JP2007181849A - Method for linearly heating steel sheet - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for linearly heating a steel sheet, which method can stably achieve a large deformation angle. <P>SOLUTION: The method for linearly heating the steel sheet is used for heating the steel sheet so as to simultaneously satisfy both of the following conditions: 700≤Ts≤1,000; and 27×(IYS/1,000)<SP>0.7</SP>×t<SP>-0.32</SP>-30≤Tr≤27×(IYS/1,000)<SP>0.7</SP>×t<SP>-0.32</SP>+30, where Ts(°C) shows a maximum arrival temperature on the right side surface of the steel sheet in the linear heating; Tr(°C) shows a maximum arrival temperature on the back surface of the steel sheet in the linear heating; IYS(K N/mm<SP>2</SP>) shows an integrated value of the yield strength of the steel sheet with respect to temperature from an ordinary temperature to 600°C; and t(mm) shows the thickness of the steel sheet. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a linear heating method in which a steel plate is linearly heated to be deformed into a predetermined shape.

  When a flat steel plate is deformed to form a steel plate having a final target shape, a method is used in which the steel plate is linearly heated using a gas burner or the like to be deformed into a predetermined shape. Compared to mechanical processing using a press or the like, linear heating does not require expensive equipment, and is still widely used today.

  When plastic deformation occurs due to thermal expansion when the steel plate is heated, the steel plate is deformed if it remains even if it returns to room temperature after cooling. If the thermal stress caused by the temperature rise is greater than the yield strength of the steel plate at that temperature, plastic deformation occurs. Thermal stress in one part is generated by restraint from other parts, but if the heated part is not restrained from the surroundings, it becomes free thermal expansion and no thermal stress is generated. That is, plastic deformation does not occur. On the other hand, when the constraints from the surroundings are large, the amount of final deformation becomes large by introducing a large plastic deformation.

  In the case of linear heating, as compared with welding, heating is performed relatively slowly by a burner, so that heat is transmitted to a wide area other than the heated portion. That is, as viewed from the heated portion, the constrained portion is also usually accompanied by a significant temperature rise. For this reason, in order to obtain a larger deformation, it is effective to use a steel material in which the surrounding constraints are increased, that is, the yield strength does not become so low even if the temperature rises.

  For example, in the invention relating to the linear heating method described in Patent Document 1, a steel plate is used in which the decrease in yield strength accompanying temperature rise is small in the temperature range from room temperature to 600 ° C. This is because even if the temperature rises, the yield strength does not decrease so much, and even if the temperature of the restraining portion rises, the maximum temperature portion can be restrained sufficiently and a large deformation can be obtained. In the invention described in Patent Document 1, the steel plate surface maximum temperature on the heating side is set to 600 ° C. or higher, the temperature gradient inside the steel plate is increased, and water cooling is performed to increase the temperature gradient and introduce plastic strain. The water cooling is stopped at a surface temperature of 200 ° C. or lower. Thereby, the angular deformation that occurs when the steel plate is linearly heated can be increased.

In order to obtain a steel sheet with a low decrease in yield strength due to temperature rise, it is a component containing Nb and Mo, the heating temperature during hot rolling is set to 1100 ° C. or more, and a solid solution amount of Nb and Mo is sufficiently secured. Precipitation strengthening in the heat history of the shape heating is made possible, and the lower limit temperature of rolling is set to 850 ° C. to suppress precipitation during rolling.
Japanese Patent Laid-Open No. 08-103824

  By using the invention described in Patent Document 1, it becomes possible to increase angular deformation in linear heating. However, the amount of angular deformation is not constant only by controlling the surface temperature during heating, and in some cases, a sufficient amount of deformation cannot be obtained, and it has been difficult to stably shorten the work time.

  Then, an object of this invention is to provide the linear heating method of the steel plate which can obtain the target amount of angular deformation stably while obtaining a large amount of angular deformation.

  In the invention relating to the linear heating method described in Patent Document 1, a steel plate having a small decrease in yield strength accompanying temperature rise in a temperature range from room temperature to 600 ° C. is used, and the steel plate surface temperature after heating and cooling is set to a predetermined value. The amount of bending deformation was increased by controlling the range. However, it has been found that the bending deformation due to linear heating changes in a complex manner depending on the temperature distribution in the thickness direction of the steel sheet and the high temperature strength characteristics of the steel sheet.

