JPH01225720A - Heat treatment for steel plate having variation in plate thickness - Google Patents

Abstract

PURPOSE:To control mechanical property of the whole steel plate regardless of the plate thickness even if thick part and thin part exists in the steel plate such as the steel plate having variation in the plate thickness by using induction heating device and executing input control in accordance with the plate thickness. CONSTITUTION:A heat treatment apparatus is constituted with an induction heating coil 3 arranged around the material 2 to be heated, an induction heating power source part 4 for supplying the power into this coil 3 and a computer 5 for controlling input power under the constant carrying speed to the plate thickness of the material 2 to be heated or controlling the carrying speed under the constant input power. At first, the plate thickness to the plate length is beforehand inputted in the computer 5 and also the aimed temps. at each part is inputted. Then, the position at each part in the material 2 to be heated is sensed with a PLG, etc., attached to a table roll and the electric power or carrying velocity is controlled so as to become the aimed temp. to each position. Therefore, heating corresponding to the plate thickness can be executed.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention relates to a heat treatment method for a steel plate whose thickness varies, such as a differential thickness steel plate or a tapered steel plate.

[Prior Art] In recent years, this type of variable thickness steel plate has been widely used as bulkheads that are subjected to hydrostatic pressure in ships, pressure tanks, and the like.

Heat treatment for such steel plates with variable thickness involves charging them into a heat treatment furnace and holding them for a certain period of time to bring the thin and thick parts to a uniform temperature, as with conventional materials of the same thickness, and then cooling them in water or cooling them by air. This method is commonly used.

[Problem to be solved by this invention] However, when implementing such a conventional method, the amount of heat dissipated from the same surface area is the same regardless of the plate thickness at the same surface temperature, so the cooling curve is different depending on the thickness of the thin and thick plates. will be different. (Refer to the graph in Figure 7.) As a result, there is a difference in the cooling rate between thin-walled areas and thick-walled areas, with the cooling rate being high in thin-walled areas and slow in thick-walled areas. As shown, there is a difference in the tissue composition ratio between the thin-walled part and the thick-walled part. As a result, differences arise in the mechanical properties obtained by mechanical tests performed on thin-walled portions and thick-walled portions.

On the other hand, a method of water cooling a differential thickness steel plate or a tapered steel plate (see Japanese Unexamined Patent Publication No. 59-232606) is also known, but in this method, the cooling rate is uniformly determined for the plate thickness. Therefore, if the cooling rate for thick-walled areas is specified, the cooling rate for thin-walled areas is automatically specified, or vice versa, so it is impossible to determine the cooling rates for thin-walled areas and thick-walled areas separately. Therefore, the mechanical properties are also subject to uniform limitations.

This invention was devised in view of the above-mentioned circumstances, and its purpose is to provide a method to arbitrarily set the mechanical properties of a steel plate whose thickness varies, that is, to make the mechanical properties of thin and thick parts equal. It is an object of the present invention to provide a heat treatment method that can determine mechanical properties regardless of the plate thickness of thick and thin parts.

[Means for Solving the Problems] According to the method of the present invention, an induction heating device is used for heat treatment of a steel plate whose thickness varies in the longitudinal direction or the width direction, and the manual control is performed by adjusting the thickness of the steel plate. This will be done by performing heating corresponding to the plate thickness.

The heating temperature can be controlled according to the plate thickness, not only when the plate thickness is uniform, but also for plate thickness variable plates such as steel plates whose plate thickness changes in a fixed pattern (tapered steel plates). Any mechanical test value can be obtained by In other words, the mechanical properties can be determined regardless of the thickness of the plate, such as thick and thin parts.

(Example) The processing method of the present invention will be described below with reference to illustrated examples.

First, an induction heating apparatus 1 for carrying out the method of the present invention will be explained with reference to FIG. It consists of a power supply section 4 and a computer 5 that controls the input power by keeping the feed rate constant with respect to the thickness of the material to be heated 2, or controls the feed rate by keeping the input power constant. In this figure, reference numeral 6 is a table roller, and 7 is a pulse generator (PL).
G).

