JPH01260A - Steel materials for chemical conversion treatment and their manufacturing method - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to a steel material intended for chemical conversion treatment, particularly zinc phosphate treatment, and a method for producing the same. This relates to a technique for forming a nitride layer.

[Conventional technology]

Phosphate (zinc) coatings play an important role as a coating base for steel materials or as lubricating coatings for plastic working, and the formation reaction is the dissolution reaction of iron as described in (1) 2 below, and ( 2) It goes through the process of phosphate film precipitation reaction of formula.

Fe+ToPOa=Fe(HzPO4)z+Ht↑
・(11) In recent years, cold-rolled steel sheets intended for painting, such as cold-rolled steel sheets for automobiles, have been treated with a trace amount of T in order to provide ultra-deep drawability.
In some cases, i is added to steel, but in this case, Ti dissolved in steel during chemical conversion treatment causes a reaction to become TiO□, and during this reaction process, the pH of the interface is lowered, thereby inhibiting the chemical conversion reaction. Therefore, there is a problem that the phosphate chemical conversion treatment properties are poor.

Also, when adding Cr for the purpose of obtaining high corrosion resistance, Cr oxides are formed on the steel sheet surface and inhibit the dissolution of Fe.
Due to its poor chemical conversion properties, it is only used for limited purposes.

Furthermore, cold-rolled ordinary steel sheets such as fully hardened sheets that are not subjected to recrystallization annealing and hot-rolled pickled sheets are worth painting and using to take advantage of the mechanical properties of the material. When a phosphate base treatment is applied as a base treatment, the chemical conversion reaction is extremely poor due to the Fe hydroxide film present on the surface of the steel sheet, so the current situation is that it is not possible to expand the use of the steel sheet.

In addition, in the manufacturing process of low-alloy cold-rolled steel sheets,
Recrystallization annealing is often performed in a reducing atmosphere,
This process produces a concentrated layer of selectively oxidized Mn oxide on the surface of the steel sheet, and since this concentrated layer dissolves well in the chemical conversion treatment solution, it promotes the chemical conversion reaction, which improves the chemical conversion treatment properties. However, in cold-rolled steel sheets to which Ti and Cr are added,
Despite the formation of a Mn oxide-concentrated layer on the surface, as the amount of these elements added increases, chemical conversion treatment properties tend to deteriorate for the same reasons as described above.

By the way, it is well known that Cr-containing steel materials exhibit high corrosion resistance. However, Cr-containing steel materials are inert (hard to dissolve) in the phosphate treatment solution even under treatment conditions that can form a phosphate film on ordinary steel materials that do not contain Cr, and are even more difficult to dissolve. In the case of Cr-containing steels, no phosphate coating can be formed.

For this reason, Cr-containing steel materials have been unavoidably subjected to manganese phosphate treatment or oxalate treatment.

In this case, fluorine additives may be used to increase the cutting power.

On the other hand, several attempts have been made to improve the chemical conversion treatability. For example, in the case of Ti-added steel, a method in which solid solution C and solid N are present in the surface layer during the cooling process during continuous annealing (Japanese Patent Application Laid-open No. 1986-149729) has been proposed.

[Problem that the invention seeks to solve]

However, this published method is effective only for continuous annealing, and in cold-rolled steel sheets that contain a relatively large amount (several percent) of Cr, a large amount of Cr nitrides are formed on the surface, which causes a reaction. spoil one's sexuality.

On the other hand, for fully hard plates and hot-rolled pickled plates that do not require annealing, the above method cannot improve the chemical conversion properties.

Furthermore, even after annealing, the surface of a cold-rolled steel sheet may be ground depending on how it is used after that, and even if the chemical conversion treatment property is improved by annealing, it becomes ineffective.

By the way, as mentioned above, Cr-containing steel materials have poor phosphate treatment properties. In the case of low Cr content steel (Cr55%),
It is not impossible to form a phosphate film under special processing conditions such as strong acidity, high temperature, and steel engine processing.In recent years, low Cr content steel materials have been considered for application in automobile applications.
In the case of automobiles, if the phosphate treatment is applied to the car body together with ordinary steel, the special treatment conditions mentioned above cannot be applied. Therefore, low Cr1
When JL material is used for automobiles, like ordinary steel material,
It is desirable to perform phosphate treatment in advance.

