JPS6323246B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a continuous heat treatment method aimed at improving the surface quality of metal sheets, in particular cold rolled steel sheets. In industrial practice, cold rolled steel sheets are prepared by pickling hot rolled steel strips (after steel production and hot rolling into ingots or slabs);
It is then cold rolled to the desired thickness and finally annealed to restore the mechanical properties of the steel and skinned to give it the desired final surface appearance and eliminate the horizontal portion of the tensile curve. - Obtained by performing pass processing. All these operations following hot rolling adjust the final surface condition of the steel sheet. Therefore, insufficient rinsing after pickling leaves seeds for subsequent contamination on the surface. Similarly, the selection of rolling oil is extremely important as this oil will not be removed from the steel plate surface unless the annealing is well adapted. Several authors have shown that the cleanliness of the steel plate surface, especially the amount of deposited carbon, determines the suitability of this steel plate to phosphate coating treatments and its resistance to corrosion by salty fog after painting. It has been sufficiently shown that this is an important parameter for
The cleanliness of this surface can be determined by various methods, such as the Scotch tape test. In this method, a transparent adhesive tape is applied to the surface of a steel plate, and the tape containing deposits on the steel plate is removed. Deposits on the surface of the steel plate are quantified by measuring the absorption of light passing through the Scotch tape. Such methods measure the amount of surface deposits of any nature, e.g. traces of dust, carbon, shavings, etc. Another similarly widely used surface property measurement method aims to quantify the total amount of carbon present on the steel surface. This method uses an inorganic tampon to wash the surface of a steel plate with hydrochloric acid, reacts it with the acid, and measures the amount of CO ₂ liberated. In this way, the total amount of carbon present in various forms on the surface of the steel is determined in mg/m ² . Similarly, cleaning with hydrochloric acid may be carried out in advance by spray degreasing for the purpose of standardizing the test. This is to remove any protective oil that may be present and to bring the steel to its as-formed condition and prior to phosphating and final painting. This method is a well-known test method called the "ford test." Other methods for analyzing steel surfaces include ion microanalyzers and Augier spectrometers. They can detect any chemical elements present at the surface and changes in their content at depth. These techniques can detect contamination that can occur with elements other than iron, ie, elements originating from the baths used (washing, pickling, rinsing, degreasing) or from the steel itself. As mentioned above, it is now well known that the presence of carbon on the surface of steel reduces the resistance of painted steel sheets to corrosion by salty fog. This carbon comes mostly from rolling oil. In current practice, the rolling oil is not removed from the surface of the steel sheet after rolling, but evaporates during the annealing (batch annealing) process in a bell furnace. However, when measuring the carbon content of the surface after such annealing, unnoticeable contamination is observed, resulting in undesirable consequences of phosphate coatings and painting (exposure to salt-laden mist). Significant advances can be made, for example, by prior electrolytic degreasing during welding of sodium orthosilicate, followed by continuous annealing of the product. In simple continuous annealing, the heating is actually
It is carried out in an atmosphere of N ₂ /H ₂ and the oil does not have time to evaporate. This is because heating is very fast. On the contrary, in some known processes, the continuous annealing is preceded by a degreasing operation, which is mostly carried out in an alkaline medium. Since the rolling oil is removed before entering the furnace, surface cleanliness is clearly superior. And for a very clean steel plate made in a static furnace, e.g.
The cleanliness is particularly good with respect to the total carbon present on the surface, which is between mg/m ² and 8 mg/m ² . However, as mentioned above, although such a reduction in the amount of surface carbon improves the durability of the paint according to various authors, the improvement is still found to be small. The purpose of the present invention is to correct precisely this situation. The inventors have filed an application relating to steel sheets for automobile bodies, including continuous annealing by open flame heating, phosphate coating and painting, in the manner often used for continuous electroplating of steel strips. Summer. This mode of heating is known to be very suitable for preparing surfaces from an electroplating point of view, which requires inter alia that the surface be free of any traces of oxides at the entrance to the zinc bath. Depending on the type of furnace,
This mode of heating is to some extent oxidative, and any oxides that may result from passing through the furnace must be reduced with atmospheric gaseous hydrogen during the subsequent annealing-electroplating step. The method of the invention shows that undegreased strip steel, i.e. in which the rolling oil has been simply combusted or evaporated in an open flame furnace, is significantly better than strip steel that has been annealed after alkaline degreasing. This is based on the completely unexpected evidence that the material has excellent behavior towards phosphate coatings and coatings. Pursuing their research, the inventors have discovered that by carefully regulating the combustion, a very clean, unoxidized and phosphate coated product with particularly good resistance to salty fog after coating and painting has been discovered. It has been noted that certain strips of steel can be produced. The following examples and drawings are produced by degreasing in an alkaline bath before annealing, and which is believed to inherently retard the phosphate coating reaction.
It will help to understand this unexpected effect due to the absence of SiO ₂ film. The present method, which is the object of the present invention, involves continuous heat treatment including a heating operation on a thin metal sheet, followed by a rapid cooling operation _.
A heating operation is carried out at a temperature below the transformation temperature and is applied to the undegreased sheet, i.e. the sheet at least partially reapplied with rolling oil, and that this heating is carried out, at least initially, in an open-flame furnace. , preferably in a furnace with incomplete combustion, and that the quenching comprises a residence step in an aqueous medium, preferably at a temperature of 75° C. or higher. The expression "aqueous medium" as used herein does not, in a restrictive manner, mean a bath of pure water, which may be used to store substances in solution and/or for any purpose.
Of course, the same extends to any aqueous medium, saturated or unsaturated, in which it is contained or in suspension. The aqueous medium may consist of a static bath, a water spray pipe or a spray pipe, or preferably two or three of these means, regardless of their order. The temperature of the aqueous medium should be above 75° C. in order to ensure that the mechanical properties are uniformly distributed throughout the sheet and that the smoothness of the sheet is excellent. The upper limit of this temperature is also the boiling point, since the aqueous medium must remain liquid even when in the form of a mist spray. The application of the method according to the invention can also be adapted to different products to be treated, as required. Similarly, in some cases the unexpected effects of open flame furnace treatment of undegreased sheets can be further improved. In particular with regard to mechanical properties, following the quenching, the sheets may be subjected to a precipitation step at temperatures of 200 to 500°C. Besides this already important first improvement of the surface quality of the sheet metal, the invention exhibits further advantages. It is known that when heating steel in an oxidizing medium (even at very weak levels), certain elements, such as manganese, chromium, phosphorus, etc., which are more easily oxidized than iron, migrate to the surface. There is. In this way, a surface enrichment of the steel occurs with certain elements, even those present in extremely small amounts in the steel. In this way, mild steel containing 0.3% manganese can exhibit a manganese content of around 15% at the outermost surface after annealing in a bell furnace. Moreover, even if the annealing
Low dew point and low _O2 under protective atmosphere of _N2 / _H2
The same is true even if it is done based on the content. gaseous H ₂ O
The residual content of and O ₂ was sufficient to attract manganese to the surface, and the very long time (several hours) and high temperature (700 °C) made this phenomenon extremely pronounced. In principle, in the case of continuous annealing, the residence time at high temperatures is very short (a few minutes), so that the contamination of the surface with volumetric elements will be very low. However, the inventors have demonstrated that a reduction in enrichment at the surface can only be obtained without alkaline degreasing. This is because alkaline degreasing also has the effect of leaving a film of residual silica on the surface which creates an oxidation potential due to the separation of alloying elements present in the volume. Figure 1 shows that steel that has been continuously annealed after alkaline degreasing has a surface affected by an unnoticeable enrichment of manganese, but is not degreased beforehand and is continuously annealed in an open-flame furnace. In similar tempered steels
This explains well the fact that the Mn content shows only a slight increase. This figure (in which the number of NCN electrons is shown on the ordinate and the binding energy EV is shown on the abscissa) shows the normalized values measured at the top surface for two continuously annealed mild steels. The spectrum is shown. One is degreased before annealing (A steel),
The other is degreasing carried out in an open flame furnace according to the method of the invention (Steel B). The inventors have identified the unexpected effect of reducing this enrichment. In fact, the steel sheets obtained by the method of the invention resisted atmospheric corrosion and rust pinholes very well during the storage process. Such improvements are very important since the presence of corrosion pinhole debris on steel plates prepared for delivery to customers is significant. A special embodiment of the method of the invention provides further improvements in surface quality. This embodiment consists of a treatment in an acidic medium during or after heating. Such treatments virtually eliminate all traces of surface contamination, whether they originate from residual carbon, from rolling oil, or from residual enrichment of elements coming from the volume of the steel. Can be completely removed. Such operations are conveniently carried out after an oxidation step of annealing (quenching in an aqueous medium, exposure to oxidizing gases for a limited duration). Removal of the thin oxide layer produced by this process makes it possible to reach an already averaged level of surface contamination. The acid used is preferably an organic acid, particularly formic acid or having formic acid as the main component. It is advantageous to carry out this after rapid cooling or after final cooling. The following example shows the results of this operation after a treatment already claimed separately by the inventors (consisting of immersion in a boiling water bath). In Table 1,
The surface of the steel, which was already very slightly contaminated after the quenching (and thus also before the quenching, since the quenching was carried out in distilled water), was further clearly improved by the pickling treatment used. It can be seen that In this example, the amount of carbon present on the outermost surface was determined by the Ford test method.

