JPS59182911A - Heating method of steel ingot - Google Patents

Abstract

PURPOSE:To prevent surely generation of a heating crack in the surface part near a transformation temp. by heating a steel ingot at the heating rate regulating the specific temp. just before the surface temp. of the ingot attains the AC1 of this kind of steel or below then heating the ingot at the max. heat efficiency. CONSTITUTION:A steel ingot of the steel kind with which the time for attaining the nose part of the end line of pearlite transformation in an isothermal transformation chart is >=15 minutes and which is particularly liable to crack and has >=0.5m diameter or subtence distance is heated in the following way: The steel ingot is heated at the heating rate expressed by the equation in about 100 deg.C prior to starting the AC1 transformation of this kind of steel or below. The D(m) in the equation is the diameter or subtense distance of the steel ingot. The steel ingot right after the heating is then heated at the max. heat efficiency to prevent surely the crack in the stage of heating and to avoid wasteful slow heating as in the prior art, by which the heating efficiency is improved and the heating time is reduced.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an efficient method for heating steel ingots that are prone to thermal cracking. Medium to low-alloy steel that is prone to hardening
Large steel ingots often develop deep horizontal cracks on the surface when heated, rendering them unusable.

Conventionally, this type of heating cracking occurs when the surface compression-internal tensile stress that occurs when heating and temperature increases passes through the transformation point.
It was presumed that this was caused by a temporary reversal and a change from surface tension to internal compression. However, in actual heating of steel ingots,

The present invention has been made in view of the above considerations, and its gist is that the diameter or distance across opposite sides of the steel type that takes 15 minutes or more to reach the nose of the end line of pearlite transformation in the isothermal transformation diagram is 0.5 m or more. When heating a steel ingot, the surface temperature of the steel ingot is approximately 100°C just before reaching point t of the relevant steel type.
This is a method of heating a steel ingot, which is characterized by raising the temperature at a rate of (110-40D') °C/hr or less, and then performing heating with maximum thermal efficiency. Here, D is the diameter or distance across opposite sides of the steel ingot. That is, the inventors collected data on the type of heating furnace, steel type, steel ingot type, furnace atmosphere temperature curve, and presence or absence of cracking from the heating furnace operation records that had been accumulated in the past, and entered this data into a computer. Then, the surface temperature and center temperature of the steel ingot are estimated and calculated from the ambient temperature.
I also drew a graph showing the changes over time.

Conventionally, these calculations and graphing have not been performed because the calculations are extremely troublesome. The steel ingot surface and center temperatures were calculated from the ambient temperature by solving the following equations.

×'(To-T,)...(3) Here, Kd° standard thermal conductivity, S, internally generated heat amount, 089
Surface heat absorption amount, ψcg: overall heat absorption rate, αC: heat transfer coefficient. As a result of analyzing these many graphs, it was found that for the size of the steel ingot within the range of investigation, the temperature difference between the surface layer and the center of the steel ingot is greater than a certain temperature, regardless of the size of the steel ingot. We found that cracks do occur in some cases, but only for a certain period before the start of transformation, and after that there is a tendency for cracking to occur even at high heating rates = 3 - This was confirmed through additional experiments. However, the heat removal period should be approximately 100 °C just before the surface layer reaches point A, and the heat removal rate can be determined by changing the diameter or opposite side of the steel ingot to D.
(m), then (11°-40I)2)°C/h, and the present invention was completed by discovering that it is possible to form a -film. Table 1 shows examples of the present invention together with conventional examples and comparative examples. Also, those in Table 1 are shown in Figures 1 to 8.

A graph of temperature over one hour in each case is shown. In the case of the embodiment of the present invention, no cracking occurred, and in the conventional case of Table 1, heat was removed during the progress of transformation as shown in Fig. 5, but in the present invention, as shown in Fig. 6, heat was removed after the transformation started. It can be seen that the heating rate is significantly faster, that is, maximum efficiency heating is achieved, and the heating time is shortened. It goes without saying that in the present invention, careful heating is required in the low-temperature brittle region, taking care to avoid cracking due to thermal stress and strain, as in the past. In addition, the surface temperature is measured on a specific steel ingot in a specific furnace.
If you find the relationship with the furnace gas temperature, you won't have to actually measure it every time.

In the present invention, the time required to reach the nose of the end line of pearlite transformation in the isothermal transformation diagram is 1.
The test time was limited to 5 minutes or more because cracks often occur in these steel types. Specifically, Ni-Cr-Mo steel, which is a structural low alloy steel, and Ni
-Cr steel +7) High Ni content corresponds to this. There are various theories as to why these steel types crack more easily than other fast-transforming steel types, but it has not yet been clarified.
Ha + art also ancient nu η I ten together m 幇0
．． The reason why the length was limited to 5 m or more is because if the length is 0.5 m or less, cracks hardly occur no matter how fast the heating rate is.

As has been made clear above, the heating method of the present invention is characterized by heating a steel ingot that is prone to cracking due to heating near the transformation temperature. This method has many advantages, such as avoiding wasteful heat removal, improving heating efficiency, and shortening heating time.

[Brief explanation of drawings]

FIGS. 1 to 6 are temperature rise curve diagrams of a conventional example, a comparative example, and an example of the present invention. A: Ambient temperature, B: Steel ingot surface temperature, C: Steel ingot center temperature, D: Temperature difference between surface and center Patent applicant: Sanyo Special Steel Co., Ltd.

Claims (1)

[Claims] Or, when heating a steel ingot with a distance across flats of 0.5 m or more, the surface temperature of the steel ingot is approximately 10°C, just before reaching AC- of the steel type.
If 0°C is the diameter of the steel ingot or the distance across opposite sides is D (m), then (110-40D') Heating of a steel ingot is characterized by raising the temperature at a rate of 0°C/h or less, and then heating the steel ingot with maximum thermal efficiency. Method.