-30 m,
It is sufficient to analyze the temperature distribution data on the upper surface of the steel sheet and to predict the flatness defect.

Although the place where the temperature is to be measured is described as the position of the hot straightening machine following the forced cooling step, it may be located immediately before or immediately after the hot straightening machine. The reason for this is that hot straightening is performed by reciprocating the steel sheet into and out of the hot straightening machine.

Next, a method of predicting buckling from the above-mentioned measured temperature will be described.

The measured temperature distribution of the steel sheet and the residual stress distribution at room temperature correspond one to one. However, direct calculation of the residual stress requires an enormous amount of calculation processing, and it is very difficult to perform online calculation and buckling prediction on a steel sheet manufacturing line. Therefore, it is necessary to predict the distribution of the residual stress from the measured temperature distribution without performing a detailed stress calculation.

A relatively low temperature portion of the temperature distribution of the steel sheet becomes a compressive stress due to the residual stress. However, in order to determine whether the temperature is relatively low, a reference temperature for comparison is first required. . If the temperature distribution is constant along the length of the steel sheet,
The average temperature of the entire surface of the steel sheet (in this case, the same as the average temperature within the width of the steel sheet) can be used as a reference for comparison, and a portion lower than the average temperature becomes the compressive stress.

However, usually, the temperature also changes in the length direction of the steel sheet. In this case, the average temperature of the entire surface of the plate cannot be used as an index. This is because, for example, the temperature distribution at the front end of the steel sheet does not affect the generation of residual stress at the rear end of the steel sheet. Therefore, in the present invention, the steel sheet is divided into partial regions, and a low temperature region serving as a compressive residual stress region is obtained from the difference between the average temperature in the partial region and the temperature of each part of the steel plate.

Next, a low-temperature region that can be regarded as being continuous in the length direction is extracted from the low-temperature regions in the partial region.
This is because flatness failure does not occur unless the compressive stress region exists to some extent in the length direction. At this stage, a continuous low temperature region is defined in each subregion. Usually, one or more continuous low-temperature regions exist in the width direction. Therefore, it is checked whether or not each continuous low-temperature region is a low-temperature region having a size enough to cause flatness failure. By performing this operation on all the partial regions, it is possible to determine whether or not flatness occurs in the steel sheet.

FIG. 1 is a calculation and control block diagram of a correction control device used for carrying out the method of the present invention.

Referring to FIG. 1, the procedure for predicting flat defects of the present invention will be described in detail below.

(1) The steel sheet is equally divided into a grid having M grids in the width direction and N grids in the length direction, and the steel sheet temperature is measured on each grid point area (FIG. 1, S). -50). This
Here, as a rough guide, M is about 20 to 60, and N is M
It is appropriate to set it to about 3M.

(2) A partial area (block) consisting of M grids in the width direction and n + 1 grids in the length direction is defined on the grid. A region having a temperature lower than the average temperature in the partial region (hereinafter referred to as “partial region average temperature”) can be regarded as a compressive residual stress region at room temperature (S-5).
1).

(3) By moving this partial area by k lattices in the length direction, from the front end to the rear end of the steel sheet (N
−n) / k + 1 partial areas, (1)
From equation (2), the temperature difference (ΔT) ijr between the temperature T (i, j) of the lattice point (i, j) and the average partial area temperature TBr of the partial area r.
Is calculated (S-52). Here, as a rough guide,
n is an integer value of about M / 10 to M / 4, and k is 1/2 to n.
It is appropriate to use an integer value of about 1/4.

[0067]

(Equation 1)

(4) The lattice point where (ΔT) ijr is negative is a portion where a compressive residual stress occurs at room temperature. The average of (ΔT) ijr is determined by the following equation (3) in a lattice row in the length direction of the steel sheet at the position i in the width direction of the steel sheet (hereinafter referred to as the i-th row), and is defined as (ΔTac) ir.

[0069]

(Equation 2)

If (ΔTac) ir is negative, the ith column can be considered as a low temperature part on average, but if only some rows are low temperature and most of the others are high temperature, It is believed that no bowing occurs.