  Therefore, the present inventors have conducted various experiments and analyzes, and found that a large amount of angular deformation can be stably obtained by applying optimum heating conditions according to the strength characteristics of the steel sheet.

This invention is made | formed based on the said knowledge, The place made into the summary is as follows.
(1) Ts (° C.) is the maximum temperature reached on the steel sheet surface during linear heating, Tr (° C.) is the maximum temperature on the back surface, and IYS (K · N / mm ² ), and the thickness of the plate is t (mm), and heating is performed under the conditions that satisfy the following formulas at the same time.

700 ≦ Ts ≦ 1000
27 × (IYS / 1000) ^0.7 × t ^−0.32 −30 ≦ Tr ≦ 27 × (IYS / 1000) ^0.7 × t ^−0.32 +30
(2) When the difference in yield strength between normal temperature and 600 ° C. is ΔYS (N / mm ² ), a steel plate satisfying ΔYS ≧ 230 and IYS ≦ 200000 is used. Method for linear heating of steel plates.

  In the linear heating method of the present invention, a large amount of angular deformation can be stably obtained by controlling the front and back surface temperatures during heating to an appropriate range according to the strength characteristics of the steel sheet used. .

In the present invention, the surface side of the steel sheet refers to the side that is heated by linear heating. Moreover, the back surface side of a steel plate means the opposite side to the said surface side. As shown in FIG. 1, heating is performed from the surface side of the steel plate 1 with a burner 2 or the like, and the heating position is sequentially moved in the direction of the heating direction 4 indicated by an arrow to be heated linearly. After the heating by the burner 2, cooling is performed by supplying cooling water to the heating part using the cooling water nozzle 3. The cooling water nozzle 3 also moves sequentially so as to keep a constant distance between the burner 2 and the cooling water nozzle 3. As a result, as shown in FIG. 2, the steel plate is angularly deformed in the plate thickness direction. Δ shown in FIG. 2 is an angular deformation amount. Here, the angular deformation amount δ is calculated as follows: δ = (1/2) × sin ⁻¹ (2d / w) where w is the dimension in the horizontal direction of heating of the steel sheet 1 and w is the deformation amount d in the vertical direction. It can be obtained using an equation.

  When a steel sheet is heated in the linear heating method, compressive plastic strain occurs on the steel sheet surface side that has reached the maximum temperature, and angular deformation can occur due to tensile stress generated when the surface side contracts during the subsequent cooling process. .

  In order to increase the amount of angular deformation, it is necessary to increase the compressive plastic strain during heating as much as possible. Usually, the yield strength on the surface of a steel sheet during linear heating is reduced to about 1/3 or less of the normal temperature regardless of the steel sheet used. Therefore, in order to increase the compressive plastic strain, it is necessary to control the yield strength on the back surface side within an appropriate range by controlling the back surface temperature of the steel sheet.

  The optimum steel plate back surface temperature at the time of heating largely depends on the strength of the steel plate to be used from room temperature to high temperature, and generally requires higher temperature for higher strength steel. However, if the temperature is too high, the compressive plastic strain is reduced, and the amount of angular deformation is also reduced. As an index for characterizing the strength characteristics of the steel sheet, not the yield strength at a specific temperature, but the temperature integrated value (IYS) of the yield strength from room temperature to 600 ° C. is suitable. The larger the IYS, the higher the optimum back surface temperature (Trc). Further, as the plate thickness increases, Trc decreases. Based on the above trend, the following equation was derived by multiple regression analysis.

Trc = 27 × (IYS / 1000) ^0.7 × t ^−0.32
In addition, normal temperature means the temperature of the steel plate before linear heating, and is the range of −10 to + 80 ° C. from the extremely cold region in winter to the high temperature in summer.

