JPH0756042B2 - Method for manufacturing thin web H-section steel - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial field of application) A manufacturing method for preventing generation of web waves when a thin web H-section steel having a large web height and a thin web thickness relative to a flange thickness is produced by hot rolling. It is about.

(Prior Art) As is well known, the thin-walled web H-section steel, which has a large section modulus and is more economical than strength, has not been supplied to the market due to the problem of web waves in the manufacturing method by rolling.

Further, the build-up H-section steel produced by the welding method still has problems such as a problem of welding distortion and high cost.

Now, according to the rolling manufacturing method, in general, the generation of the web wave is because a compressive internal stress exceeding the buckling limit of the web is generated in the web due to the residual stress caused by the temperature difference in the cooling process between the flange and the web. Therefore, cooling means for equalizing the temperatures of the flange and the web have been proposed.

However, in the thin web H-section steel, the flange is relatively thick, the web is thin, and the height of the web is high. Therefore, it is very difficult to reduce the temperature difference between the flange and the web, and the residual stress is inevitably large. On the other hand, it is extremely difficult to suppress the web wave because the buckling limit of the web is low.

Further description will be given below with reference to the drawings.

FIGS. 5 (a) and 5 (b) are schematic explanatory views regarding the web wave of the thin web H-section steel. In the rolled H-section steel 1a having the web 1 and the flanges 2a and 2b as shown in FIG. 5 (a), when the web thickness W _t is thinner than the flange thickness F _t and the web height W _H is higher, for example, As shown in FIG. 2B, hot rolling is possible, but as described above, the web wave 3 is generated in the web 1 and it is difficult to obtain a product. Therefore, in the present invention, the thin web H-section steel means the ratio W _{t of the} web thickness W _t and the web inner width u.
It is empirically found that when / u is 0.017 or less, and when the ratio Wt / u exceeds 0.017, a web wave is hard to be generated, and when it is 0.017 or less, a web wave is generated. That is, all the rolled H-section steels referred to as standard sizes in the current JIS and other standards are included in the range where the above Wt / u is larger than 0.017, and no web wave was generated.

The web wave is generated as a result of web buckling, and the web buckling stress is approximately proportional to the square of the ratio Wt / u, that is, (Wt / u) ² . In the manufacturing process that goes through the normal rolling-air cooling process, the thickness of the flange and the web is also related, but in the conventional product size flange-web thickness ratio (<2),
It is also known that when this (Wt / u) ² becomes 3 × 10 ⁻⁴ or less, a web wave is generated.

Next, generation of residual stress in rolled H-section steel will be qualitatively described.

FIG. 6 (a) shows changes in the flange temperature 4 and the web temperature 5, and the temperature difference 6 between the flange and the web with respect to the elapsed cooling time after the rolling is finished, and FIG. , 7 indicates the stress of the flange, 8 indicates the stress of the web, 9 indicates the yield stress of the web, and 10 indicates the yield stress on the tensile side of the flange. The temperature transition during cooling is unevenly cooled at each part of the cross section, so that there is a time lag in the start of transformation, and the transition situation is complicated. The major feature is that the temperature difference between the flange and the web is 6 near the time A at which the flange temperature 4 finishes the transformation.
Shows a peak, and then gradually decreases while the temperatures of the flange and the web approach each other. Since the web restrains the transformation expansion of the flange in the vicinity of the time A at which the flange finishes the Ar ₁ transformation, the flange stress 7 is the compressive stress.
The web stress 8 becomes a tensile stress.

After the flange has completed the Ar ₁ transformation, pulling on the flange as the temperature difference 6 between the flange and the web becomes smaller,
Although compressive stress accumulates on the web, the cross-sectional area of the web is generally smaller than the cross-sectional area of the flange, and therefore the absolute value of the compressive stress of the web is larger than the tensile stress of the flange. When the cooling further progresses and the compressive stress 8 of the web reaches the yield stress 9 at point B at time B, after that, it changes at a value almost equal to the yield stress. At room temperature C, residual stress is applied to the web and the flange at the stresses indicated by points B and C, respectively.