Therefore, the number of lattice points satisfying (ΔT) ijr <0 is checked at j = 1 to n + 1 in the i-th column. This score is the number of lattice points (n + 1) in the partial area in the steel plate length direction.
Is greater than half and (ΔTac) ir is negative,
The i-th row in the width direction of the partial region r is determined to be a continuous low-temperature region (hereinafter, referred to as a continuous low-temperature region) in the length direction. The continuous low-temperature range obtained in this manner is used as a buckling determination target (S-5)
3).

FIG. 6 is a graph showing the distribution of the compressive stress region by calculating the residual stress at room temperature by the finite element method using the temperature measurement result at the position of the hot straightening machine, and the continuous low temperature region by the method of the present invention. It is a figure which illustrates the part determined.

The shaded area in FIG. 6 indicates a compressive stress area or a continuous low temperature area.

The residual stress when the temperature distribution measured on a steel plate having a width of 3180 mm, a thickness of 30.5 mm and a length of 27000 mm reaches a uniform temperature (room temperature) is divided into finite elements, and the thermoelastic finite element method is used. Fig. 6 shows the calculated result.
(A). Regarding the temperature distribution of the same steel sheet, in the method of the present invention, a grid of M = 41 points in the steel sheet width direction and a grid of N = 61 points in the length direction is used, and the number of grid points in the steel sheet length direction of the partial region is (n + 1) = 8. , The number of moving grids in the partial area is k =
6, and the temperature of the portion determined to be the continuous low-temperature region in the partial regions ((T) and (B)) in FIG. 6A (FIG. 6 (b-T) and (b-B)) is shown in FIG. (C-T) and (c-B)
It was shown to. As shown in FIG. 6, since the temperature distribution and the residual stress distribution correspond well, if only the temperature distribution is known, the method of the present invention can accurately determine the portion that becomes the compressive residual stress without performing the stress calculation. You can see that.

(5) The average temperature difference (ΔTa) p in the continuous low temperature range p is calculated by the following equation (4).

[0076]

(Equation 3)

Here, Δx is the lattice spacing in the width direction (Δx = W
/ (M-1)).

For each continuous low-temperature region p obtained in each partial region r, the barycentric position gp in the sheet width direction of each continuous low-temperature region is calculated according to the following equation (5).

[0079]

(Equation 4)

Here, Sp and Ep are the numbers of the lattice rows at both ends in the width direction of the continuous low temperature range p, W is the plate width, and xi is the distance from one width end of the i-th lattice row.

The reason why it is necessary to calculate the average temperature difference and the position of the center of gravity in the continuous low temperature region is that the average temperature difference in the continuous low temperature region is extremely effective as an index for determining buckling, The buckling limit temperature difference (ΔTa) cr described later is
This is because it changes depending on the width direction position of the compressive stress region, that is, the position of the center of gravity of the continuous low temperature region.

When a plurality (pn) of continuous low-temperature areas exist in the width direction, the center of gravity gp (p = 1 to p
From (n), the average center of gravity position G of all the continuous low-temperature regions distributed in the width direction is calculated by equation (6).

[0083]

(Equation 5)

(6) The buckling limit temperature difference (ΔTa) cr is determined as follows.

The average temperature difference in the continuous low temperature range of interest is (ΔT
a) Let p. Σ is the compressive residual stress generated at room temperature in this continuous low temperature range, and σcr is the critical compressive stress at which buckling occurs.
, The following equations (7) and (8) are obtained.

Σ∝ (ΔTa) p (7) σcr∝∝ (ΔTa) cr (8) The critical buckling stress when the compressive stress region is one in the width direction is determined by the present inventors. As a result of conducting a theoretical analysis, is expressed by the following equation (9).

[0087]

(Equation 6)

Here, E is the longitudinal modulus of elasticity of the steel sheet, ν is the Poisson's ratio, t is the sheet thickness, B is the width of the compressive stress region,
(G) is a function of the equation (10) defined by the center of gravity g of the compressive stress region.

[0089]

(Equation 7)

Here, G 'is the position of the center of gravity of the compressive stress starting from the center of the plate width.

As described above, the width B of the compressive stress region and the center of gravity g of the compressive stress region can be considered as the width Bp of the continuous low temperature region and the center of gravity gp of the continuous low temperature region.