In addition, an oxalate film is used as a lubricating film during cold forging of steel containing Cr, such as stainless steel, but if it were possible to generate a phosphate film instead of this, it would be possible to ) The same lubrication treatment as ordinary steel is possible, (ii
) It can be expected to have significant benefits, such as better lubrication properties than oxalate coatings.

Therefore, the main purpose of the present invention is to treat steel materials with poor chemical conversion properties, such as Cr-containing steel materials, without impairing their mechanical properties.
The purpose is to improve chemical conversion treatment properties.

[Means for Solving the Problems] The gist of the first invention for solving the above problems is characterized in that the steel material has an iron-based nitride layer on its surface.

In addition, the manufacturing method of the second invention can prepare the surface of the steel material from room temperature to 70°C.
It is characterized in that an iron-based nitride layer with a thickness of 100 Å or more is formed on the surface by ion nitriding the steel material at a temperature of 0°C.

[For production]

According to the present invention, when an iron-based nitride layer is formed on the surface of a steel material, phosphate chemical conversion treatment property becomes extremely good. Moreover,
Even with Cr-containing steel, forming an iron-based nitride layer provides excellent chemical conversion treatment properties.

Furthermore, in the method of the present invention, the surface of the steel material is nitrided by the ion nitriding method at a steel material temperature of 700° C. or lower, so that the chemical conversion treatment property can be improved without impairing the mechanical properties of the steel material. Furthermore, according to the ion nitriding method, compared to the gas nitriding method using ammonia gas, the production rate of the iron-based nitride layer is faster and the productivity is improved. Moreover, even if the amount of nitrogen infiltration is the same, the ion nitriding method provides better surface activity characteristics.

[Specific structure of the invention]

The present invention will be explained in more detail below by giving further experimental examples.

The present invention forms an iron-based nitride layer on the surface of steel,
In the case of a steel material that does not substantially contain Cr, an ion nitriding method within a relatively low temperature steel material temperature range of room temperature to 700[deg.] C. is preferably employed when forming the steel material.

When the steel material is a Cr-containing steel material, the iron-based nitride layer generated on the surface thereof is a nitride layer mainly composed of Fe and Cr. According to the present invention, a phosphate chemical conversion coating can be formed not only on low Cr-containing steel materials but also on steel materials with a 30% Cr content. The ability to form a phosphate chemical conversion coating on Cr-containing steel materials with a Cr content of 0.5 to 5% means that
As mentioned earlier, this will open up applications as a steel material for automobiles. In addition, the fact that phosphate conversion treatment can be applied to low Cr, high Cr containing steel in the same way as ordinary steel does not require special treatment conditions, which greatly reduces manufacturing costs. resulting in a reduction. In the case of Cr-containing steel, ion nitriding is preferably performed at a relatively high steel temperature of 350 to 600°C.

Specifically, the following experiment was conducted to investigate the relationship between ion nitriding conditions, iron nitride layer thickness, and chemical conversion treatment properties.

The test steel material was a fully hardened cold-rolled steel sheet made of vacuum-melted material having the chemical composition shown in Table 1 (vacuum-melted, hot-rolled, 3.
0III11 thickness, pickled and cold rolled to 0.8 mmt) was used.

This full hard material was subjected to ion nitriding, and the degree of nitriding was varied to form an iron nitride layer on the surface, and chemical conversion treatment properties were investigated. Note that different methods were used to analyze the thickness of the iron nitride layer depending on whether it was very thin or relatively thick. The very thin nitride layer thickness was analyzed using a photoelectron spectrometer (generally called ESC^ or xPS) and expressed as the depth from the surface until the nitrogen concentration reached lat%. On the other hand, the thickness of the thick nitride layer was calculated from the chemical analysis value of the N amount after identifying the products on the surface of the steel plate using an X-ray diffraction method. In addition, the chemical conversion treatment is performed by dipping using a commercially available zinc phosphate-based chemical treatment solution, and the evaluation is performed by observing the density, thickness, size, etc. of the chemical crystals using a scanning electron microscope. Ta. The results are shown in Table 2.

