KR20040038381A - Surface Modification Method for Manufacturing High Corrosion Resistance Steel Using Two Furnaces - Google Patents

Abstract

PURPOSE: A surface modification method for manufacturing high corrosion resistance steel by using two furnaces is provided to increase productivity, greatly reduce purging gas and improve corrosion resistance and aesthetic characteristics by performing post oxidation treatment promptly and effectively using nitriding and post oxidation furnaces. CONSTITUTION: The surface modification method comprises a step of nitriding the ε-iron nitride or ε-iron carbonitride using N2 gas or purging nitrocarburizing gas onto the ε-iron nitride or ε-iron carbonitride after forming ε-iron nitride or ε-iron carbonitride on the surface of the steel part by nitriding or nitrocarburizing a steel part in first furnace; a step of controlled cooling in the air after taking out the steel part from the furnace; and a step of cooling the post oxidized steel part before using the cooled steel part or black finish treating the cooled steel part after post oxidizing the steel part in the second furnace by charging the controlled cooled steel part into second furnace that is set to the post oxidation conditions in advance to carry out post oxidation.

Description

Surface Modification Method for Manufacturing High Corrosion Resistance Steel Using Two Furnaces}

The present invention relates to a surface modification method for producing a high corrosion resistant steel using two furnaces, in particular nitriding by treating the nitriding or nitrification process and the post-oxidation process using a dedicated nitriding furnace and a post oxidation furnace, respectively. The cooling time from the processing temperature to the post-oxidation temperature can be greatly shortened, so that the phase change of the surface nitride layer formed in the nitriding process can be prevented by rapid cooling, and the uniform cooling of the steel parts and the post-oxidation of appropriate The present invention relates to a surface modification method for manufacturing a high corrosion resistant steel using two furnaces having excellent corrosion resistance by forming a solid, dense and uniform black iron oxide film on the surface by post-oxidation treatment at a temperature.

Hydraulic and pneumatic piston rods, automotive shock absorber rods and damper rods are chromium plated or surface treated for corrosion resistance, abrasion resistance, and uniform and dense surface coating.

In particular, in order to have excellent corrosion resistance in an environment where corrosion by chlorine ions (Cl) such as water and brine are severe, conventionally, after nitriding or nitriding and post-oxidation treatment are carried out after nitriding treatment in a single furnace. Oxidation treatment was performed.

When nitriding or nitriding is carried out as described above, only the ε-Fe _2-3 (N, C) phase is generated in the compound layer according to the treatment method as shown in FIG. 1 or in some cases, the ε-Fe _2-3 (N, C) Only a mixed phase and a γ'-Fe ₄ N phase or a γ'-Fe ₄ N phase may be formed. However, these two phases have little corrosion resistance to salt water and the like.

Therefore, in order to impart corrosion resistance, post-oxidation is necessary. When post-oxidation is performed, oxygen is contained only in the ε-Fe _2-3 (N, C) phase, in which case γ'-Fe ₄ N The phase is almost corrosion resistant even after post-oxidation.

In general if the nitriding treatment or a carbo-nitride, _{ε-Fe 2-3 (N, C} ) phase and the γ'-Fe ₄ when passing on the mixed phase obtained N gas and temperature control during the treatment on the outermost surface of the compound layer, ε-Fe _{2- Only three} (N, C) phases can be formed. In this case, ε-Fe _2-3 (N, C) phase is formed on the outermost surface, and ε-Fe _2-3 (N, C) phase and γ'-Fe ₄ N phase coexist below it. The post oxidation treatment is also performed on the mixed phases of the Fe _2-3 (N, C) phase and the γ'-Fe ₄ N phase to have corrosion resistance.

Therefore, neither the ε-Fe _2-3 (N, C) phase or the mixed phase of the ε-Fe _2-3 (N, C) and γ'-Fe ₄ N phases formed by nitriding or nitriding carburization is applied to the brine. There is little corrosion resistance.

