JPH05140642A - Method for dehydrogenizing hot rolled steel material - Google Patents

Abstract

PURPOSE:To dehydrogenize a hot-rolled steel material without changing the mechanical properties at a low cost by slowly cooling the steel material after completing hot-finish rolling and rapidly reheating it to neat the recrystallizing temp. at a specific heating rate in the slow-cooling stage. CONSTITUTION:The steel material is hot-rolled, and is slowly cooled to <=about 200 deg.C with air-cooling after completing the finish rolling. This steel material during slow-cooling stage is rapidly reheated to near the recrystallizing temp., e.g. to about 500 deg.C at >=1 deg.C/sec heating rate. It is preferable that the reheating is executed by using the induction coil. By this reheating, diffusion and discharge of the hydrogen in the steel material is made active and the dehydrogenization is completed till developing the delayed fracture. In this way the steel material can be dehydrogenized on line within a short time and at a low initial cost and low running cost.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a dehydrogenation method for removing hydrogen from hot rolled steel.

[0002]

2. Description of the Related Art It has been known that in some steel materials, defects (delayed fracture) due to hydrogen in steel are likely to occur. Methods for reducing hydrogen in steel are roughly classified into (1) a vacuum degassing treatment method for reducing hydrogen at the molten steel stage, (2) steel for reducing hydrogen at the stage of blooming, billet, slab or rolling thereof. Ingot / steel degassing method. The method of the present invention belongs to a steel ingot / steel material degassing treatment method.

Vacuum degassing treatment methods include RH degassing method and L
There are a D degassing method, an ASFA-SKF method and the like, and dehydrogenation is promoted by lowering the hydrogen partial pressure in the atmosphere.
The steel ingot / steel material degassing treatment method includes a steel ingot zag press bonding, dehydrogenation gradual cooling method, and the like, in which hydrogen in the steel is diffused and removed by the high temperature holding by the zag press bonding and gradual cooling.

[0004]

The conventional dehydrogenation methods include:
Although there are few problems related to quality function of dehydrogenation, there are problems in terms of initial cost and running cost.

An object of the present invention is to provide a dehydrogenation method for hot-rolled steel, which is more cost effective than conventional methods.

[0006]

According to the present invention, the above object is to rapidly cool a steel material in a slow cooling stage after finishing rolling of hot rolling to a temperature close to a recrystallization temperature at a heating rate of 1 ° C./sec or more. This is achieved by the method of dehydrogenation of hot-rolled steel material which is reheated.

[0007]

In the method of the present invention, after finishing rolling, the steel material which has been slowly cooled and packaged is about 500 at the slow cooling step.
Rapid reheating to ° C activates the diffusional release of hydrogen, which is released in about 30 minutes and thus before delayed fracture occurs.

[0008]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A preferred embodiment of the hot-rolled steel dehydrogenation method according to the present invention will be described below with reference to the drawings. FIG. 1 shows the steps of hot rolling, air cooling, and packing of steel materials in order. (A) shows the slabbing process of hot rolling, (b) shows the finishing process of hot rolling, (c) shows the slow cooling (air cooling) process after hot rolling, and (e) shows The packing process performed when the steel material is cooled to about 200 ° C. or lower is shown.
In the present invention, the rapid reheating step (d) is provided between the steps (c) and (e), and the steel material in the slow cooling stage is heated from a temperature of around 200 ° C at a heating rate of 1 ° C / sec or more. Rapidly reheat to a temperature as high as possible up to near the recrystallization temperature.

As a rapid reheating means, as shown in FIG. 1, an induction coil 2 is provided, and a rolled steel material 4 is passed through it,
An eddy current is generated in the rolled steel material 4, and the rolled steel material is rapidly heated by the generated Joule heat.