From Table 2, it was found that when the thickness of the iron nitride layer formed by ion nitriding was 100 Å or more, the chemical conversion processability was significantly improved. For example, in sample MN13, the iron nitride layer thickness = 8
0, and the chemical conversion treatability is normal (△), that is, at the same level as before. However, when the iron nitride layer thickness is 100 people, as in 4, the chemical conversion treatment properties are significantly superior. It becomes a thing (◎).

In this experiment, the compounds formed on the surface and responsible for improving chemical conversion treatment properties were mainly γ'-Fe and N.
When present in a thickness of 00 Å or more, chemical conversion treatment properties are significantly improved.

For comparison, Table 2 also includes the case of gas nitriding at high temperature (Nll 2.13), but as a result of testing with a photoelectron spectrometer, no N was detected on the surface. The reason why this sample has good chemical conversion treatment properties is thought to be due to the selective oxidation layer of Mn, as described above.

By the way, the reason why the presence of a very thin layer of about 100 layers mainly composed of γ' -F and eaN on the surface of the steel material improves chemical conversion treatability is currently beyond the realm of speculation, but -
You can think like a fiery person.

In general, chemical conversion reactions involve the dissolution of iron at the anode and the precipitation of phosphate crystals due to the increase in interfacial pH associated with hydrogen generation at the cathode, as shown in reaction equations (1) and (2) above. It can be considered separately. In many cases, poor chemical conversion treatment properties are caused by the iron dissolution reaction not occurring promptly. On the other hand, γ'-Fe4. Iron nitrides such as N exhibit active dissolution in the acidic range of pH 2 to 5 in the chemical conversion bath, accompanied by hydrogen generation. From this, it is considered that iron nitrides present on the surface promote the anodic reaction of the chemical conversion reaction.

Next, a preferred numerical range in the present invention will be explained.

The thickness of the iron-based nitride layer on the surface of the steel material is 10 mm as described above.
It is preferably 0 Å or more. On the other hand, although the upper limit is not limited, an increase in the amount of nitrides produced (:) increases the processing time (results in
It is determined by taking into account that (ii) surface hardening deteriorates processing characteristics such as elongation and bending of the steel material, and (iii) increases cost.

Furthermore, the reason why the temperature of the steel material is kept below 700°C during the ion nitriding process is that if the temperature exceeds 700°C, not only will the original mechanical properties of the steel material change significantly, but the nitriding rate will be faster than necessary, resulting in unnecessary thickening. This is because a thin nitride layer will be formed. 150~ for steel materials that do not substantially contain Cr
It is desirable to perform the ion nitriding treatment at a steel material temperature of 400°C. On the other hand, in the case of Cr-containing steel materials, a temperature range of 350 to 600°C is suitable.

〔Example〕

Next, an example will be described.

(Example 1) As shown in Table 3, a full hard cold-rolled steel plate to which Ti and Cr are added and a reducing atmosphere combustion plate (780°C
Short-time ion nitriding of 7% H2-NzlD-P (<30°C) was performed at room temperature for 30 seconds, and the chemical conversion treatment property was evaluated.

As can be seen from Table 4, the chemical conversion treatability of full hard plates and annealed plates to which Ti and Cr are added deteriorates as the amount of these added elements increases, but the iron nitride layer By forming a layer with a thickness of 100 Å or more, chemical conversion treatment properties became good regardless of the amount of added elements. For example, as shown in Table 3, as Al-A2-A3 becomes T
As the i content increases and as B1 → B2 → B3, the Cr content increases, but along with this, the chemical conversion treatment property (
1) has also deteriorated, but the chemical conversion treatment properties after ion nitriding (2) have also deteriorated.
) were all good when the thickness was 100 Å or more.

(Example 2) A hot rolled steel plate having the chemical composition shown in Table 5 was heated with 10% HCI.
After descaling, ion nitriding was performed and chemical conversion treatment properties were evaluated.