Therefore, equations (8), (9) and (1)
The following equation (11) is obtained from the equation (0).

(ΔTa) cr = c · K (gp) · t ² / (Bp · W) (11) where c is a coefficient representing a material constant.
It is clear that the equation can be used when the continuous low temperature region is one in the width direction (S-54).

By comparing the average temperature difference (ΔTa) p in the continuous low temperature region obtained in this way with the buckling limit temperature difference (ΔTa) cr determined by the position and width of the low temperature region, the following (A) If the formula is satisfied, it can be predicted that buckling will occur in the natural cooling process after the correction.

(ΔTa) p> (ΔTa) cr (A) Usually, one or more continuous low-temperature regions are often distributed in the width direction. To determine the buckling limit temperature difference (ΔTa) cr, Further, the following measures are required.

When the continuous low temperature region is singular in the width direction,
Apply equation (11).

When there are a plurality of continuous low-temperature regions in the width direction, the center of gravity gp (p = 1, 2) of the continuous low-temperature region on both sides of the average gravity center G
Is used to define the buckling limit.

(A) The continuous low-temperature region on both sides of G is the following (1)
When the expression (2) is satisfied, the buckling coefficient K (gp) becomes extremely close to that expressed by the expression (10) because the interval between adjacent compressive stress regions is narrow.

| G−gp | /W≦0.1 (12) Therefore, in this case, assuming that one compressive stress region, that is, a single continuous low-temperature region, the buckling limit temperature difference is obtained by Expression (11). It is possible to give a value. However, in equation (10), G is used for gp, and the sum ΣBp of the widths of the continuous low-temperature regions on both sides of G is used for Bp.

(B) If the continuous low-temperature region on both sides of G does not satisfy the expression (12), it cannot be treated as a single compressive stress region, and the following cases are further classified.

(A) For the average center of gravity G, 0.45 ≦
When G / W ≦ 0.55 holds, it can be considered that the continuous low-temperature region is distributed moment-wise symmetrically.

FIG. 7 is a diagram showing the relationship between the buckling coefficient K (gp) and the position of each compressive stress region in the case of two compressive stress regions where the overall center of gravity G is located at the center of the width.

In this case, the buckling coefficient K in the equation (11) is obtained.
(Gp) includes the following equation (13) shown in FIG.
4) Use the function of equation.

[0104] 0 ≦ gp / W ≦ 0.25: K (g p) = 0.24 · exp (12.88gp / W) ··· (13) 0.25 ≦ gp / W ≦ 0.4: K (Gp) = 50.exp (-8.57 gp / W) (14) In Equation (11), the sum of the widths of the respective continuous low-temperature regions is used as Bp.

(B) 0.45 ≦ G for the whole center of gravity
If /W≦0.55 does not hold, the smaller one is selected for each continuous low-temperature range using equation (11).

The coefficient c in the above equation (11) is obtained by applying the above-mentioned method to various steel sheets and performing regression analysis so that the actual occurrence of flat defects can be best explained. It was decided as follows.

C = 190 · (490−TBr) TBr <420 ° C., c = 13300 TBr ≧ 420 ° C. (15) As described above, the quality of the flat shape of the product steel sheet is determined based on the above equation (A). Is performed, and as necessary in the subsequent steps,
Measures such as re-straightening (hot and cold), refining, splitting and quality grading are taken.

[0108] Among the processes after the above-mentioned judgment, the best measure is to continuously perform a re-correction treatment on the steel plate in which buckling is predicted to occur.

Next, a method for dealing with a steel sheet in which buckling is predicted by the above equation (A) will be described with reference to FIG. 1 (FIG. 1, S-55).

After the steel sheet has been hot-straightened, in the process of allowing it to cool naturally, in a continuous low-temperature region where buckling is predicted to occur,
The reason for correcting again within the range where the average temperature difference (Ta) p 'satisfies the following equation (B) will be described.

(ΔTa) p− (ΔTa) cr <(ΔTa) p ′ <(ΔTa) cr (B) FIG. 8 illustrates the range of the average temperature difference in the continuous low-temperature region in which re-correction is to be performed. FIG.

(1) First, the lower limit (ΔTa) p− (ΔT
a) cr is determined from the following requirements.

