JP2000345366A - Manufacturing method of steel material with excellent weather resistance - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent flowing rust and to thickly form stable rust at a short time even in use in a coastal region by coating the surface of steel with iron powder of specified grain size and thereafter subjecting it to heat treatment of executing heating and cooling to specified temp. ranges and rust promoting treatment as well. SOLUTION: The surface of steel is coated with iron powder of 1 to 100 μm average grain size. As the iron powder to be used, pure iron powder such as reduced iron powder is preferable, and this is uniformly dispersed into water and is thereafter applied by a doctor blade method, or the like. Next, the steel is subjected to heat treatment of executing heat treatment at 500 to 1000 deg.C and cooling after that to a room temp. In this way, the iron powder and the surface of the steel and each powder are slightly sintered to form an iron powder layer on the surface of the steel. The thickness of the iron powder layer is suitably controlled to 10 to 100 μm. Moreover, the steel is subjected to rust promoting treatment. This treatment is executed preferably in such a manner that the steel is retained in an atmosphere in which the dew temp. is fixedly held, and the temp. of the steel repeatedly varries between the temp. range of >=(the dew point temp. +5 deg.C) and the temp. range off(the dew point -5 deg.C).

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a weather-resistant steel material, and more particularly to a method for producing a weather-resistant steel material having a weather-resistant stable rust formed on the surface of the steel material at an early stage. Note that the weather-resistant steel is a steel specified in JIS G 3114. The steel material in the present invention includes a thick steel plate, a thin steel plate, a bar steel, and a shaped steel.

[0002]

2. Description of the Related Art Weather-resistant steel in which alloying elements such as P, Cu, Cr, and Ni are added to steel to improve corrosion resistance in the atmosphere is a stable rust that is hardly permeated by oxygen and water that cause corrosion outdoors. A rust that forms in a few years. This stable rust is dark brown compared to rust in ordinary steel, and is harmonized with the scenery, and since this stable rust has the effect of suppressing subsequent corrosion, the application of rust preventive paint is unnecessary,
Weather-resistant steel is widely used as an inexpensive and highly corrosion-resistant material that can be used barely.

However, since it takes several years for stable rust of weathering steel to be formed, when used in a rural area, flow rust called rust soup is generated during that time, and there is a problem with the landscape. Was. On the other hand, when used in the coastal zone, stable rust is hardly formed due to the action of flying sea salt particles, corrosion is not suppressed, and peeling rust called floating rust also occurs to a considerable extent The problem remains.

To cope with such a problem, a method of applying a paint to the surface of a steel material and shielding the steel material from a corrosive environment by a coating film is widely used. However, in the method of applying a paint to the surface of a weather-resistant steel material to form a coating film, the corrosion resistance of the coating film is high, and it takes several decades to form a stable rust layer. Due to the problem of peeling due to deterioration of the film, it is unsightly in appearance, and if the deterioration of the coating film is particularly severe due to the environment, it is necessary to apply it again, furthermore, due to the difference in the degree of progress of corrosion under the coating film Also, there is a problem that color unevenness occurs.

[0005] In order to solve these problems, several methods have been proposed for forming a stable rust at an early stage. For example, Japanese Patent Publication No. 7-37672 discloses a rusting weather-resistant steel sheet treated with a rusting treatment solution having a special composition. Japanese Patent Application Laid-Open No. 1-142088 discloses a surface treatment method for weather-resistant steel in which the surface of a steel sheet is treated with an acidic solution containing iron ions, and then a phosphate film is formed.

However, each of these methods is a method for forming a stable rust directly from a weather-resistant steel sheet, and there is a limit in quickly producing a stable rust having a sufficient thickness.

[0007]

DISCLOSURE OF THE INVENTION The present invention advantageously solves the above-mentioned problems, prevents the occurrence of flow rust, and forms a thick stable rust in a short time to improve weatherability even in use in coastal areas. An object of the present invention is to propose a method for producing a steel material having excellent weather resistance.

[0008]

Means for Solving the Problems In order to achieve the above-mentioned object, the present inventors have intensively studied a method for forming a stable rust quickly and thickly on a weather-resistant steel material. As a result, the present inventors have found that corrosion proceeds on the surface, and in order to form a rust layer quickly and thickly, it is effective to increase the surface of the steel material in a three-dimensional manner. Has come to the conclusion that it is important that they are connected to the steel surface. Based on this idea, the present inventors focused on iron powder, applied fine iron powder to the surface of steel material, sintered the surface of the steel material by heat treatment, and then performed rust acceleration treatment, the same rust acceleration treatment, It has been found that even when applied, the corrosion progresses rapidly, and that stable rust adhered to the steel surface can be formed quickly. In addition, it has been found that a stable rust can be formed thick by appropriately increasing the thickness of the sintered iron powder.