From Table 6, compared to the previous example (Table 4), the amount of iron nitride layer formed was smaller even with ion nitriding under the same conditions;
When an iron nitride layer with a thickness of 100 Å or more was formed, chemical conversion treatment properties became good.

(Example 3) In particular, a Cr-containing steel plate was investigated for its chemical conversion properties and paintability. The composition of the sample material, the ion nitriding conditions, and the results are as shown in Table 7.

<Discussion> As a comparative example, the results when a Cr-containing steel plate was subjected to phosphate treatment without being nitrided are also shown in llh C8, C9, and C10.

In the case of Cr<3%, many areas not covered with phosphate crystals ("sketches") were observed, and the coating film peeled off in the coating film adhesion test. When the content of Cr was 3%, no phosphate crystals were formed at all, and the adhesion of the coating film was extremely poor.

In all of the invention examples (Ilil C1 to C7), a significant improvement in coating film adhesion was achieved based on a dramatic improvement in phosphate treatment properties. The surface of the steel plate subjected to ion nitriding treatment has
Even in stainless steel, fine phosphate crystals were densely formed over the entire surface.

In addition, post-painting corrosion resistance was significantly improved by ion nitriding. When a phosphate film is not formed as in the comparative example, blisters are extremely likely to form and grow due to damage to the paint film, but by applying ion nitriding treatment, when a phosphate film is formed, blisters do not form or grow. Growth was severely restrained.

(Example 4) Next, a cold forging test of the stainless steel wire was conducted.

The cold forging test conditions were as follows.

・Test material: Material 5US304 23φ wire drawing through the following processing steps A material was prepared and subjected to a cold forging test.

■ Pickling (Nitrofluoric acid, Rol, 10 minutes) → Ionic nitriding (50% NdHt* 5 Torr650
°C, 5s) - Zinc phosphate treatment (total acidity "q120 points, 70 °C
, 10 minutes dry powder lubricant application, wire drawing/forging: forward multi-stage extrusion test was conducted.

(Pass schedule: 23φ→21.26φ→19.9
φ→18.35φ→16.92φ→15.6φ→14.
35φ→13.26φ) ・Evaluation: Judging by the presence or absence of burn-in.

○: No burn-in, ×: Burn-in occurrence results are shown in Table 8.

Discussion As the area reduction rate increased, seizure occurred in the conventional ferrous oxalate treatment. On the other hand, the area reduction rate in this test was 66.8%.
In the above range, no seizure occurred in the ion nitride-zinc phosphate treated products.

It has been found that the stainless steel wire having Fe and Cr-based nitrides formed on the surface layer according to the present invention has good cold forgeability when subjected to lubrication treatment using zinc phosphate treatment.

〔Effect of the invention〕

As described above, according to the present invention, steel materials with poor chemical conversion treatment properties,
For example, it is possible to significantly improve the chemical conversion treatability of fully hardened cold-rolled steel sheets, poorly chemically treated materials with addition of Ti and Cr, hot-rolled pickled plates, etc., without impairing their mechanical properties. can.

In addition, even for annealed low-alloy cold-rolled steel sheets that had good chemical conversion treatment properties when the steel was manufactured, the surface coating may be scraped off during press forming, leading to deterioration in chemical conversion treatment.
Even in such a case, chemical conversion treatment properties can be improved by performing ion nitriding while maintaining the shape after press molding.

Patent applicant: Sumitomo Metal Industries, Ltd.

Claims (4)

[Claims] (1) A steel material for chemical conversion treatment characterized by having an iron-based nitride layer on the surface of the steel material. (2) The steel material for chemical conversion treatment according to claim 1, wherein the iron-based nitride layer is formed with a thickness of 100 Å or more. (3) The chemical compound according to claim 1, wherein the steel material contains 0.5 to 30% by weight of Cr, and the iron-based nitride layer is a nitride layer mainly composed of Fe and Cr. Steel materials for processing. (4) A method for producing a steel material for chemical conversion treatment, which comprises forming an iron-based nitride layer with a thickness of 100 Å or more on the surface of the steel material by subjecting the surface of the steel material to ion nitriding treatment at a steel material temperature of room temperature to 700°C.