Generally, when this residual stress becomes larger than the buckling stress of the web, a web wave is generated during cooling.

Conventionally, in order to reduce the residual stress, as a means for reducing the temperature difference between the web and the flange by the time the transformation end time A is reached, for example, the web heat retention method disclosed in JP-A-50-133110 or JP-A-49- There is the flange water cooling method of 43810. However, when manufacturing a thin web H-section steel, sufficient effects cannot be obtained by simply reducing the temperature difference between the web and the flange. Further, the applicant of the present invention has previously described that the method for producing a thin web H-section steel is disclosed in Japanese Patent Laid-Open No.
Although JP-A No. 60-248818 provides a means for cooling H-shaped steel while restraining it, it has a drawback that equipment for restraining it becomes large in scale.

(Problems to be Solved by the Invention) An object of the present invention is to provide a manufacturing method capable of preventing the generation of a web wave by a simple means when manufacturing a thin web H-section steel by rolling.

(Means and Actions for Solving the Problem) In a method for producing a thin-walled web H-section steel by forcibly cooling the flange of H-section steel immediately after hot finish rolling, in a method for producing a thin web H-section steel The lower limit of the temperature difference between the flange and the web and the upper limit of the temperature difference between the flange and the web immediately after water cooling, in which the thermal stress of the web after forced cooling to room temperature is less than the buckling stress of the web, A method of forcibly cooling the flange within the upper and lower limits of the temperature difference, or the upper limit of the water cooling time during which the web wave does not occur during forced cooling, and the web up to normal temperature after forced cooling The lower limit of the water cooling time at which the thermal stress becomes less than the buckling stress of the web is previously obtained for each size of H-section steel and the cooling water amount density, and the flange is forcedly cooled within the upper and lower limits of the water cooling time. Gist It is what
The means and action of the present invention will be described in detail below.

First, the present inventors investigated how the change of the cooling condition of the flange affects the temperature of the flange and the web, and further the web stress with the passage of time. FIG. 7 (a) shows an example of cooling curves of H-section steel after finish rolling in air cooling and water cooling, and FIG. 7 (b) shows the thermal stress of the web corresponding to the temperature change in FIG. 7 (a). It shows the change.
The horizontal axis indicates the temperature in the central portion in the width direction of the web, and the right side of the horizontal axis indicates the state of transition from the high temperature to the low temperature region with the passage of time. Curve 11 is a flange cooling curve for air cooling, curve 12 ~
14 is a cooling curve of the flange when the water cooling of the flange is started from the time when the web temperature is D, 12 is water cooling for a short time (up to the temperature E when the water cooling ends), and 14 is water cooling for a long time (the temperature G ends when the water cooling ends. Up to 13) and 13 are cooling curves of the respective flanges when the cooling is performed for a water cooling time between the two (up to the temperature F when the water cooling ends). The straight line 15 is the cooling curve of the web, which is common to both air cooling and water cooling. Curves 16 to 19 in FIG. 7 (b) show changes in the thermal stress of the web corresponding to the cooling curves 11 to 14 described above. Further, the line 20 in the figure shows the buckling stress of the web, which becomes smaller as the temperature becomes higher. By the way, since the web wave is generated when a compressive internal stress exceeding the buckling limit of the web is generated in the web as described above, the thermal stress 16 in the case of air cooling shown in FIG. Buckling stress 20 is reached at point D, and a web wave is generated. Curves 17 to 19 showing the thermal stress of the flange water-cooled material have in common that the compressive stress increases as the temperature difference between the flange and the web decreases during water cooling, but once the water cooling ends, it changes to the tension side. After that, it changes to the compression side again. This is because the temperature difference between the flange and the web, which has been reduced by water cooling, is once enlarged and reduced after water cooling. The compressive stress of the web during water cooling is greater as the water cooling time is longer, as indicated by points G, F, and E, and conversely, the compressive stress at room temperature is
As shown in L and C, the longer the water cooling time, the smaller it becomes. Among the above-mentioned short, medium and long water cooling time conditions, the stress during water cooling of the stress transition curve 19 having a long cooling time reaches a buckling stress 20 at a point R, and a web wave is generated during water cooling. Further, in the case of the stress curve 17 having a short cooling time, the peak point e of the thermal stress during water cooling is a buckling stress of 20 or less, a web wave does not occur during water cooling, but on the way to normal temperature after water cooling is completed. It was found that the buckling stress reached 20 at the point of and the web wave was generated.
That is, when the water cooling degree is too strong, the web wave is generated during the water cooling, and when the water cooling degree is too weak, the web wave is generated between the water cooling and the normal temperature. The thermal stress transition 18 in the case of an intermediate water cooling time is equal to or less than the buckling stress during the water cooling and after reaching the normal temperature after the water cooling, and it becomes possible to prevent the web wave under this condition. That is, in the case of thin web H-section steel,
Web waves cannot be prevented only by flange water cooling for reducing the temperature difference between the flange and the web as in the residual stress reduction method for conventional H-section steel.