[0009] When rust accelerating treatment is performed by simply allowing iron powder to exist on the steel sheet surface without sintering the iron powder on the steel sheet surface, rust is formed, but most of the rust is floating rust. And many of them were dropped off by subsequent washing, and rust unevenness was generated. The reason for this has not been clarified at present, but if iron powder is merely present on the steel surface and does not adhere firmly to the steel surface, corrosion occurs at the contact surface between the iron powder and the ground iron. It is considered that the progress has progressed and the adhesion between the ground iron and the iron powder cannot be secured.

The present invention has been completed based on the above-mentioned findings and further studies. That is, according to the present invention, after applying iron powder having an average particle size of 1 to 100 μm to the surface of a weather-resistant steel material, the weather-resistant steel material is subjected to a heat treatment of heating and cooling to a temperature range of 500 to 1000 ° C. The present invention also provides a method for producing a steel material having excellent weather resistance, characterized by further performing a rust accelerating treatment.In the present invention, an iron powder layer having a thickness of 10 to 100 μm is formed on the surface of the weather resistant steel material. It is preferable to apply iron powder so that

In the rust accelerating treatment of the present invention, the weather-resistant steel material is held in an atmosphere in which the dew point temperature is kept constant, and the temperature of the weather-resistant steel material is set to (dew point temperature + 5 ° C.) in the atmosphere. It is preferable that the temperature is repeatedly changed between the above temperature range and the temperature range of (dew point temperature −5 ° C.) or less,
In the rust accelerating process of the present invention, the atmosphere is preferably set to an oxygen gas concentration of 15 to 50% by volume,
Further, the temperature of the weather-resistant steel material is set at a rate of 0.1 to 2 ° C.
/ Min, and the temperature decreasing rate may be varied as 0.01 to 2 ° C / min.

In the rust accelerating process of the present invention, the temperature of the weather-resistant steel is set to (dew point temperature +5) in an atmosphere where the dew point temperature is kept constant and the oxygen gas concentration is 15 to 50% by volume.
℃) and the temperature range below (dew point -5 ℃), the rate of temperature rise is 0.1 ~ 2 ℃ / min, the rate of cooling is 0.01
The temperature may be periodically varied as ~ 2 ° C / min.

[0013]

DETAILED DESCRIPTION OF THE INVENTION In the present invention, iron powder having an average particle size of 1 to 100 .mu.m is applied to the surface of a weather-resistant steel material. The iron powder to be used is preferably a pure iron powder such as reduced iron powder, water atomized iron powder, gas atomized iron powder, carbonyl iron powder and the like. Further, other than pure iron powder, low alloy steel powder containing some alloying elements may be used as long as stable rust can be formed at an early stage.

If the average particle size of the iron powder exceeds 100 μm, the effect of increasing the surface area is reduced, and the rust layer formed is not uniform. On the other hand, when the average particle size of the iron powder is less than 1 μm, the effect of promoting the formation of the rust layer is saturated, and the powder becomes expensive, which is economically disadvantageous. For this reason, the average particle size of the iron powder is set to 1 to 100 μm. As a method of applying iron powder to the weather-resistant steel material surface, iron powder of the above average particle diameter, liquid,
Preferably, it is added to water, stirred, uniformly dispersed, and then, for example, applied to a steel material surface by a doctor blade method or the like. The thickness can be arbitrarily adjusted to a desired thickness, which is preferable. Examples of the liquid to which the iron powder is added and dispersed include alcohol and acetone in addition to water. After application, drying is preferred.

[0015] The weather-resistant steel material coated with iron powder on the surface is then heated at 500 to 1000 ° C in order to sinter the iron powder on the steel material surface.
Heat treatment for heating and cooling to a temperature range of If the heating temperature is lower than 500 ° C., the sintering of the iron powder and the steel surface and between the iron powders does not proceed, and the adhesion between the iron powder and the steel surface is insufficient, so-called floating rust often occurs. On the other hand, 1000 ℃
If it exceeds sintering, the sintering of the iron powders excessively progresses, and the effect of increasing the surface area is lost. Therefore, the heating temperature for heat treatment is 500
It should be in the range of ~ 1000 ° C. In addition, preferably, 600 to 800
° C.

