JPH11181517A - Method for dehydrogenating steel - Google Patents

Abstract

PROBLEM TO BE SOLVED: To effectively remove hydrogen in a steel without deteriorating the mechanical properties of the steel by heating a steel from an ordinary temp. and executing heating in a specified temp. range for >= specified time. SOLUTION: By holding a steel at 200 to 500 deg.C, at 200 to 300 deg.C and at 400 to 500 deg.C for above the time (t) at least satisfying the relation of (D.t)<1/2> =(1/2 the minimum thickness of the product) (D denotes the diffusion coefficient of hydrogen in each holding temp. in α iron), hydrogen in a relatively movable state trapped in the dislocation of the steel is released at >=200 deg.C, and hydrogen in a state of being strongly trapped in precipitates and grain boundaries is released at >=400 deg.C respectively. The temp. range of 200 to 500 deg.C hardly exerts influence on heat history in heat treatment or the like for maintaining the mechanical properties of the steel product. In consideration of the productivity of the steel, each holding time in each temp. range is preferably regulated to <=(t)+2 hr.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for dehydrogenating steel to prevent delayed fracture of a steel product and maintain a predetermined strength and hardness.

[0002]

2. Description of the Related Art In order to prevent defects and delayed fracture immediately after production due to diffusible hydrogen in steel, conventionally, degassing of molten steel, slow cooling of cast steel pieces, slow cooling of steel products, and dehydrogenation by reheating / reheating / holding have been performed. Has been done.

For example, Japanese Patent Laid-Open Publication No. Sho 56-163217 discloses a method of gradually cooling a cast steel slab to 500 ° C. in a closed pit. Also, Japanese Patent Application Laid-Open No. Sho 52-117231
In the official gazette, after the cast billet is sufficiently transformed by cooling,
A method is shown in which reheating is performed just below the Ac ₁ transformation point and the temperature is maintained.

On the other hand, as a method for dehydrogenating steel products, there is a method of stacking steel sheets in order to enhance the slow cooling effect as described in Japanese Patent Application Laid-Open No. 52-156709, for example.

As described above, in the conventional dehydrogenation method, the steel material is gradually cooled or reheated and kept at a relatively high temperature just below Ac ₁ (about 600 to 700 ° C.).

[0006]

As a method for dehydrogenating steel, slow cooling and reheating / retaining at high temperature immediately below Ac ₁ are effective. At this time, the dehydrogenation of the cast steel slab is because the thickness of the steel material is large, there are minute voids, and gasified hydrogen in the voids is difficult to escape from the steel. The effect is greater. However, with respect to dehydrogenation of steel products, it is necessary to satisfy mechanical properties such as strength and hardness at the same time.In other words, the steel products have undergone thermal histories such as heat treatment to maintain mechanical properties. In many cases, the thermal history considering only the dehydrogenation treatment described above cannot maintain the mechanical properties. Therefore, it is an issue to carry out dehydrogenation effectively while maintaining mechanical properties.

[0007]

SUMMARY OF THE INVENTION In order to achieve the above object, the present invention studies dehydrogenation in steel, and the hydrogen in steel is in a relatively mobile state trapped in dislocations and in the form of precipitates and crystals. There are those that are strongly trapped in the grain boundaries,
When the temperature of the hydrogen is increased from room temperature, hydrogen trapped in dislocations is released at 200 ° C. or higher, and when the temperature is further raised, hydrogen trapped in precipitates and grain boundaries is released at 400 ° C. or higher. I found that. That is, the present inventors have found that even if the temperature is not higher than 600 ° C. immediately below Ac ₁ used in the conventional dehydrogenation, the hydrogen trapped in the steel can move at the above temperature.

[0008] The present invention has been completed based on the above findings, and the gist of the present invention is to heat a steel product from normal temperature, and to heat the steel product from 200 to 50.
At 0 ° C., 200 to 300 ° C. or 400 to 500 ° C., at least (D · t) ^1/2 = (1/3 of the minimum product thickness)
The dehydrogenation treatment of steel is characterized in that the steel is held for a time t or more that satisfies the relationship 2) (where D is the diffusion coefficient of hydrogen in α-iron). Since the dehydrogenation treatment is performed in a temperature range of 200 to 500 ° C., dehydrogenation can be performed without substantially affecting the heat history such as heat treatment for maintaining the mechanical properties of the steel product.

[0009]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, the present invention will be described in detail.

It is well known that in order to release hydrogen which has become free from diffusion in steel out of the steel, it must be moved by a distance corresponding to the square root of the product of the diffusion coefficient and time at each temperature. In order for hydrogen to be released out of steel, hydrogen existing in the center of steel products moves to both sides,
It has to move at least half the thickness of the product.
Therefore, it is necessary to hold the time t that satisfies at least (D · t) ^1/2 = ( ^１ ／ of the minimum product thickness) for each time. At this time, the longer the retention time, the better the dehydrogenation effect, and there is no upper limit, but considering productivity, the retention time is preferably t + 2 hours or less. Here, D is the diffusion coefficient of hydrogen in the α steel at each temperature. The conventional dehydrogenation method is also basically a method using the effect of the time required for this diffusion.