[Table] The table shows the case of steel exposed outdoors in the shade for 48 hours in summer. The first sample (A) was alkaline degreased and subjected to N ₂ /H ₂ in a conventional furnace equipped with a radiant tube.
Heating at 700°C, holding at this temperature for 1 minute, cooling to 500°C with air blowing (jet cooling), slow cooling between 500 and 400°C for 3 minutes, and cooling to room temperature with air blowing. It underwent a continuous process consisting of a final cooling. The second sample (B) underwent similar treatment, but the first cooling was water quenching and 450°C.
Replaced by reheating. Three other samples (C), (D), (E) were prepared according to the method of the invention. Namely, an undegreased steel plate was heated in an open-flame vertical furnace (combustion was controlled to produce reducing smoke due to lack of combustion air), rapidly cooled, overaged for 1 minute at 450°C, and heated to room temperature. This is the final cooling of the Rapid cooling was performed in three different ways. a. Jet cooling in air (Sample C) (very weak oxidation); b. Treatment with water (D) (weak and irregular oxidation); c. Immersion in a water bath at near boiling temperature (E) (thickness 100
~600 Å homogeneous oxidation). The samples were pickled according to a method separately claimed by the inventors, for example by immersion for 5 seconds in a formic acid bath at a concentration of 1 g/liter. Resistance to corrosion according to the above test is 0
Rated on a scale from (very good resistance) to 10 (poor resistance).