After being heated to the above temperature range, it is cooled to room temperature. Cooling conditions are not particularly limited. Note that the heat treatment is preferably performed in a non-oxidizing atmosphere to prevent surface oxidation of the steel material. The non-oxidizing atmosphere is preferably a nitrogen gas, an inert gas, or a hydrogen gas atmosphere. By this heat treatment, the iron powder, the surface of the steel material, and the iron powder are lightly sintered, the iron powder is fixed in a layered manner on the surface of the steel material, and an iron powder layer is formed on the surface of the steel material. The thickness of the iron powder layer formed on the steel material surface is preferably in the range of 10 to 100 μm. Since the thickness of the iron powder layer is slightly reduced by the heat treatment for sintering, it is preferable to adjust the thickness of the iron powder applied to the steel material surface so as to have this thickness. In addition,
The thickness of the iron powder layer after sintering is flat at the tip (for example, 3 mm
With a micrometer having a contact of φ), the thickness of the steel material before and after the application of the iron powder and after the sintering is measured at least at five points each, and the difference between the average value of the latter and the average value of the former is measured. Ask. If the thickness of the iron powder layer is less than 10 μm, the formed rust layer is too thin to exhibit sufficient weather resistance, while if it exceeds 100 μm, oxygen diffuses into the ground iron during the rust acceleration treatment. The corrosion of the base iron does not progress, and the adhesion between the rust layer and the base iron decreases.

Next, the weather-resistant steel material having the iron powder layer formed on the surface is subjected to a rust accelerating treatment. In the rust accelerating treatment, the temperature of the weather-resistant steel material is maintained in an atmosphere in which the dew point temperature is kept constant, and in the atmosphere, a temperature range of (dew point temperature + 5 ° C.) or more and (dew point temperature −5 ° C.) or less It is preferable to perform a process of repeatedly changing the temperature range between the above-mentioned temperature ranges. The value of the dew point kept constant does not need to be particularly limited as long as it is in a range where dew can be formed on the steel material. The adjustment of the dew point is preferably performed by spraying water into the atmospheric gas. In order to obtain a predetermined stable rust, it is preferable to use pure water as the water to be sprayed.

It is preferable that the atmosphere for holding the steel material keeps the dew point constant and further keeps the oxygen gas concentration at 15 to 50% by volume. If the oxygen gas concentration is less than 15% by volume,
The rate of progress of the corrosion is slow, and it takes a long time to form a stable rust, and the production efficiency decreases. If the oxygen gas concentration exceeds 50% by volume, corrosion does not proceed. For this reason, the oxygen gas concentration in the atmosphere is preferably set to 15 to 50% by volume.
It is preferable that oxygen gas be mixed with nitrogen gas to adjust the partial pressure of oxygen gas. The oxygen gas to be mixed should be as pure as possible to obtain a predetermined stable rust.

The steel material is held in the above-mentioned atmosphere, and the temperature of the steel material is set to a temperature range of (dew point temperature + 5 ° C.) or more and (dew point temperature −5).
(° C) or lower. By reciprocating between a temperature range of (dew point temperature + 5 ° C) or higher and a temperature range of (dew point temperature −5 ° C) or lower, the steel surface repeats dew condensation and drying so that corrosion progresses quickly and stable rust can be formed in a short time. . Holding in each of the above temperature ranges is not particularly required, but holding for 1 to 60 minutes does not cause any problem as long as the number of repetitions within a certain time is reduced.

When the temperature of the steel material is less than (dew point temperature + 5 ° C.) or more than (dew point temperature −5 ° C.), dew condensation on the surface of the steel material,
Since the drying is unstable, the corrosion does not proceed, and it is difficult to form a stable rust layer. Further, the temperature rise and fall of the steel material temperature between the temperature range of (dew point temperature + 5 ° C) or more and the temperature range of (dew point temperature −5 ° C) or less are performed at a rate of 0.1 to 2 ° C./min. Is preferably 0.01 to 2 ° C./min. If the rate of temperature rise is less than 0.1 ° C./min, it takes too much time to raise the temperature, which is not efficient. On the other hand, if it exceeds 2 ° C./min, a rust layer is hardly formed on the steel surface. In addition, the temperature drop rate is 0.01 ° C / mi
If it is less than n, it takes too much time to lower the temperature, which is not efficient. On the other hand, if it exceeds 2 ° C./min, dew condensation is difficult, and a rust layer is hardly formed on the surface of steel.

As described above, when the rust accelerating treatment is applied to the weather-resistant steel having the iron powder layer formed on the surface, corrosion proceeds on the surfaces of the ground iron and the iron powder. FIG. 1 schematically shows the progress of corrosion. 1A to 1C show the case where iron powder is applied, and FIGS. 1D to 1F show the case where iron powder for comparison is not applied. In the present invention, the average particle size is 1 to 100
The use of fine iron powder of μm has the advantage that the area of the corrosion reaction is increased and the corrosion progresses faster even when the same rust accelerating treatment is performed. And iron corrosion,
Since the precipitation reaction is due to elution and oxidation of iron ions,
The rust formed as shown in (b) also has the effect of filling voids between the iron powders and forming a dense rust layer. Further, since the iron powders are lightly sintered and the iron powders are fixed in layers on the surface of the steel material, the formed rust layer has good adhesion to the steel material. Further, since the thickness of the sintered iron powder is increased, as shown in FIG.
The rust layer can be made thicker than when iron powder is not applied (FIG. 1 (f)).

