JPS6235465B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for modifying the surface properties of cold rolled steel sheets (including steel strips, hereinafter referred to as cold rolled steel sheets). Cold-rolled steel sheet is one of the most common steel materials used in a wide range of applications such as automobiles and home appliances.In recent years, however, in the field of automotive steel sheets, high-quality steel sheets have been developed to reduce the weight of car bodies and improve safety. The development of cold-rolled steel sheets with high tensile strength is being promoted, and much effort is being put into studying component design and manufacturing processes to obtain materials that have both high strength and high workability. Further, in the case of mild steel sheets, workability is further improved by fixing nitrogen and carbon in the steel with additive elements such as Al and Nb. However, some of the additive elements used for the purpose of improving the mechanical properties of cold-rolled steel sheets concentrate on the steel sheet surface during annealing and form an oxide film, which is one of the important properties of cold-rolled steel sheets. Many of them impair certain surface properties, such as phosphate chemical conversion properties, paintability, and appearance (temper color). This kind of action is due to its strong affinity with oxygen.
This problem of surface property deterioration can be said to be very serious when designing components for improving mechanical properties, as it is particularly noticeable in important elements such as Al, Si, and Mn, which are important for improving the mechanical properties of steel. As a result of intensive studies to solve this problem of surface quality deterioration, the inventors found that after cold rolling,
By applying iron plating containing a high concentration of oxygen to the steel sheet surface prior to recrystallization annealing, it is possible to effectively prevent elements in the steel from diffusing and concentrating on the surface during annealing. This discovery led to the completion of this invention. That is, this invention applies iron plating containing 5000 ppm or more of oxygen to a hot rolled, descaled, and cold rolled steel sheet in a pre-process of recrystallization annealing to a thickness of 0.01 μm or more and 2.0 μm or less. The surface properties of the cold-rolled steel sheet have been advantageously improved by applying it within a range. Japanese Patent Publication No. 54-29976 discloses a known method of modifying surface properties by applying heat treatment to steel materials, but in this case iron plating is applied. By applying heat treatment after plating, the purpose is simply to make the surface treatment properties of the steel constant regardless of the properties of the base material, and according to this, iron plating under normal conditions. Si during annealing and recrystallization,
In order to reduce surface concentration of Mn, etc. to a level that does not harm chemical conversion treatment properties or surface appearance, Si and Mn in steel must be
When the amount of addition is large, the iron plating thickness is 2
Since it was necessary to make it about μm or more, it was extremely difficult to implement it industrially considering the cost and implementation process. Therefore, we investigated the effect of various iron plating conditions on the surface concentration suppression effect, and found that by performing iron plating under conditions that contain 5000 ppm or more of oxygen during iron plating,
It was found that the surface concentration of elements in steel, such as Mn and Si, was significantly suppressed. In other words, when iron plating is performed under these conditions, even if a considerable amount of Mn, Si, etc. are added to the steel, the iron plating thickness is as thin as 0.1 to 0.5 μm, and the surface concentration during annealing is reduced. can be reduced to a sufficient level. The reason why iron plating containing a high concentration of oxygen strongly suppresses the surface concentration of elements in steel such as Mn and Si during recrystallization annealing is that these elements are trapped by oxygen during iron plating. It depends. In other words, elements such as Mn and Si, whose concentration on the surface is a problem, have a strong affinity for oxygen, so they are selectively oxidized on the surface of the steel sheet during annealing, resulting in concentration on the surface. However, if iron plating containing a high concentration of oxygen is applied in the pre-annealing process,
During the annealing process, oxygen in this iron plating becomes Mn,
Combined with Si, fine Mn and other particles are formed in the iron plating layer.
This is to effectively prevent these elements from diffusing to the surface by forming an oxide of Si. In order to have this effect, the higher the oxygen concentration in iron plating, the better.