Next, a heat treatment method according to the present invention using this induction heating apparatus 1 will be explained.

First, the computer 5 preliminarily stores the plate thickness T (
x), and also input the target temperature Temp(
x) as T emp(x) = f (x , T(x) + a
+ β,...) Enter α = material coefficient and β = thermal conductivity.

Then, the position of each part of the material to be heated 2 is determined using the PLO attached to the table roll, and the target temperature Temp(
Control the power or feed rate so that x).

A tapered steel plate having the composition shown in the table of FIG. 2 will be explained. This tapered steel plate is
Slab dimensions = 210 x 900 x 1820+++n+
, rolling dimension = 13/22.5x 1890X 10
250mm.

As a result of rolling conditions (850° C. or lower at T=41 mm+3, 730° C. or lower before 3 baths, and 710±10° C. in finishing), the rolled shape shown in the graph of FIG. 3 was obtained.

And each point A, B, C, D shown in FIG.

That is, the distribution of mechanical test values at four points in the longitudinal direction of the steel plate is shown in the club of FIG. Here, YP is 7. at point A and point D.
The difference is 3 kgf/mm2.

Next, this steel plate was divided into two parts in the width direction, and one part had a rated output of 10
By the method of the present invention using an induction heating device 00,
Heating was performed to obtain a temperature distribution as shown in the graph of FIG. As a result, the difference between YP point A and D point is 1.4
kgf/mm”.

In addition, the other half of the two halves was inserted into a conventional heat treatment furnace, and the
Extraction was carried out after holding at 0"C for 30 minutes. At this time, the temperature distribution at the time of extraction was almost constant. As a result, the difference between the YPOA point and the D point was 5. 4
kg 17M”.

As described above, according to the heat treatment method of the present invention using the induction heating apparatus I, the mechanical test value distribution at each point A, B, C, and D can be made more uniform compared to the heat treatment method using the conventional heat treatment furnace. can do. That is, in a variable thickness steel plate such as a tapered steel plate, the mechanical properties of the entire steel plate can be uniformly determined regardless of the thickness of the thick portion and thin portion.

〔Effect of the invention〕

This invention method heat-treats a steel plate whose thickness varies in the longitudinal direction or width direction by using an induction heating device and controlling its input according to the thickness of the steel plate. It is characterized by carrying out corresponding heating.

According to this configuration, the heating temperature can be adjusted not only when the thickness is uniform, but also for variable thickness steel plates such as tapered steel plates whose thickness changes in a fixed pattern. By controlling it, arbitrary mechanical test values can be obtained. That is, even if there are thick portions and thin portions as in a variable thickness steel plate, the mechanical properties of the entire steel plate can be determined regardless of the thickness.

[Brief explanation of the drawing]

Fig. 1 is a schematic cross-sectional view showing an induction heating apparatus used for carrying out the method of this invention, Fig. 2 is a composition table showing the composition of the steel plate actually treated by the method of this invention, and Fig. 3 is its rolled shape. 4 is a graph showing the distribution of mechanical test values of the rolled steel plate. FIG. 5 is a graph showing the state of the rolled steel plate heat treated by the method of the present invention. FIG. Figure 7 is a graph showing the cooling curve of carbon steel, Figure 8 is CCT.
It is a graph showing a curve. DESCRIPTION OF SYMBOLS 1... Induction heating device, 2... Heated material, 3... Induction heating coil, 4... Induction heating power supply section, 5... Computer 16... Table roller, 7... pulse generator. Figure 1 L−−−−−−−−−−−−−−−−+−−−++
-''Figure 5

Claims (1)

[Claims] A heat treatment method for a steel plate whose thickness varies in the longitudinal direction or the width direction, by using an induction heating device and controlling its input according to the thickness of the steel plate. A method for heat treatment of a variable thickness steel plate, characterized in that heat treatment is performed by performing heating corresponding to the thickness of the steel plate.