At the stage where the temperature of the steel sheet is naturally cooled and lowered, the temperature difference (ΔTa) p generated immediately after the hot straightening.
Also gradually decreases. Assuming that the average temperature difference in the continuous low-temperature region during the natural cooling process is (ΔTa) p ′, FIG.
As shown in (2), the thermal stress increases as (ΔTa) p ′ decreases, and a thermal stress σ ^* occurs at room temperature (RT). This σ ^*
If the stress exceeds the critical buckling stress σcr, the steel plate will buckle.

On the other hand, the average temperature difference at which the stress σcr at RT is (ΔTa) cr has already been derived.

Therefore, σ ^* − (ΔTa) p ′ shown in FIG.
On the diagram, the average temperature difference TL when the stress reaches σcr is ΔTL = (ΔTa) p-(ΔTa) cr. Once buckling occurs in the steel sheet, it becomes difficult to transport and re-correct the steel sheet.
It is preferable to perform re-correction in a temperature range of TL or higher, and ΔT
L is the lower limit value of the average temperature difference for performing the re-correction defined by the formula (B).

(2) On the other hand, the upper limit was determined according to the following requirements.

Even if the internal stress of the steel sheet is released by performing re-correction, the average temperature difference in the continuous low-temperature region at that point is (ΔT
a) If the value exceeds cr, buckling will occur at the stage where the natural cooling is repeated again.
) The upper limit ΔTU of p ′ needs to be (ΔTa) cr.

Now, the timing of performing the re-correction as described above should be based on the value of the average temperature difference in the continuous low-temperature region due to the mechanism of buckling. It is very troublesome to constantly measure the temperature distribution of the sample. Since an almost perfect linear relationship is established between the average temperature difference in the continuous low-temperature region and the steel plate temperature in the natural cooling process, in the actual operation, as shown schematically in FIG. The average temperature TAVE of the following (1)
The re-correction may be performed in the range of TL to TU shown in equation (6) (FIG. 1, S-56).

TL = △ TL · ｛(TAVE−RT) / (△ Ta) p｝ + R.T., TU = △ TU · ｛(TAVE−RT) / (△ Ta) p｝ + R.T. (16) When the average temperature difference (ΔTa) p in the continuous low-temperature region is extremely large, occurrence of buckling may not be prevented by re-correction only once. In this case, as shown in FIG. 8C, re-correction is performed once in a temperature range R1 in which the amount of decrease in the average temperature difference is equal to or less than (ΔTa) cr, for example, the temperature TR1, and the average temperature difference (ΔTa) p in TR1 is obtained. By replacing (ΔTa) p in equation (B) with 'R1, correction can be performed again in the temperature range R2 determined again from equation (B) (FIG. 1, S-57). .

In the above method, re-correction is performed based on the temperature of the steel sheet. However, the temperature of the steel sheet at this time does not need to be measured every time, and it is sufficient to estimate the temperature using, for example, the following equation (C).

Θ (t) = θf + (θ0−θf) · exp (−mt), m = 2α / (c · γ · h) (C) where θ (t) is Temperature, θf is the temperature of the external atmosphere, θ0 is the temperature of the continuous low-temperature region of interest immediately after the start of natural cooling, that is, immediately after the completion of hot straightening, c is the specific heat of the steel
γ is the specific gravity of the steel sheet material, h is the plate thickness, and α is the heat transfer coefficient in natural cooling. It is necessary to determine the value of α such that the temperature of the steel sheet is measured and the difference between the temperature θ (t) according to the above equation (C) and the temperature measurement result is minimized.

By using α thus determined, the natural cooling time until the steel sheet temperature decreases to the predetermined temperature can be calculated back from the above equation (C), and the time corresponding to TL and TU can be calculated. If re-correction is performed within the set time,
Buckling can be prevented.