Here, the reason why 200-300 ° C. and 400-500 ° C. are particularly effective among 200-500 ° C. will be described. In other words, relatively movable hydrogen trapped weakly by dislocations is dissociated at 200 ° C. or more and starts diffusing, but the effect is the same even at 300 ° C. or more. Therefore, when only hydrogen trapped in the dislocations or the like is diffused, it is preferable to set the temperature to 300 ° C. or less, because unnecessary heating is not required, and it is economically desirable, and also from the viewpoint of preventing a reduction in material strength.

At a temperature of 400 ° C. or more, hydrogen firmly trapped in precipitates and crystal grain boundaries is also dissociated and begins to diffuse, overlapping the above-described effects.

On the other hand, it is well known that the strength of a steel material is reduced by tempering. Therefore, in order to obtain a desired strength, tempering is omitted or tempering is performed at a low temperature. In such a case, according to the conventional dehydrogenation method, the target strength and hardness cannot be maintained if the temperature is maintained immediately below Ac ₁ for a long time. For this reason, in the present invention, the upper limit temperature is set to 500 ° C.

Therefore, 200 to 40 hardly affects the reduction in strength or hardness, which is the gist of the present invention.
Hydrogen diffusion treatment at a temperature lower than 0 ° C, preferably 200 to 300 ° C, or 400 to 50 corresponding to a low tempering temperature.
The hydrogen diffusion treatment at 0 ° C. enables dehydrogenation while maintaining the strength of the target steel product.

Here, the term "holding" refers to performing heat treatment at a predetermined temperature for a predetermined time.

In the present invention, 200-500 ° C., 200-500 ° C.
Within the temperature range of 300 ° C. or 400 to 500 ° C., the temperature may change during the holding.

[0017]

EXAMPLES Examples of the present invention will be described below by taking rail steel as an example and comparing with a conventional method.

The rail steel used was a steel having two types of components A and B shown in Table 1 and was rolled and deformed into an AREA 132 pound rail by a conventional method.

The minimum head thickness of this rail (steel) is 45
mm.

[0020]

[Table 1] Table 2 shows the conditions of the above-mentioned rail dehydrogenation method, the amount of hydrogen in steel before and after dehydrogenation, and the hardness.

The amount of hydrogen in the steel was measured by a temperature-raising gas chromatograph method, and the hardness was measured by a Vickers hardness test method of JIS Z 2244. The diffusion coefficient of hydrogen in steel can be found, for example, in “Metal Data Book” edited by the Japan Institute of Metals.
(Maruzen, published on January 30, 1984), that is, the self-diffusion coefficient; D ₀ = 4.2 × 10 ⁻⁸ (m ² / s)
And activation energy; Q = 3.85 (kJ / mo)
using l), was determined from the equation _{D = D 0 exp (-Q /} RT). At this time, R is a gas constant (= 8.3 J / deg · m)
ol), and T is the absolute temperature at the time of retention.

As shown in Table 2, in the method of the present invention, 200
The dehydrogenation treatment was performed while maintaining the temperature within the range of ~ 500 ° C. In any case, as is clear from the numerical values of the amount of hydrogen before dehydrogenation and the amount of hydrogen after dehydrogenation, hydrogen was effectively removed. Removed. In addition, there was almost no difference between the hardness (Hv10) before dehydrogenation and the hardness (Hv10) after dehydrogenation as mechanical properties of steel.

On the other hand, when dehydrogenation was carried out at a temperature of 650 ° C. as a comparative method, the dehydrogenation amount was as good as in the method of the present invention, but the hardness after dehydrogenation was dehydrated. The hardness was significantly lower than the hardness before bare.

From these results, it was confirmed that according to the method of the present invention, hydrogen in steel can be effectively removed without deteriorating the mechanical properties of steel.

[0025]

[Table 2]

[0026]

According to the present invention, it is possible to effectively remove hydrogen caused by delayed fracture of steel while maintaining the mechanical properties of the steel product.

Claims (3)

[Claims] 1. A steel material is heated from room temperature, and
A method for dehydrogenating steel, wherein the temperature is maintained at a temperature t or more that satisfies the following (Equation 1). (D · t) ^1/2 = h / 2 (Equation 1) where D: diffusion coefficient of hydrogen in α-iron at the holding temperature h: minimum thickness of steel 2. A steel material is heated from room temperature to a temperature of 400 to 500.
A method for dehydrogenating steel, wherein the temperature is maintained at a temperature t or more that satisfies the following (Equation 1). (D · t) ^1/2 = h / 2 (Equation 1) where D: diffusion coefficient of hydrogen in α-iron at the holding temperature h: minimum thickness of steel 3. A steel material is heated from room temperature, and
A method for dehydrogenating steel, wherein the temperature is maintained at a temperature t or more that satisfies the following (Equation 1). (D · t) ^1/2 = h / 2 (Equation 1) where D: diffusion coefficient of hydrogen in α-iron at the holding temperature h: minimum thickness of steel