In order to confirm the existence of the appropriate conditions, FIG. 1 (a) shows heating and cooling experiments using a thin web H-section steel formed by welding as a test material, and shows the generation of web waves generated during cooling. The size of the test material is H504 x 195
× 6/19, Steel type is SS41. This test material was heated in a heating furnace at 850 ° C. and extracted, and when the temperature at the center of the web reached 630 ° C., the water density was set to 150 l / m ² · min and water cooling of only the outer surface of the flange was started. The horizontal axis represents the temperature difference between the flange and the web at the end of water cooling, and the temperature was measured with a thermocouple attached to the center of the thickness of the flange and the center of the web width direction. The vertical axis represents the height a of the web wave 21 shown in FIG. 1 (b) (amplitude × 2). Originally, the steepness, that is, (web wave height a / wavelength λ) should be adopted to accurately represent the degree of the web wave, but as a result of actual measurement, the wavelength λ was almost equal in all test materials. Therefore, the degree of the web wave was evaluated simply by the height of the web wave. The ● marks show the results of the measurement of the web waves immediately after water cooling, and the ○ marks show the results of the measurements of the web waves after cooling to room temperature. The broken line 22 and the solid line 23 show the respective trends. According to Fig. 1 (a), when strong cooling is performed so that the temperature difference between the flange and the web at the end of water cooling is about 25 ° C or less, a web wave is generated during water cooling, and the web wave remains even after cooling to room temperature. It is clear that it remains. In the region where the temperature difference is negative, the wave at room temperature is smaller than the wave during water cooling because the temperature of the flange and the web are reversed, so the amount of shrinkage of the web after water cooling is larger than that of the flange. This is because Further, when the temperature difference is about 75 ° C. or more and the temperature is light, the web wave does not occur during water cooling, but the web wave remains at room temperature, indicating that water cooling is insufficient. After all, it can be seen that when the temperature difference between the flange and the web at the end of water cooling is in the range of about 25 to 75 ° C, there is no or very little web wave. As described above, the gist of the present invention is to prevent web waves by cooling control so that the temperature difference between the flange and the web at the end of water cooling falls within a certain range.

The conditions for air cooling control differ depending on the temperature conditions at the start of water cooling, the water amount density, and the size of the H-section steel. In actual operation, it is also practical to control the temperature by water cooling time in addition to the method of grasping the temperature immediately after the forced cooling. The specific procedure will be described below.