[0123]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A forced-cooled steel sheet subjected to the above-mentioned additional test of the prior art, that is, a plurality of upper surfaces of a steel sheet were conveyed from a temperature of 766 ° C. to 786 ° C. at a speed of 38 m / min to 56 m / min after hot rolling. The lower surface of the steel plate was forcibly cooled to a range of 452 ° C. to 520 ° C. by a plurality of water sprays by a lamina water film flowing down from the slit nozzles, width 2200 mm, plate thickness 30.7 mm, length 19000 mm
16 sheets of steel, width 3180mm, thickness 30.5mm,
For 20 sheets of 27000 mm length, the temperature of the entire upper surface of the sheet was measured by a surface thermometer at the position of the hot straightening machine following the forced cooling device, and the flatness at room temperature was predicted using the method of the present invention. .

In this embodiment, the temperature measurement and the comparison operation were performed using the above-described forced cooling steel plate rolling apparatus having the configuration shown in FIG. That is, the upper surface temperature of the steel plate was measured by the surface thermometer 5U provided in front of the hot straightening machine 4. The result of the temperature measurement by the surface thermometer 5U is temporarily transferred from the surface thermometer to the personal computer 6U for controlling the timing of measurement sampling by the surface thermometer, and immediately thereafter, the engineering workstation which is the correction control device 9 which performs the comparison operation. (Hereinafter referred to as EWS) via a communication network (Ethernet).

The resolution of the surface thermometer is approximately 80 in the width direction of the steel sheet.
Pixels: Approximately 300 pixels in the length direction of the steel plate. However, due to the limitation of the EWS magnetic disk capacity for storing temperature measurement data, only 41 points in the width direction of the steel plate and 61 points in the length direction need to be stored. These temperature measurement data were used. However, since it is difficult to detect the edges of the steel plate strictly in the four laps of the steel plate, the grid rows and grid rows corresponding to the edges are deleted by one grid row and one grid row on the left, right, front and rear of the steel plate. For the prediction, 39 grid columns in the width direction and 59 grid rows in the length direction were used. The partial area length was given approximately equal to the width of the steel sheet. That is, the number of grid rows constituting the partial region was set to 8 in both steel plates. Therefore, width 2
The partial area length for a 200 mm steel plate is 2217 mm,
The partial area length for a steel plate with a width of 3180 mm is 3150 m
m.

Assuming that k = 2, this partial area is moved in every two lattice rows in the length direction of the steel sheet, and the continuous low-temperature area and the average temperature difference are calculated for all 26 blocks for one steel sheet. Then, the buckling limit temperature difference corresponding to the form of the continuous low-temperature region was calculated, and the occurrence of the flatness failure was predicted by comparing with the average temperature difference. The average temperature difference obtained in this way exceeds the buckling limit temperature difference and has the largest difference with the buckling limit temperature difference, or the average temperature difference in any partial region indicates the buckling limit temperature difference. If not exceeded, the value of the average temperature difference closest to the buckling limit temperature difference was extracted.

FIG. 9 is a diagram showing the measurement and calculation results of this embodiment in the relationship between the average temperature difference (ΔTa) p in the continuous low temperature region and the buckling limit temperature difference (ΔTa) cr. Here, the mark of the average temperature difference is indicated by blacking out the mark where the flat shape has a defect.

As can be seen from FIG. 9, the region where the above equation (A) is satisfied, that is, the region where the average temperature difference (ΔTa) p exceeds the buckling limit temperature difference (ΔTa) cr described in the method of the present invention, This is in good agreement with the record of occurrence of defects, and it was confirmed that the method of the present invention is effective for predicting the occurrence of flat defects and determining the quality of flat shapes.

Next, when it was predicted that buckling would occur by the method of the present invention, the steel sheet which was forcibly cooled under the same conditions as in the above example was re-rectified and re-rectified according to the formula (B). The results are shown in Tables 1 and 2 below.

Table 1 shows Examples showing the flatness of a steel sheet when re-correction is performed by the method of the present invention in a natural cooling process after hot correction, and Comparative Examples thereof.

Table 2 shows an example in which in the natural cooling process after hot straightening, re-correction according to the method of the present invention and further subsequent re-correction were necessary.

[0132]

[Table 1]

[0133]

[Table 2]

In the case where re-correction is not performed, re-correction or re-re-correction should be performed within the scope of the method of the present invention even for a steel sheet having a temperature difference condition in which flatness occurs as shown in FIG. As a result, it was confirmed that buckling can be completely prevented.