However, in particular _{ε-Fe 2-3 (N, C} ) on the _{(ε-Fe 2-3 (N, C} ) only a pore is formed ε-Fe _2-3 to oxygen is mainly breaking through the pores ( Oxygen is contained in the N, C) phase and the pore layer is filled with an oxide.) When oxygen is contained, the corrosion resistance is greatly improved, and the Fe 2 O 4 oxide layer is formed on the compound layer to further improve the corrosion resistance.

In FIG. 1, the materials constituting the compound layer are ε-Fe _2-3 (N, C) phase and γ'-Fe ₄ N phase, and all regions except γ'-Fe ₄ N phase are ε-Fe _2-3 ( N, C) phase.

Conventionally, the surface layer is formed of epsilon iron nitride or epsilon iron carbonitride (ε-Fe _2-3 (N, C)) on the surface of steel parts at a temperature of 570 to 580 ° C. After continuous post oxidation process to form black iron oxide (Fe ₃ O ₄ ) film, the surface of steel parts is first nitrided or nitrocarburising in a single nitriding furnace. After purging the nitriding or nitrification atmosphere in the furnace with N ₂ gas, the temperature of the components is cooled in the furnace to the post-oxidation temperature and then post-oxidation is performed in the same furnace. Since the nitriding or nitrification process and the post-oxidation process are processed continuously in a single furnace, it is necessary to cool to a temperature suitable for post-oxidation after nitriding.

However, in the conventional treatment method using a single furnace, the method of continuously nitriding or nitriding carburizing, N ₂ gas purging, temperature cooling, and post-oxidation treatment in the same furnace is described as follows when using a large furnace for mass production. It causes the same serious problem.

In other words, the time required to lower 40 ° C in the field production furnace (1200mmφ × 2200mmH) with about 2 tons of steel parts to be treated takes about 1 hour and 30 minutes or more, so it is cooled to the appropriate post-oxidation temperature of 400 to 540 ° C. It takes a minimum of three hours to cool down. In this case, since N ₂ purging gas is consumed 10 m 3 / hr or more, a large amount of purging gas is consumed.

In addition, as described above, when the furnace cooling time is extended, nitrogen and carbon in the compound layer formed on the surface of the steel part are denitrated and decarburized, thereby changing the chemical composition of the ε-iron nitride or ε-iron carbonitride in the compound layer, thereby reducing the corrosion resistance. That is, the nitride formed by nitriding or carburizing _{ε-Fe 2-3 (N, C} ) phase of the top layer when kept for a long period of time at least 500 ℃ temperature in a nitrogen atmosphere, not a nitride or carbo-nitriding atmosphere ε-Fe _2- C and N present in the ₃ (N, C) phase escape into the N ₂ gas atmosphere, and the ε-Fe _2-3 (N, C) phase is changed into a γ'-Fe ₄ N phase. In this case, the γ'-Fe ₄ N phase does not contain oxygen even after post-oxidation, so corrosion resistance is not imparted. Therefore, when the chemical composition is changed, the corrosion resistance is rather reduced.

In addition, the iron oxide when the post-oxidation by steam at 540 ℃ or more should be produced Fe ₃ O ₄ Fe ₂ O _{3 is} also produced in small amounts will adversely affect the corrosion resistance.

Furthermore, in the conventional method, when steel parts are cold-cooled, cooling of the steel parts does not proceed uniformly according to the furnace position of the steel parts, resulting in a long furnace cooling time or an uneven black iron oxide film at the time of post-oxidation. It will adversely affect the corrosion resistance.

Therefore, the surface treatment method of the corrosion resistant steel by the conventional nitriding method remains a problem to be solved, such as the formation of a non-uniform and not beautiful post-oxidation layer due to low productivity, corrosion resistance and temperature unevenness of the product.

Therefore, the present invention has been devised in view of the problems of the prior art, and its object is to carry out a nitriding or nitriding carburizing process and a post-oxidation process by using a dedicated nitriding furnace and a post-oxidation furnace, respectively. The cooling time to the oxidation treatment temperature can be shortened by 2 to 40 times, preventing rapid phase change of the surface nitride layer formed in the previous process by rapid cooling, and uniform cooling of steel parts and proper post-oxidation temperature. It is possible to provide a surface modification method for producing a high corrosion resistant steel using two furnaces having excellent corrosion resistance by forming a solid, dense and uniform black iron oxide film on the surface by post oxidation treatment.