  Practically, if the back surface maximum temperature (Tr) during heating is controlled within the range of Trc ± 30 ° C., a large amount of angular deformation can be obtained as shown in FIG. With respect to the surface temperature, it is effective to control the back surface temperature as described above and to make it as high as possible within a range that does not adversely affect the material. In the present invention, the maximum surface temperature (Ts) during heating is limited to a range of 700 to 1000 ° C. If Ts is less than 700 ° C., the steel sheet as a whole has sufficient rigidity so that the amount of deformation does not increase. On the other hand, when it exceeds 1000 ° C., excessive baking occurs on the surface side by subsequent water cooling, and ductility and toughness are deteriorated.

  After linear heating, water cooling is usually performed so that the next process can be quickly performed, but it is not essential in securing a large amount of angular deformation, and is not limited in the present invention.

Since the steel plate used in the present invention is a general thick steel plate, it is not necessary to limit the components and the production method. For any steel sheet, a certain amount of angular deformation can be obtained by controlling the heating temperature as described above. However, if the purpose is to obtain as much angular deformation as possible, it is preferable to use a steel sheet having a large difference in yield strength (ΔYS) between normal temperature and 600 ° C. and a small IYS. The standard is ΔYS ≧ 230 N / mm ² and IYS ≦ 200000 K · N / mm ² .

As described above, a large amount of angular deformation can be stably obtained by performing linear heating in which the front and back surface temperatures are controlled according to the strength characteristics of the steel sheet.
(Example)
A steel plate was produced from a steel piece having the components shown in Table 1, and the linear heating method of the present invention was applied in the manner shown in FIG. The calculation method of the angular deformation amount δ is as shown in FIG. Table 2 shows the plate thickness of the steel plate, the mechanical properties of the base material, the linear heating conditions, the amount of angular deformation, and the toughness of the heated portion. No. 1-8 are examples of the present invention, No.1. 9 to 16 are comparative examples. Table 3 shows the outline of the linear heating test.

As is apparent from Table 2, Example No. of the present invention. In Nos. 1 to 8, the maximum temperature at the front and back surfaces at the time of linear heating satisfies the predetermined condition of the present invention, so that the amount of angular deformation due to linear heating is as extremely high as 4 × 10 ⁻² radian or more. .

  On the other hand, Comparative Example No. In Nos. 9 to 16, since either the surface temperature or the back surface temperature deviates from the predetermined range, the angular deformation amount is small.

  Comparative Example No. In No. 9, since the maximum surface temperature at the time of heating was lower than the lower limit of 700 ° C. of the present invention, the steel sheet had sufficient rigidity, could not give sufficient compressive plastic strain, and the amount of angular deformation was small. Comparative Example No. Nos. 10 and 12 had small amounts of angular deformation because the highest temperature reached on the back surface was high. Comparative Example No. No. 11 has a small amount of angular deformation because the back surface temperature was low.

  Comparative Example No. Since the front and back surface temperatures of both 13 and 15 were too low, the amount of angular deformation was small. Comparative Example No. In No. 14, both the front and back surface temperatures were high, so the amount of angular deformation was small, and the toughness of the heated part was also lowered. Comparative Example No. In No. 16, since the surface temperature was high, the amount of angular deformation was large, but the toughness of the heated portion was lowered.

It is a figure which shows the point of the linear heating method which concerns on this invention. It is a figure which shows the calculation method of angular deformation amount (delta). It is a figure which shows the relationship between the back surface highest achieved temperature Tr and the angular deformation amount (delta).

Explanation of symbols

1 Steel plate 2 Burner 3 Cooling water nozzle 4 Heating direction

Claims (2)

The maximum temperature reached on the steel sheet surface during linear heating is Ts (° C), the maximum temperature reached on the back surface is Tr (° C), and the temperature integrated value of the yield strength from room temperature to 600 ° C of the steel sheet is IYS (K · N / mm ² ). A method for linear heating of a steel sheet, characterized in that heating is performed under conditions that simultaneously satisfy the following formula when the plate thickness is t (mm).
700 ≦ Ts ≦ 1000
27 × (IYS / 1000) ^0.7 × t ^−0.32 −30 ≦ Tr ≦ 27 × (IYS / 1000) ^0.7 × t ^−0.32 +30 The steel sheet wire according to claim 1, wherein a steel sheet satisfying ΔYS ≧ 230 and IYS ≦ 200000 is used when a difference in yield strength between normal temperature and 600 ° C. is ΔYS (N / mm ² ). Heating method.

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