FIG. 2 shows an example of an apparatus for carrying out the method of the present invention. The present invention can be basically carried out only by providing a flange water cooling device 28 immediately after the finishing universal rolling machine 25. For comparison with the case where preliminary water cooling is performed before water cooling, cooling devices 29 and 30 are provided on the front surface of the intermediate universal rolling machine 26 and the rear surface of the edging rolling machine 27 that performs flange width reduction, respectively. FIG. 3 (a) illustrates the relationship between the web temperature at the start of water cooling and the water cooling time in the state of generation of web waves when only water cooling is performed immediately after finish rolling.
FIG. 3 (b) shows a case where the flange is pre-water-cooled by intermediate rolling and then water-cooled immediately after finish rolling. When the web temperature at the start of water cooling was 650 ° C or less, the group A changed the temperature at the start of water cooling such that after the finish rolling, a slight waiting time was provided until the start of cooling, and the group B started cooling immediately after the finish rolling. . Target size is H504 × 195 × 6/19,
The steel grade was SS41, the water quantity density in the water cooling device on the rear surface of the finishing rolling mill was 200 l / m ² · min, and cooling was performed so that the cooling water collided with approximately 90% of the flange width. In the graph, a circle indicates that no web wave is generated, and a cross indicates that a web wave is present. The proper range of water cooling time that can prevent web waves is the third
It is shown between the curves 31 and 32 in the figure (a) and between the curves 33 and 34 in the figure 3 (b). That is,
In the water cooling time at the ends of the curves 31 and 33, the web wave does not occur during water cooling, but it is a so-called undercooling range in which the web wave is generated before reaching room temperature after water cooling, while the curves 32 and 34
When the water cooling time exceeds, the web wave is generated during water cooling, and the web wave remains at normal temperature in a so-called supercooling range. That is, in the present invention, the lower limit water-cooling time that does not generate a web wave after reaching the room temperature after water cooling means curves 31 and 33, and the upper limit water-cooling time that a web wave does not occur during water cooling is a curve. It means 32 and 34. Also, as shown in FIG.
According to (b), the appropriate water cooling time range is wider as the temperature at the start of water cooling is higher, and if preliminary water cooling is performed in the intermediate rolling stage to reduce the temperature difference between the flange and the web in advance, the appropriate water cooling time will increase.・ It can be seen that both lower limits move to the short side. Preliminary water cooling can shorten the length of the water cooling equipment after finishing rolling in the case of a water cooling method that only passes through the cooling zone in one direction, or the passing speed can be increased, so the effect of improving productivity can be expected. . Incidentally, in place of the flange preliminary water cooling in the intermediate rolling stage, or in addition to the flange preliminary water cooling, during heating from the intermediate rolling stage to the finish rolling, well-known heating of the web portion and heat retention of the web have exactly the same effect. Can be obtained.

The appropriate water cooling time for preventing the generation of the web wave is determined by the size of the H-section steel and the cooling water density density condition in addition to the web temperature condition at the start of water cooling as described above. It is only necessary to grasp the relationship shown in FIG. 3 for each water density. Fig. 4 illustrates how to determine the water cooling time control point when flange water cooling is performed immediately after finish rolling in an actual production line. Water cooling is performed within the range of arrow 35 indicating the flange water cooling start web temperature range. The minimum value t ₁ of the upper limit water cooling time 36 in which no web wave is generated and the maximum value t ₂ of the lower limit temperature 37 in which no web wave is generated until the temperature reaches the room temperature after water cooling are calculated in advance, and an intermediate time between the two is obtained. According to the one-point control with the control point as the control point, it is not necessary to measure and determine the water cooling start temperature for each rolled material of the size. In the example of FIGS. 3 (a) and 3 (b), when the water cooling time was set by single point management, it was about 45 seconds and 30 seconds. In the graph of FIG. 3, the proper water cooling range is obtained based on the actual measurement data of the actual rolling equipment. In addition to the means based on the actual measurement data, heat transfer and thermal stress analysis methods by computer simulation are used. Of course, it is also good.

(Example) As a result of investigating the state of generation of web waves for thin web H-section steel of H500 × 200 × 6/19 and steel grade SS41, with or without preliminary water cooling before finish rolling or with or without flange water cooling immediately after finish rolling Is shown in Table 1. The water quantity density of the flange immediately after the finish rolling was 200 l / m ² · min. The setting of the water cooling time in this example was performed by the above-mentioned one-point control, but there was no particular problem in terms of accuracy.

