JPS59126715A - Direct testing method of hardenability in rolling - Google Patents

Abstract

PURPOSE:To perform a hardening test with reliability by sandwiching a hot- rolled steel sheet with the respective cooling surfaces of a pair of plural cooling boxes to cool locally the steel sheet so that the steel sheet positioned in the intermediate part of the cooling boxes is cooled by heat conduction and the cooling rate is thus changed continuously. CONSTITUTION:Two sets a pair of cooling boxes 4A, B, 5A, B of which the cooling faces are positioned oppositely to each other are disposed at suitable intervals and are moved by movable mechanisms 6A, B, 7A, B such as air cylinders so that a hot-rolled steel sheet 1 is sandwiched by the respective cooling surfaces P. A cooling medium C is ejected to the inside of the boxes 4A, B, 5A, B and the rolling surfaces of the sheet 1 facing the surfaces P are cooled. The parts of the sheet 1 which are not sandwiched by the boxes 4A, B, 5A, B are kept air cooled, and the cooling rate is continuously changed by the heat conduction in the rolling direction of the sheet 1.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for testing the hardenability of hot rolled steel sheets.

It is well known that the properties of steel depend on its metallographic structure, especially its transformation products, and in order to obtain good material quality by optimizing the chemical composition and manufacturing process of steel, it is necessary to accurately understand the transformation characteristics. It is necessary. As a simple test method suitable for this purpose, the single-end quenching method as shown in P2O0, edited by the Japanese Standards Association, JIS Handbook of Steel (1980), is widely used.

This method involves heating and holding a cylindrical test piece to a predetermined temperature and then cooling only one end surface with water so that the cooling rate changes continuously in the axial direction due to heat conduction in the axial direction of the cylinder. There are characteristics. According to this method, the hardness or metallographic structure of steel materials when quenched at various cooling rates can be determined from X number of test pieces, so it is possible to study steel materials manufactured by conventional reheating treatment. This is an extremely useful test method for development.

On the other hand, in recent years, much research and development has been conducted on processes for manufacturing steel materials by subjecting them to controlled cooling treatment or direct quenching treatment after controlled rolling. The properties of the steel produced by these new processes are different from those produced by reheating, and the transformation characteristics of the new process cannot be determined by reheating test methods such as quenching. Since this cannot be determined, a test method is needed to evaluate the hardenability of the hot-rolled steel sheet itself.

Conventionally, as shown in FIG. 1, a one-end quenching method has been reported in which the side end 2 of a hot-rolled steel plate 1 is water-cooled (W,
B, Davis+ C, C, Ealey and
E. A. AImond: JISl, 210 (1972)
In general, the shape and surface texture of the side end surfaces of steel plates differ for each individual steel plate, so the reproducibility of the amount of heat transfer in the water-cooled part is poor, and therefore it is difficult to reproduce the distribution of the cooling rate within the steel plate due to heat conduction. 1. This method lacks reliability as a hardenability testing method. Furthermore, even if the dimensions and shape of the steel strip before rolling are the same, the width of the steel strip after rolling will be different if the rolling conditions are different, and the deformation during rolling is non-uniform in the width direction. This is a difficult point.

The present invention overcomes the above-mentioned difficulties and provides a hardenability testing method that is as reliable as the reheating and single-end hardening method. , which was established for hot-rolled steel sheets, is characterized by local cooling of the steel sheet.

The present invention will be explained below with reference to the drawings.

Fig. 2 (a) is a front view showing an example of the method of the present invention, Fig. 2 (b) is a side view, and Fig. 2 (c) is a plan view; Each pair of cooling boxes is arranged at an appropriate interval of 7 and their cooling surfaces P face each other. Also, this cooling box 4A, 4B, 5A
, 5B are connected to a cooling medium supply mechanism (not shown) that spouts a cooling medium cy such as water toward the cooling surface P.

Note that the dotted line T in the figure indicates the direction of heat conduction.