[0022]

EXAMPLE A test piece (5 mmt.times.50 mmw.times.100 mml) cut from a weather-resistant steel sheet having the composition shown in Table 1 was subjected to shot blasting, and pure iron powder having an average particle size shown in Table 2 was added to water. After being uniformly dispersed by stirring, it was applied to the surface of the steel sheet by a doctor blade method. After application, after drying, heat treatment is performed at 800 ° C in an Ar atmosphere and allowed to cool, fixing the iron powder to the steel sheet surface and sintering the iron powder together to form an iron powder layer on the steel sheet surface. did.

Next, each test piece was subjected to a rust accelerating treatment. Rust acceleration treatment is performed in an atmosphere where the dew point temperature is kept constant (temperature: 30 ° C, humidity: 90%, dew point temperature: 25 ° C, 25%
The test piece temperature was repeatedly fluctuated between (dew point temperature −5 ° C.) and (dew point temperature + 10 ° C.) in an O ₂ -remainder N ₂ gas atmosphere to form rust on the test piece. The rate of temperature rise between the temperature range below the dew point and the temperature range above the dew point is 1 ° C / s, and
And The holding in a predetermined temperature range was set to 10 minutes.
One cycle was 4 hours, and the treatment period was 20 days.

[0024]

[Table 1]

As a comparative example, a steel sheet was coated with iron powder and then subjected to a rust accelerating treatment without heating (steel No. 12). A plate that had only been treated (Steel Sheet No. 13) was also prepared. The thickness of the rust layer of the test piece subjected to such treatment was measured, and the test piece was further exposed for 3 months in the countryside and the coastal zone, and the occurrence of flow rust and floating rust was investigated. Table 2 shows the results. In addition, the generation | occurrence | production state of the flow rust was visually observed about the contamination state of the concrete installed under the test piece. The occurrence of floating rust on the surface of the test piece was visually observed.

[0026]

[Table 2]

Table 2 shows that the test pieces treated under the conditions in the range of the present invention have a thick rust layer,
No rust was found in any of the coastal areas. No floating rust was generated. In steel sheet No. 9 in which the thickness of the iron powder layer was out of the preferred range of the present invention, occurrence of floating rust was slightly observed. On the other hand, steel sheets No. 10 and No. 11 in which the average particle size of the iron powder used is out of the range of the present invention.
In, the formation of the rust layer became non-uniform, and generation of flow rust and floating rust was observed. In addition, in the steel sheet No. 12 which was only coated with iron powder and not subjected to heat treatment, a rust layer was not formed uniformly, and floating rust and flow rust were generated. In steel sheet No. 13 which had not been coated with iron powder and had only been subjected to rust accelerating treatment, the thickness of the rust layer was thin, and floating rust and flow rust were observed.

[0028]

According to the present invention, it is possible to prevent the occurrence of flow rust and floating rust, and furthermore, it is possible to quickly form a thick weather-resistant stable rust for improving the weather resistance even when used in a coastal zone, It is possible to manufacture a weather-resistant steel material that can withstand use at a low cost, and this has a remarkable industrial effect.

[Brief description of the drawings]

FIG. 1 is an explanatory view schematically showing a rust layer formation state ((a) to (c)) in a steel material of the present invention and a rust layer formation state ((d) to (f)) in a conventional steel material. It is.

Continuation of the front page (72) Inventor Yasuyoshi Yamane 1-chome, Kawasaki-dori, Mizushima, Kurashiki-shi, Okayama Pref. No address) Inside Kawasaki Steel Corporation Mizushima Steel Works (72) Inventor Kenichi Amano 1-chome, Mizushima Kawasaki-dori, Kurashiki-shi, Okayama Prefecture (No address) Inside Kawasaki Steel Corporation Mizushima Works F-term (reference) BC02 BC09 CA53 CA62

Claims (2)

[Claims] An average particle size of 1 to 10 on the surface of a weather-resistant steel material.
After applying 0 μm iron powder, 500-
A method for producing a steel material having excellent weather resistance, comprising performing a heat treatment of heating and cooling to a temperature range of 1000 ° C. and further performing a rust accelerating treatment. 2. The surface of the weather-resistant steel material has a thickness of 10 to 100 μm.
The method according to claim 1, wherein the iron powder is applied so as to form an iron powder layer having a thickness of?