[0135]

According to the method of the present invention, in the manufacturing process of a steel sheet, at the stage of hot straightening following forced cooling, the occurrence of flatness failure at room temperature is predicted beforehand, and the quality of the flat shape of the steel sheet is determined. It becomes possible. Further, by performing the hot straightening again only on the material for predicting the occurrence of the flat defect before the occurrence of the flat defect, it is possible to efficiently prevent the occurrence of the flat defect.

[Brief description of the drawings]

FIG. 1 is a calculation and control block diagram of a correction control device used for implementing a method of the present invention.

FIG. 2 is a configuration diagram after a rolling mill of a forced cooling steel sheet manufacturing apparatus in which the method of the present invention is performed.

FIG. 3 is a diagram showing a correspondence between a maximum temperature difference between a width end portion and a width center portion of a steel sheet measured at a position of a hot straightening machine after a hot steel sheet is forcibly cooled and a flat defect occurrence record.

FIG. 4 shows the temperature measurement data of the steel sheet shown in FIG.
It is a figure which shows the relationship between what averaged the temperature difference of the width end part and the width center part in the steel plate length direction, and the flat defect occurrence result.

FIG. 5 is a diagram illustrating a relationship between a maximum temperature difference in a width direction of a steel sheet and a record of occurrence of flat defect.

FIG. 6 is a diagram showing the distribution of the compressive stress region calculated by the finite element method based on the residual stress at room temperature using the temperature measurement result at the position of the hot straightening machine, and it is determined that the region is a continuous low temperature region by the method of the present invention. FIG.

FIG. 7 is a diagram showing the relationship between the buckling coefficient K (gp) and the position of each compressive stress region in the case of two compressive stress regions where the overall center of gravity G is located at the center of the width.

FIG. 8 is a diagram for explaining a range of an average temperature difference in a continuous low-temperature region in which re-correction is to be performed.

FIG. 9 shows the results of measurement and calculation of the embodiment of the present invention, in which the average temperature difference (ΔTa) p and the buckling limit temperature difference (ΔTa) in the continuous low temperature range are obtained.
FIG.

FIG. 10 is a diagram illustrating a temperature distribution of a steel sheet generated by forced cooling.

[Explanation of symbols]

REFERENCE SIGNS LIST 1 steel plate 2 rolling mill (roll stand) 3 forced cooling device 4 hot straightening machine 5 and 7 surface thermometer (infrared radiation surface thermometer) 6 and 8 temperature measurement control device (personal computer) 9 straightening control device (engineering work) station).

──────────────────────────────────────────────────続 き Continued on the front page (58) Field surveyed (Int. Cl. ⁷ , DB name) C21D 9/52, 9/56 C21D 9/573, 11/00 B21B 37/00, 45/02

Claims (2)

(57) [Claims] In a manufacturing process of a steel sheet in which a hot steel sheet is forcibly cooled, hot-straightened and then naturally cooled, a temperature distribution of the entire surface of the steel sheet after the completion of the hot straightening is measured, and the measured temperature distribution is measured. A partial region r having a length equal to 1/2 to twice the width of the steel plate and the same width as the steel plate width is formed while being moved in the longitudinal direction of the steel plate, and the average temperature TBr in these partial regions r and each part of the steel plate are formed. And in the partial area, in the length direction of the steel sheet, detects all the continuous low-temperature areas p in which the steel sheet temperature is continuously lower than the average temperature TBr, and detects the width of the continuous low-temperature area and the width in the width direction. Buckling limit temperature difference (ΔTa) cr defined from position
And the average value (ΔTa) p of the difference between the steel sheet temperature in the continuous low temperature range p and the partial area average temperature TBr, and judge the quality of the flat shape of the product steel sheet based on the following equation (A). Manufacturing method of steel sheet. (ΔTa) p> (ΔTa) cr (A) 2. In the method for manufacturing a steel sheet according to claim 1, when the formula (A) is satisfied, an average temperature difference (ΔTa) p ′ in the continuous low-temperature region p in the natural cooling process is calculated. A method for producing a steel sheet, wherein the straightening is performed again while the value of (ΔTa) p ′ is within the range shown by the following equation (B). (ΔTa) p− (ΔTa) cr <(ΔTa) p ′ <(ΔTa) cr (B)