It is another object of the present invention to purge after nitriding or nitriding and carburizing by carrying out the post-oxidation process in a separate post-oxidation furnace in the case of having a post-oxidation process subsequent to nitriding or nitrification process in a single furnace in mass production. It is to provide a surface modification method for manufacturing high corrosion-resistant steel using two furnaces that can greatly reduce the N ₂ purging gas because the after-oxidation furnace is used to take out from the nitriding furnace immediately after purging with gas.

1 is a schematic cross-sectional view showing the surface structure of a steel part when the steel part is nitrided or nitrided.

In order to achieve the above object, the present invention, after the nitriding or nitriding carburizing treatment of the steel parts at 550 ~ 600 ℃ in the first furnace using two furnaces and purged with nitriding or nitrocarburizing gas with N ₂ gas Next, the steel parts were removed from the furnace, controlled cooling in air for 30 seconds to 1 hour, and then charged into the second furnace for post-oxidation at 400 to 540 ° C., which was set to the post-oxidation condition in advance, for 1 minute. It provides a surface modification method for the production of high corrosion-resistant steel using two furnaces, characterized in that the post-oxidation is carried out for ~ 5 hours, followed by air cooling or water cooling.

The post-oxidation treatment is to contain oxygen in the compound of the ε-Fe _2-3 (N, C) phase and further form Fe ₃ O ₄ oxide in the outermost surface layer to improve corrosion resistance and to obtain a beautiful black surface. The lower the post-oxidation temperature, the thinner the Fe ₃ O ₄ layer and the smaller the amount of oxygen entering the ε-Fe _2-3 (N, C) phase. Therefore, if the post-oxidation treatment for 1 minute to 5 hours in the temperature range of 400 ~ 540 ℃ Fe ₃ O ₄ oxide layer thickness is only about 0.1 ~ 2㎛.

If post-oxidation is performed at 540 ° C. for more than 5 hours, the thickness of the Fe ₃ O ₄ oxide layer is increased by post-oxidation. In this case, steel parts requiring high corrosion resistance without black finishing after post-oxidation are applied. Suitable. That is, when the thickness of the Fe ₃ O ₄ oxide layer becomes thick, there is a problem in that the oxide layer is peeled off during the black finishing process, and the finish roughness deteriorates.

In addition, in the case where the post-oxidation treatment is 400 ° C. for 1 minute or less, the thickness of the Fe ₃ O ₄ oxide layer is too thin, and the amount of oxygen entering the ε-Fe _2-3 (N, C) phase is also low, resulting in poor corrosion resistance. . Therefore, it is preferable to form the thickness of the Fe ₃ O ₄ oxide layer to be less than 1 μm to perform black finishing.

In addition, although the time required for the control cooling is set to 30 seconds to 1 hour, it is preferable that an appropriate cooling time is set in consideration of the size (thickness, diameter, weight, etc.) of the steel component to be post-oxidized. For example, a plate made of 0.8 mm thick is cooled to post oxidation temperature in about 30 seconds, and 100 mmφ × 2000 mm H rod takes about 1 hour to cool down.

The isothermal post-oxidation method of the present invention is applied to an atmosphere in which nitriding or nitriding is carried out by a gas method, a plasma method (ion method), a fluidized bed method, a plasma-gas combined treatment method, or the like in the first furnace. A compound layer composed of ε-iron nitride or ε-iron carbonitride is formed, and oxygen is contained in this layer, and isothermal post-oxidation is performed in a second post oxidation furnace to form a Fe ₃ O ₄ iron oxide layer on the compound layer.

