JPH0442089B2 - - Google Patents

Description

[Detailed description of the invention]

(Industrial Application Field) The present invention relates to an accelerated cooling method for a thick steel plate, and more particularly, to an accelerated cooling method for a thick steel plate that has excellent strip-cutting properties. (Prior art and its problems) In order to manufacture thick steel plates with high strength, high toughness, and excellent weldability, the accelerated cooling method is used to water-cool steel plates immediately after hot rolling, including steel plates for shipbuilding. It has been put to practical use in the production of various high-tensile steel plates. In such accelerated cooling, it is most important to uniformly cool the entire surface of the steel plate in order to ensure uniformity of the material and maintain a good shape of the steel plate. However, in actual manufacturing, even if there are no defects in the cooling system, uneven cooling may occur due to the plate crown, the temperature distribution of the steel sheet immediately before cooling, and the water riding effect during cooling, resulting in the temperature of the steel sheet after cooling. Heterogeneity may not be avoided. Therefore, conventionally, in order to reduce the distortion after cooling, a certain recuperation time is allowed after cooling is completed, and the steel sheet is straightened when the temperature difference in the thickness direction disappears (see Japanese Patent Publication No. 55-38406). To prevent ear waves and non-uniformity of mechanical properties across the width of the sheet, after accelerated cooling, online straightening and reheating to temperatures ranging from 500℃ to ₁ point Ac should be performed in this order or in the reverse order. Method performed by
(see Publication No. 2003) has been proposed. However, in the first method described above, the steel plate is simply straightened after cooling and reheating time until the temperature difference in the plate thickness direction disappears to ensure a good large plate shape. When cut, the problem remains that large lateral bends occur. In addition, in the second method, apart from the cost problem of installing a new reheating furnace, the heating time is only a few minutes, so temperature non-uniformity tends to remain in the width direction of the sheet. It is difficult to satisfy the tolerance required to ensure a good strip shape, and the first method described above has a problem in that large lateral bending occurs when strips are cut in the same way. (Point of View of the Present Invention) In order to solve the drawbacks of the above-mentioned conventional methods, the present invention has conducted intensive research and found that in accelerated cooling steel sheets that are water-cooled after hot working, the temperature non-uniformity of the steel sheet before cooling is caused by cooling. Compared to conventional steel sheets, which are air-cooled or normalized after rolling, the residual stress generated during the water-cooling and subsequent air-cooling processes tends to be larger. On the other hand, in the strip shape, the temperature non-uniformity in the width direction (the difference between the highest value and the lowest value per strip width in the temperature distribution curve in the strip width direction) △T and the amount of lateral bending δ has the following relationship. It can be approximated by the formula: △T=8・W・δ/α・I ² (W: strip width, α: linear expansion coefficient, I: steel plate length), and as can be understood from this relational expression, Since the stress difference in the range where strips are cut occurs as lateral bending, the temperature non-uniformity in the width direction (the difference between the highest and lowest values in the temperature distribution curve in the width direction of the steel sheet, hereinafter referred to as the width It was discovered that if the directional non-uniformity (direction non-uniformity) exceeds the allowable value, even if the shape of the large steel plate before strip cutting is good, large lateral bending will occur when strip cutting is performed. Ivy. (Summary of the Invention) The gist of the present invention is to provide a manufacturing method in which a thick steel plate is accelerated cooled after hot rolling or hot straightening, and then the thick steel plate is straightened online. The surface temperature distribution of a thick steel plate is measured, and the final correction in the online straightening process is performed when the difference between the highest value and the lowest value in the plate width direction temperature distribution curve becomes 10°C or less. "A method for accelerated cooling of thick steel plates." In the present invention, the reason why the final correction is performed in the online correction process is to release the residual stress that has been generated immediately before the correction. The difference between the highest and lowest values in the temperature distribution curve in the sheet width direction is measured in the width direction of the accelerated cooling steel sheet (thickness 50 mm, sheet width 2175 mm, sheet length 144550 mm) immediately before hot leveler straightening as shown in Figure 1. The temperature distribution (center of the steel plate) and the steel plate shown in Figure 2 are cut into strips with a width of 400 mm.
As can be seen from the amount of lateral bending when cutting in mm and the amount of lateral bending estimated from the temperature distribution in the width direction,
Temperature distribution in the width direction (in the center of the steel plate) immediately before hot leveler straightening of the accelerated cooling steel plate and the width across the steel plate
This is in good agreement with the amount of lateral bending when cutting at 400 mm, and this is because the smaller the unevenness of the temperature distribution in the width direction (center part of the steel plate) immediately before straightening with the hot leveler, the smaller the amount of lateral bending after strip cutting. . Such measurements are preferably carried out during the straightening process after accelerated cooling, and are carried out by measuring the surface temperature distribution of the steel plate after accelerated cooling of the steel plate, immediately before straightening, or during straightening. The reason why the temperature non-uniformity in the width direction is limited to 10°C or less just before the final straightening is due to the relationship between the temperature non-uniformity in the width direction and the amount of lateral bending (plate thickness 30°C) shown in Figure 3.
mm, board length 16000mm and 25000mm, strip width
400 mm) to ensure that the permissible amount of horizontal bending in shipbuilding longitudinal materials is less than 1 mm/m, the temperature non-uniformity in the width direction needs to be less than 10°C. Since the purpose of the straightening temperature in the present invention is to release residual stress generated before straightening, it is preferable to carry out the straightening at a temperature as high as possible.Although it depends on the capacity of the straightening machine, the straightening temperature is preferably 400°C or higher. If it takes several minutes to reduce the temperature non-uniformity to 10°C or less, it is better to use a cooling bed, which is usually located at the rear of the straightening machine, in consideration of productivity. Hereinafter, the present invention will be explained in more detail based on Examples. (Example) Comparing the amount of lateral bending of a steel plate manufactured by a conventional method under the conditions shown in Table 1 below with the amount of lateral bending of a steel plate manufactured by applying the method of the present invention, it is found that in the conventional example, While lateral bending of 30 mm/16.5 m occurred, in the present invention, only half or less of the lateral bending occurred.