[Table] The * mark is according to the present invention In the present invention, the open flame furnace and the absence of degreasing always bring about an improvement. This improvement becomes even more pronounced if one oxidation step is added to the process, and especially if the resulting oxide is uniform and 100-600 Å thick.

[Brief explanation of drawings]

Figure 1 shows the final results of Steel A, which was continuously annealed after alkaline degreasing, and Steel B, which was continuously annealed in an open flame furnace without prior degreasing by the method of the present invention. The difference in Mn content on the surface is shown. Steel B manufactured by the method of the present invention shows a slight increase in Mn.

Claims (1)

[Claims] 1. A method for continuously annealing a thin metal sheet contaminated with rolling oil, comprising: (a) a method for continuously annealing a thin metal sheet contaminated with rolling oil, _which method comprises: (b) rapid cooling of the heated sheet metal, the rapid cooling being a reaction between the metal sheet and the aqueous medium; (c) immersing the cooled metal sheet in an organic acid aqueous solution. . 2. A method according to claim 1, in which, following rapid cooling, the sheet is subjected to a carbon deposition step at a temperature of 200°C to 500°C. 3. The method according to claim 1, wherein the organic acid is formic acid. 4. Claims in which the aqueous medium is composed of a static bath, a water spray pipe, or a spray pipe, or a combination of any two or all of these three means, and the order thereof is arbitrary. The method according to any one of paragraphs 1 to 3.