In order to conduct a hardenability test on a hot rolled steel plate by the method of the present invention, first, cooling boxes 4A, 4. B and cooling box 5A,
5B are arranged at a predetermined interval. Therefore, a high-temperature steel plate that had been rolled was brought in, and the pair of cooling boxes 4A, 4B, 5A, and 5B were moved by appropriate movable mechanisms such as air cylinders 6A, 6B, 7A, and 7B. , 4B, 5A, and 5B cooling surfaces P make the steel plate■
Find 2 pieces. Furthermore, cooling boxes 4A, '4B, 5A,
A cooling medium C such as water is ejected from a cooling medium supply mechanism (not shown) into each of the steel sheets 5B to cool the rolled surface of the steel plate 1 facing the cooling surface P.

As a result, the steel plate 1 is locally rapidly cooled. Also at this time, the cooling boxes 4A and 4B for the steel plate 1.

The parts not sandwiched between 5A and 5B are air-cooled or heat-insulated, and the cooling rate is continuously changed by heat conduction in the rolling direction of the steel plate. In this case, the rolled surface of the steel plate is almost flat and the oxide film is dense, so the heat transfer coefficient during water cooling hardly depends on the heating conditions or rolling conditions of the steel plate. Therefore, if the interval between the two pairs of cooling boxes is kept constant, the reproducibility of the distribution of the cooling rate of the steel plate in the rolling direction is extremely good, and is equivalent to the reproducibility of the cooling rate distribution in the axial direction of the reheating single-end quenching method. Become. Furthermore, in the center of the steel plate, deformation during rolling is uniform in the rolling direction, so it is also possible to conduct tests by continuously changing the cooling rate under the same rolling conditions.

An example of the cooling rate distribution in the rolling direction of a hot-rolled steel plate when the method of the present invention is carried out is shown in FIG. As is clear from this figure, the cooling rate changes continuously from the water-cooled region to the air-cooled region, so it is impossible to know the hardness and structure of the steel material when quenched under various conditions from a single test piece. I can do it.

In the figure, the interval between the air cooling areas is 7 Qmm, and the depth is 2 from the steel plate surface.
mrn, B indicates the center of plate thickness, and T indicates the direction of heat conduction.

Next, Example 7 according to the method of the present invention will be shown.

A hot-rolled steel plate (temperature 900C) having a length of 1 m, a width of 160111 m, and a thickness of 22+ nm was locally quenched using the method of the present invention. The thickness of each cooling surface is 1401111X
The interval between the cooling boxes of 140+11111 is g 7 Q mm and fc.

Table 1 shows the reproducibility of the cooling rate at a given location equivalent to 20 C/sec.

Table 1 *Side end surface hardening method shown in FIG. 1 Generally, as the rolling reduction ratio increases, the side end surface of a hot rolled steel plate becomes noticeably uneven and cracks on the surface become severe. Therefore, when testing a steel plate with a high rolling reduction ratio using the side hardening method as shown in FIG. 1, the reproducibility of the cooling rate at a predetermined position becomes poor.

On the other hand, in the method of the present invention, since the rolled surface is hardened as shown in FIG. 2, the cooling rate at a predetermined position is independent of the rolling reduction ratio and has good reproducibility as shown in Table 1. Therefore, the hardenability test can be carried out accurately.

As explained above, according to the present invention, it is possible to perform a hardenability test on a hot-rolled steel sheet with the same reliability as the reheating and one-end hardening method.

[Brief explanation of the drawing]

FIG. 1 is an explanatory view showing a conventionally known hardenability test, in which (a) is a front view and (b) is a side view. FIG. 2 is an explanatory diagram showing an example of the method of the present invention, in which (a) is a front view, (b) is a side view,
(C) is a plan view. Also, FIG. 3 is an example of the cooling rate distribution in the rolling direction of a hot-rolled steel plate according to the method of the present invention.
This is a chart showing. ■...Steel plate 2...Side end 3...Water cooling 4A, 4B, 5A, 5B...Cooling box 6A,
6B, 7A, 7B...-Movable mechanism P...-Cooling surface C...Cooling medium T...Heat conduction Figure 1 (A) (
υ) Figure 2 (a) - (b)

Claims (1)

[Claims] A plurality of pairs of cooling boxes with their cooling surfaces facing each other are arranged at appropriate intervals, and a hot rolled steel plate is sandwiched between the cooling surfaces of the cooling boxes to cool locally, and A rolling direct hardenability test method characterized by continuously changing the cooling rate so that the steel plate located in the section is cooled by heat conduction.