[Table] (Operations and Effects of the Invention) As is clear from the above explanation, according to the present invention, after a thick steel plate is acceleratedly cooled after hot rolling or hot straightening, the thick steel plate is straightened online. The surface temperature distribution of the thick steel plate was measured during the straightening process after accelerated cooling, and the difference between the highest and lowest values in the width direction temperature distribution curve was determined to be 10
Since straightening when the temperature drops below ℃ is the final straightening in the online straightening process, not only does the shape of the large steel plate remain good, but even when strip cutting is performed, it is not possible with conventional methods. The lateral bending that has occurred can be reduced as much as possible. Further, in the present invention, it is sufficient to measure the temperature distribution in the width direction after cooling, so there is no need to newly install a reheating furnace on-line, and there is an advantage that it can be easily implemented.

[Brief explanation of the drawing]

Figure 1 is a graph showing the temperature distribution in the width direction (center part of the steel plate) of an accelerated cooling steel plate immediately before straightening with a hot leveler, and Figure 2 is a graph showing the amount of lateral bending and the temperature in the width direction when the steel plate is cut with a strip width of 400 mm. A graph showing the amount of lateral bending estimated from the distribution, and FIG. 3 is a graph showing the relationship between the degree of temperature non-uniformity in the width direction and the amount of lateral bending.

Claims (1)

[Scope of Claims] 1. A manufacturing method in which a thick steel plate is acceleratedly cooled after hot rolling or hot straightening, and then straightened on-line, wherein the surface temperature of the thick steel plate is adjusted in the straightening process after accelerated cooling. Acceleration of a thick steel plate characterized by measuring the distribution and making the final correction in the online straightening process when the difference between the highest value and the lowest value in the plate width direction temperature distribution curve becomes 10°C or less Cooling method.