A content of 5000ppm is required. That is, when the oxygen content in iron plating is less than 5000 ppm, no particular difference is observed in the effect of preventing surface diffusion of elements in steel compared to normal iron plating. An oxygen content of 5000 ppm or more is required to have a large diffusion prevention effect and therefore to reduce the required iron plating thickness, and the lower limit of the oxygen content in iron plating in the method of this invention is shall be. Conditions for containing such a high concentration of oxygen during iron plating vary depending on the composition of the iron plating solution, the temperature of the plating solution, etc., but generally the following can be said. Generally, iron plating is carried out by cathodic electrolysis in an acidic plating solution using sulfate, chloride, fluoride, etc., but Fe ⁺⁺ ions in this plating solution are relatively small. It is unstable and tends to precipitate as hydroxide when the pH of the plating solution increases. In addition, in iron plating, the cathode current efficiency is
Although not 100%, at the cathode, along with the electrodeposition reaction of metal Fe, H ⁺ ion discharge also occurs, and this causes an increase in pH near the cathode surface (that is, the covered surface). In normal iron plating, in order to prevent precipitation of iron hydroxide from occurring due to the increase in pH near the cathode,
A relatively low range of current density is adopted, but in this invention, the above-mentioned generation of iron hydroxide near the cathode is actively utilized to deposit fine precipitates of iron hydroxide into the plating layer. By incorporating it, iron plating with high oxygen concentration can be obtained. For this purpose, iron plating may be carried out by increasing the current density as high as possible without damaging the appearance of the plating. The iron plating thickness required to improve the surface texture varies depending on the type and amount of added elements in the steel, recrystallization annealing conditions, oxygen content in the iron plating, etc., but the iron plating thickness is 0.01 μm. Since no surface property modification effect is observed below this value, this is the lower limit of the iron plating thickness. The required iron plating thickness increases as the amount of added elements in the steel and the annealing temperature and time increase, and the lower the oxygen content in the iron plating, the thicker the iron plating is required. For tensile steel, under the conditions of alloying elements of several percent or less and annealing conditions of about 850° C. or less, which are commonly used, a surface quality with no practical problems can be obtained with an iron plating thickness of about 0.1 to 0.5 μm. In cases where the amount of elements in the steel and annealing conditions are more severe,
If the requirements for surface quality are strict, the thickness of the iron plating can be further increased, but if the plating thickness exceeds 2.0 μm, the iron plating layer may deteriorate due to electrodeposition stress during plating. The upper limit of the iron plating thickness is 2.0μm as it may cause deterioration of adhesion.
shall be. There is no need to set any restrictions on the chemical composition, annealing conditions, etc. of the raw steel sheet in this invention, and the iron plating thickness and oxygen content in the iron plating should be adjusted within the above-mentioned regulatory range according to each condition. By changing it appropriately, good surface properties can be obtained. The iron plating of this invention not only does not cause any harm when applied to ordinary mild steel plates, but also improves rust resistance by covering up nonmetallic inclusions exposed on the steel plate surface. This also provides a secondary effect. The process of carrying out the iron plating of this invention is as follows:
In the continuous annealing method, it is preferable to provide electroplating equipment after the degreasing equipment on the entry side so that the process can be performed continuously, and in the case of the box annealing method, it is also preferable to perform the process continuously in the electrolytic cleaning process equipment. . A feature of this invention is that the thickness of the iron plating can be reduced by incorporating oxygen into the iron plating, and the required electroplating equipment can be compact, so in-line processing as described above is possible. It is relatively easy to install equipment that makes it possible, but if such equipment is not available,
There is no problem with a method in which iron plating is performed in a separate process before annealing. Since the iron plating of this invention is applied to the activated steel plate surface as it is cold rolled, it is possible to omit the pickling process after degreasing, but the iron plating of the steel plate surface in the degreasing process is Insufficient removal of iron powder, etc. may impede the effect of iron plating, so it is desirable to perform a light pickling treatment after degreasing. As described above, by carrying out the present invention, it is possible to modify the surface properties of cold rolled steel sheets, and examples of the present invention will be described below. Example 1 Test steel A having the composition shown in Table 1 was heated at a slab heating temperature.
Under the conditions of 1250℃, finishing temperature 850℃, and winding temperature 680℃
After hot rolling to a thickness of 3.2 mm, it was subjected to a pickling process, cold rolled to a thickness of 1.2 mm, and iron plated under the conditions shown below.