The dehydrogenation mechanism by the rapid reheating process is as follows. First, the mechanism by which hydrogen induces delayed fracture of steel materials is summarized as follows. (1) Hydrogen atoms that enter and form a solid solution in steel are trapped and accumulated in the zag portion (porous portion) of the central portion of the steel ingot or around non-metallic inclusions during the casting stage. (2) In hot rolling, it generally takes time for hydrogen to escape when the distance from the center of the steel material to the surface is long. (3) The time and the extent of delayed fracture are the amount of hydrogen,
Depending on the structure of the steel (for example, pearlite, bainite, martensite), it is 1-2 if ferrite and pearlite are mixed with bainite from conventional experience.
Cracks occur after time.

Therefore, hydrogen must be diffused and released after rolling until delayed fracture occurs. In this sense, after finishing rolling is finished, the steel materials that have been cooled and packed are reheated in the cooling process to make hydrogen diffusion and release more active (diffusion coefficient is higher at higher temperatures), and dehydrogenation occurs until delayed fracture occurs. Was completed. When induction heating is used for reheating, it is possible to heat in a short time, and it was found that by heating the steel material near the recrystallization temperature, for example, about 500 ° C, hydrogen diffusion and release can be completed in about 30 minutes. It was found that dehydrogenation was possible before the destruction occurred.

The reason why the temperature is close to the recrystallization temperature is to prevent the mechanical properties such as the yield point from changing when the recrystallization temperature is far exceeded, because the crystal grains become coarse.

A test was conducted to confirm the effect of the dehydrogenation method of the present invention. The test material is SM50A-E, and the chemical composition is 0.16C-0.236Si-1.40Mn- in% by weight.
0.015P-0.006S-0.06Cu-0.04
Ni-0.07Cr-0.01Mo-0.026Al-
0.001V-0.0084N-0.0027O-0.
It was 0004H. The shape was hot rolled flat steel having a length of 5 m and a thickness of 25 mm. As shown in FIG. 2, ten test points aj were taken. Then, as shown in FIG.
When air-cooled to 0 ° C, the three points a, b, and c were rapidly heated to 500 ° C, and the remaining seven points d-j were air-cooled from 300 ° C without heating. Therefore, 3 of a, b, and c
The method of the present invention was applied to the points, and 7 points of dj were air-cooled as in the conventional method.

An ultrasonic (UT) flaw detection test was conducted on points aj. The results are shown in FIG. That is,
UT defect echo height (100% B ₁ +20 dB)
None at points a, b, and c, but 2 at points d-j
It was 5-90%. From this, it is understood that dehydrogenation is not complete at points d-j to which the method of the present invention is applied, but dehydrogenation is not complete at points d-j, and hydrogen defects are present, which may cause delayed fracture. I understand.

The induction rapid reheating in the embodiment of the present invention is
Compared to the conventional dehydrogenation method, the equipment introduction cost (initial cost) and operating cost (running cost) are cheaper. Further, since it is possible to process online and it is not necessary to stack steel materials at a high temperature for heat retention, the movement of cargo does not stop, which is advantageous in terms of physical distribution.

[0016]

EFFECTS OF THE INVENTION According to the present invention, since the steel material in the slow cooling stage after hot rolling is rapidly reheated to near the recrystallization temperature, it is possible to dehydrogenate before the occurrence of delayed fracture, and moreover to the conventional dehydrogenation method. It can be carried out at a lower cost than that.

[Brief description of drawings]

FIG. 1 is a process diagram of a dehydrogenation method for hot-rolled steel material according to an embodiment of the present invention.

FIG. 2 is a plan view and a damage test result diagram of a test material according to the method of the present invention.

FIG. 3 is a temperature history diagram of each portion aj of the test material of FIG.

[Explanation of symbols]

 2 induction coil 4 rolled steel

Claims (1)

[Claims] 1. A method for dehydrogenating a hot rolled steel material, which comprises rapidly reheating a steel material in an annealing step after hot finish rolling to a temperature near a recrystallization temperature at a heating rate of 1 ° C./sec or more.