As a result of trying the test by changing various conditions, [web thickness
For a W-shaped steel having a small W _t / inner-web method u], that is, a web having a large degree of thinness of the web, the range of the appropriate water cooling time in the present invention becomes narrow, and it is necessary to control the water cooling time with high accuracy. understood. Further, depending on the size of the thin web H-section steel, the web wave does not occur during the water cooling as described above with reference to FIG. 4 until the minimum value t ₁ of the upper limit of the water cooling time reaches the room temperature after the water cooling. It may become smaller than the maximum value t ₂ of the lower limit temperature. For such sizes, it is not possible to manage one point of the water cooling time, but it is possible to manage one point by reducing the variation in the water cooling start temperature by temporarily waiting before or after finish rolling before the start of water cooling. In addition, in the present invention, the meaning of cooling immediately after finish rolling includes the case of once waiting before and after finish rolling as described above. The effect can be stabilized by changing the water cooling time according to the level of the water cooling start temperature as another means not to wait once.

(Effect of the invention) Since a web wave is generated in the conventional rolling method, the thin web H-section steel which has never been put into practical use has a special apparatus added to the existing rolling apparatus row according to the method of the present invention. Since it can be easily mass-produced without doing so, the economical effect is extremely large.

[Brief description of drawings]

FIG. 1 (a) is a graph showing the relationship of the web wave to the temperature difference between the flange and the web for explaining the structure of the present invention, FIG. 1 (b) is an explanatory view of the web wave, and FIG. 2 shows the present invention. FIG. 3 is an explanatory view of an example of a rolling mill row to be carried out, FIG. 3 is a graph showing the relationship between the water temperature at the start of water cooling, the water cooling time and the web wave, and FIG. 4 is a graph simplified from FIG. 5 (a) is a schematic cross-sectional view for explaining a thin web H-section steel, FIG. 5 (b) is a schematic vertical cross-section for an H-section steel explaining a web wave, and FIG. 6 (a) is a web and a flange. 6B is a graph showing the relationship between the temperature and the passage of time, and FIG. 6B is a graph showing the relationship between the internal stress of the web and the flange and the passage of time.
FIG. 7 (a) is a graph showing changes in the web temperature when the cooling conditions are changed, and FIG. 7 (b) is a graph showing changes in the web stress corresponding to the web temperature in FIG. 7 (a). 25; Finishing universal rolling mill, 26; Intermediate universal rolling mill, 27; Edging rolling mill, 28, 29, 30; Flange water cooling device.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Maki Ida 1 Tsukiko Hachiman-cho, Sakai City, Osaka Prefecture Nippon Steel Corporation Sakai Iron Works Co., Ltd. (56) References JP-B-55-43053 (JP, B2)

Claims (2)

[Claims] 1. The ratio W _t / u between the web thickness W _t and the web inner width u is set to 0 by forcibly cooling the flange of the H-section steel immediately after hot finish rolling.
In a method for producing a thin web H-section steel of 017 or less,
The lower limit of the temperature difference between the flange and the web immediately after water cooling, in which no web waves are generated during forced cooling, and the thermal stress of the web after forced cooling to room temperature are below the buckling stress of the web. A method for producing a thin web H-section steel, wherein an upper limit of the temperature difference is obtained in advance for each size of H-section steel and cooling water amount density, and the flange is forcibly cooled within the upper and lower limits of the temperature difference. 2. The ratio W _t / u of the web thickness W _t and the web inner width u is set to 0 by forcibly cooling the flange of the H-section steel immediately after hot finish rolling.
In a method for producing a thin web H-section steel of 017 or less,
The upper limit of the water cooling time during which the web wave does not occur during forced cooling,
The lower limit of the water cooling time at which the thermal stress of the web after the forced cooling to the normal temperature is equal to or less than the buckling stress of the web is obtained in advance for each size of H-section steel and the density of cooling water, and the upper and lower limits of the water cooling time are obtained. A method for manufacturing a thin-walled web H-section steel, which comprises forcibly cooling the flange in the inside.