At this time, the main cooling of the steel parts takes place in the air moving from the first nitriding furnace to the second post oxidation furnace, and when the jig loaded with the steel parts to be processed is placed on the high speed cooling fan device and operated and blown, it is loaded on the middle part of the jig. As the steel parts are easily cooled and reach the post-oxidation temperature, a Fe ₃ O ₄ oxide layer is formed on the surface.

Particularly, since the appropriate post-oxidation temperature is 400 ~ 540 ° C, the black iron oxide formed in this post-oxidation temperature range is Fe ₃ O ₄ and is more dense and firmly attached than the iron oxide formed at 540 ° C. or higher. Finishing such as lapping, polishing, or mechanical polishing yields a beautiful black surface.

The isothermal post-oxidation method using the above two furnaces can shorten the cooling time from nitriding temperature to post-oxidation temperature by 2 to 40 times, and the phase change of the compound layer due to rapid cooling as well as the reduction of N ₂ purging gas. There is almost no uniform cooling of the steel parts, and by performing post-oxidation at an appropriate post-oxidation temperature, it is possible to obtain a solid, dense and uniform black iron oxide film, thereby having excellent corrosion resistance.

Steel parts of the present invention subjected to the above treatment is dense and solid Fe ₃ O ₄ film attached to the outermost surface has almost no absorption of saline, and Fe ₂ O ₃ is not formed 400 when tested with 5% NaCl salt spray Excellent corrosion resistance over time.

Hereinafter, the present invention will be described in more detail with reference to preferred embodiments.

(Example)

Machined by grinding SM45C diameter (10mmφ) × length (190.5mmL) of the gas spring rod diameter 1200mmφ, height 2200mmH-in pit-type furnace after 10,000 charged at 580 ℃ 3 hours after the gas carburizing and nitriding treatment with N ₂ gas for 10 After purging for 10 minutes, the resultant was cooled in air for 3 minutes, charged in a second post oxidation furnace in a steam atmosphere maintained at 450 ° C., and subjected to post-oxidation for 1 hour, followed by water cooling.

After the black finish of the treated rod was subjected to a 5% NaCl salt spray test to investigate the corrosion resistance of the rod, no rust occurred for 480 hours.

In the above example, the load of about 99.9% was found to satisfy the above corrosion resistance except for loads such as defects in material itself, careless handling, and poor buffing among 10,000 rods.

In addition, when the nitriding treatment and the post-oxidation treatment were carried out continuously using a conventional furnace, no rust occurred for 48 to 120 hours. Therefore, in the conventional method, the corrosion resistance is significantly lower than that of the present invention, and the corrosion resistance variation of the component to be processed has a large problem.

As described above, according to the present invention, the use of two furnaces such as a nitriding furnace and a post-oxidation furnace enables rapid and effective post-oxidation treatment, resulting in high productivity, large saving of purging gas, corrosion resistance and aesthetic characteristics. Improved surface treatment of steel parts is achieved.

In the above, the present invention has been illustrated and described with reference to specific preferred embodiments, but the present invention is not limited to the above-described embodiments and is not limited to the spirit of the present invention. Various changes and modifications will be possible by those who have the same.

Claims (4)

Performing nitriding or nitriding on the steel part in the first furnace to form ε-iron nitride or ε-iron carbonitride on the surface of the steel part, and then purging the nitriding or nitrocarburizing gas with N ₂ gas; Removing the steel part from the furnace and controlling cooling in air; After the post-oxidation by charging the control-cooled steel parts in the second furnace for the post-oxidation set in the pre-oxidation conditions set in advance, the second stage comprising the step of cooling or using black finish processing the cooled steel parts Surface modification method for manufacturing high corrosion resistant steel using furnace. The high corrosion resistance according to claim 1, wherein the nitriding or nitriding is carried out by any one of a gas method, a plasma method (ion method), a fluidized bed method, and a plasma-gas combined treatment method. Surface modification method for steel production. The method of claim 1, wherein the post-oxidation treatment is performed at 400 to 540 ° C. for 1 minute to 5 hours. The method of claim 1, wherein the black finishing process is performed by any one of mechanical polishing, buffing, polishing, and lapping.