[Table] After performing iron plating with various oxygen contents and plating thicknesses in this way, the soaking temperature of 850
Annealing was performed at 10°C for 2 minutes in a N ₂ +10% H ₂ atmosphere (dew point -40°C), and the effect on surface properties (appearance and chemical conversion treatability) was investigated. The results are shown in Table 2.

[Table] As is clear from Table 2, the amount of oxygen in iron plating
At 600 and 2300ppm, 3μ is required to modify the surface texture.
The required iron plating thickness is 0.1 μm, which can be significantly reduced by setting the oxygen content in the iron plating to 5000 ppm or more. Example 2 Test steel B shown in Table 3 was heated to a slab heating temperature of 1150
Example 1
Iron plating with an oxygen content of 15,200 ppm was applied to various thicknesses using plating solution A, and annealing was performed at 800° C. for 1 minute to examine the effect on surface properties.

【table】 The results are shown in Table 4.

[Table] As is clear from the results shown in Table 4, when the iron plating thickness is less than 0.01 μm, no surface property modification effect is observed. If the iron plating thickness exceeds 2 μm, the appearance and chemical conversion treatment properties will be good, but if the iron plating thickness exceeds 2 μm,
In the case of μm and 5 μm, poor adhesion of iron plating was observed. As shown above in Examples 1 and 2,
0.01μ iron plating containing more than 5000ppm oxygen
By carrying out in the range of 2.0 μm or more and 2.0 μm or less, the effect of modifying the surface properties of the cold rolled steel sheet can be obtained. In addition, the hot-dip galvanizing method, which is the most common method for rust-proofing steel sheets, has high productivity because the recrystallization annealing process is carried out on the same line, and it also has sacrificial corrosion protection because the amount of plating can be easily increased. It has the advantage of being easy to increase in size, and is often applied to automotive steel sheets in areas where salt damage is severe, such as the United States and Canada.
In addition, so-called alloyed hot-dip galvanized steel sheets, in which the plated layer is alloyed with the base steel through heat treatment after plating, have the advantage of excellent paintability and weldability, and are used as surface-treated steel sheets for automobiles. It is becoming mainstream. However, when hot-dip galvanizing is applied to high-strength steel sheets, if they contain a large amount of easily oxidizable alloying elements such as Mn, Si, and Cr, these will be selectively oxidized during the reduction annealing process, resulting in the steel sheet surface being damaged. It forms a concentrated layer that impedes galvanizing adhesion and, in severe cases, can cause discoloration. It is also known that in alloyed galvanizing, material components influence the behavior of the alloying reaction. For this reason, when performing hot-dip galvanizing using the usual method, the amount of alloying elements such as Mn and Si added is limited, making composition design extremely difficult. Especially these days,
Materials with a good balance between strength and ductility or formability are required, and in order to meet this demand, if we try to strengthen by introducing a low-temperature transformed phase (composite structure strengthening) or to create a component system mainly based on solid solution strengthening, Mn, Si, etc. It is necessary to increase the amount of addition of , which results in inhibiting hot-dip galvanizing properties. However, by applying the lever invention prior to the reduction annealing process before hot-dip galvanizing and applying iron plating containing high concentration of oxygen to the surface of the steel sheet in advance, the elements in the steel can be diffused to the surface during annealing. It has also been confirmed that thickening can be effectively prevented and excellent zinc plating properties can be obtained.

Claims (1)

[Claims] 1 Iron plating containing 5,000 ppm or more of oxygen to a thickness of 0.01 μm or more and 2.0 μm or less is applied to hot-rolled, descaled, and cold-rolled steel sheets in the pre-process of recrystallization annealing. A method for modifying the surface properties of cold rolled steel sheets, characterized by: