JPS5944922B2 - Manufacturing method of cold rolled steel sheet - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention provides a method for producing a cold-rolled steel sheet, in particular, directly cold-rolling and annealing a hot-rolled steel sheet with attached scale, and then adjusting the surface of the steel sheet by acid treatment. The present invention relates to a method of manufacturing a steel plate.

Conventional cold-rolled steel sheets are produced by hot rolling → descaling → cold rolling →
It is manufactured through the process of electrolysis cleaning → box-shaped or continuous annealing → temper rolling.

The descaling process is necessary from the aspect of the surface quality of the product, and is currently carried out by pickling, but it not only requires a large amount of equipment, but also increases the equipment cost and running cost. , it is necessary to dispose of the waste acid that is discharged in large quantities.

In the cleaning process, electrolytic cleaning is performed in an alkaline bath to remove surface stains such as residual rolling oil and iron powder generated during rolling, but like the descaling process, it requires a large amount of equipment and running costs. It is said that

Furthermore, regarding conventional annealing methods, box annealing not only requires more than four days for heating, holding, and cooling, but also requires the use of reducing gas to prevent oxidation of the steel plate during annealing. .

In recent years, continuous annealing has been adopted from the viewpoint of productivity improvement, energy saving, and labor saving, but this continuous annealing method also uses a reducing atmosphere like the box annealing method, and as will be explained in detail later. There are some limitations related to the problem of increasing the heating rate and the problem of oxidation on the surface of the steel sheet.

In the manufacturing process of cold-rolled steel sheets, if it were possible to realize a manufacturing method that omitted the descaling and cleaning steps described above and did not cause deterioration of the surface properties of the steel sheets, the benefits would be enormous.

Regarding the cold rolling method of steel sheets, which involves direct cold rolling without removing hot rolling scale, the present inventor previously disclosed in Japanese Unexamined Patent Application Publication No. Sho G2-
Proposed in Publication No. 134810, and other publications
No. 245, No. 52-21246, No. 52-52157
No. 52-52158.

From this point of view, the inventors of the present invention have conducted detailed research into the manufacturing method of cold-rolled steel sheets, and as a result, they have made it possible to omit the descaling and electrolytic cleaning processes required in the conventional technology, and also to omit the annealing process. We have established a manufacturing method for cold-rolled steel sheets that has the advantages of a new variety.

The gist of the present invention is to cold-roll a hot-rolled steel plate with scale still attached to produce a cold-rolled steel plate with scale still attached, and then annealing this cold-rolled steel plate without performing a cleaning treatment. A method for producing a cold rolled steel sheet characterized by removing scale by acid treatment.

The technology of the present invention reveals that even if the descaling and electrolytic cleaning steps of the conventional technology are omitted, the surface quality of the steel sheet is no inferior to that of conventional products when the surface of the steel sheet is adjusted by acid treatment after annealing. made possible.

When a steel plate with hot rolling scale still attached is directly cold rolled by the method of the present invention, a steel plate with a black surface coated with fine powder scale is obtained.

Hot-rolled scale has poorer plasticity than base steel, so it is crushed by compressive and tensile stress within the rolling roll bite.

The scale form after rolling is a fine powder aggregate that loosely adheres to the surface of the steel sheet together with rolling oil.

At the same time, the surface area increases due to the stretching effect as the cold rolling reduction increases, and the scale exfoliation effect due to rolling causes
The thickness of the black scale covering the steel plate surface is reduced.

The following table shows the results of cold rolling a hot-rolled steel plate with a scale thickness of 9 μm with the scale still attached, and measuring the reduction ratio and scale thickness after rolling. For example, under 70% pressure, the scale thickness is 2.1 μm. is decreasing.

The scale thickness of 2.1 μm is the average scale thickness due to the aggregate of fine powder scale.

Furthermore, in detail, the scale is peeled off and separated from the base iron, as if it had been loosely crimped with a roll.

This is a situation in which fine powder scale is crimped onto a steel plate, and when the fine powder scale is removed, it has an appearance similar to a cold-rolled plate with a satin finish, and there are no scratches or the like, so there is no problem in terms of surface quality.

Further, when rolling is carried out according to the present invention, part of the scale is peeled off during rolling and mixed into the rolling oil, but since the scale is easily removed by a magnetic filter, the work can be carried out without any problem.

Next, the present invention will be explained.

First, we will discuss the differences between the pickling process, which is the main descaling method of the conventional method, and the acid treatment after annealing of the present invention.

Conventional pickling is formed using a hot rolling process with a thickness of approximately 10 μm.
In order to remove thick scale, the pickling time is as long as 30 to 60 seconds, even though the process involves contact with inorganic acids such as hydrochloric acid or sulfuric acid at a high temperature of 80 to 95 degrees Celsius. An acid is necessary, but in the case of the present invention, acid treatment is necessary because not only 20 to 50 percent of the initial scale amount is peeled off due to cold rolling, but also the scale layer is greatly reduced as the steel sheet elongates. The time is short, within 1 to 2 seconds.

As described above, since the acid treatment of the present invention is extremely simplified compared to conventional methods, it can be said to be a completely new method from the viewpoint of the manufacturing process of cold rolled steel sheets.

Next, the manufacturing process (hot rolling→cold rolling→annealing→acid treatment→temper rolling if necessary) of the cold rolled steel sheet of the present invention will be explained in more detail in order.

Direct cold rolling of scaled hot-rolled steel sheets (this cold rolling is carried out using rolling oil as usual) does not require any special scale removal operation during the cold rolling process (therefore, after rolling The surface of the steel plate is uniformly covered with black scale with a thickness of 1 μm or less.

Conventionally, iron powder mixed in the cold rolling coolant has been separated and removed in order to suppress adhesion of iron powder generated by rolling to the surface of a steel sheet, but according to the present invention, this is not necessary.

In addition, in the conventional method, thick scale is removed by pickling or the like before cold rolling, and bright annealing is performed by heating in a reducing atmosphere during annealing. Annealing a steel plate with particles still attached will cause surface stains, so in most cases cold-rolled steel plates are cleaned by electrolytic cleaning to remove rolling oil and iron powder from the surface before annealing. The product is made with the same surface condition as annealed.

In contrast, in the present invention, cold rolling is performed without removing the scale of the steel plate in the hot rolling mill, and a simple acid treatment is performed in the final step after annealing, so rolling oil residue remains on the surface of the steel plate after annealing. Even if iron powder, etc. remains, these are removed by this acid treatment and there is no problem, so there is no need to electrolytically clean the steel plate after cold rolling, and annealing can be performed in the cold rolled state. be able to.

Next, the advantages of the present invention method for annealing scaled steel plates will be described.

Annealing is performed to soften the steel sheet by recrystallizing the cold-worked structure, but currently the steel sheet is treated in a reducing atmosphere to reduce and remove oxides on the surface of the steel sheet during annealing. On the other hand, in the present invention, since the surface is adjusted by simple acid treatment after annealing, it is not necessarily necessary to create a reducing atmosphere, but rather, reducing gas can be omitted by creating a non-reducing atmosphere.

Recent continuous annealing methods aim to shorten the P length and therefore significantly reduce equipment costs by adopting the following steps: rapid heating recrystallization in a reducing atmosphere → rapid cooling → overaging treatment in a reducing atmosphere → rapid cooling. It has been done and it has happened again.

Rapid heating is a method in which fuel gas is combusted with air and the gas generated is applied directly to the steel plate and heated. However, because the heating rate is insufficient for steel plates with metallic luster in the conventional method, the black body is heated before annealing. A method of annealing after chemical treatment has also been proposed.

In contrast, in the present invention, this is not necessary at all, and a high heating rate is achieved by the thin and uniform black scale present on the surface of the steel sheet before annealing.

This improvement in heating rate is also a great advantage in box-type annealing, making it possible to shorten the processing time.

Another problem with the heating zone of the conventional continuous annealing method is that
If the air-fuel ratio (air amount/fuel gas amount) is increased, the steel plate will be excessively oxidized and the surface quality will deteriorate, so the air-fuel ratio should be set to 1.0.
Below, it is usually necessary to operate at a lower value of 0.90 to 0.95.

In such an operation, combustion efficiency is poor and there is a large loss in terms of effective utilization of fuel gas, but according to the method of the present invention, new oxidation is suppressed by the scale, so even if the air-fuel ratio is increased, the above-mentioned problem will occur. No problems at all.

Next, in the conventional method, the steel plate is rapidly cooled by water immersion or air/water spray in the rapid cooling process. There is a problem that temper color occurs, and various measures have been taken to remove this problem, but in the present invention, there is no need to take any measures to prevent temper color.

As mentioned above, the thickness of the scale layer remaining on the steel plate after annealing can be greatly reduced by cold rolling, and the surface of the steel plate can be cleaned with an extremely short acid treatment, and the phosphate treatment -The surface properties after painting (paint adhesion, paint corrosion resistance) are comparable to those of conventional methods.

As for the acid treatment method, cooling from the overaging treatment zone may be performed by immersion in an acid solution or spraying of an acid solution, or by immersion in an acid solution immediately before temper rolling.

Moreover, it is more effective to use electrolysis together.

The acid used may be an inorganic acid or an organic acid, but inorganic acids such as hydrochloric acid, 61a, and phosphoric acid, which have a high dissolution rate, are preferable.

Here, the reason why the means for removing scale after overaging treatment or box annealing in continuous annealing of steel sheets was specified as removal using hydrochloric acid, sulfuric acid, etc. will be described.

The present inventors investigated the removal of scale remaining after annealing steel sheets by dividing them into chemical treatment methods, such as acid treatment, and mechanical treatment methods, such as shot blasting, liquid honing, polishing, and grinding. .

As a result, it was found that mechanical treatment deteriorates the corrosion resistance of the steel plate after painting when it is made into a product due to the biting of abrasive grains into the surface of the steel plate and the generation of surface linear flaws.

Acid treatment, which is a chemical treatment method, not only does not have the problems of the mechanical treatment described above, but also has the advantage of reducing costs including treatment equipment costs.

As described above, the present invention has made it possible to produce a cold-rolled steel material that has new advantages such as a large number of process omissions and multiple annealing processes.

Next, an example will be described.

Example 1 A hot-rolled steel strip having a thickness of 3.2 mm was cold-rolled to a thickness of 0.8 mm at a total reduction rate of 75% with scale still attached.

The scale average thicknesses of the hot rolled steel strip and the cold rolled steel strip at this time were 9 μm and 1 μm, respectively.

Furthermore, the cold-rolled steel strip covered with black scale was subjected to continuous annealing treatment without electrolytic cleaning (recrystallization treatment at 700°C for 130 seconds by direct flame heating at an air-fuel ratio of 1.0 → air-water cooling → in N2 After performing the over-aging treatment), the surface of the copper strip was cleaned by immersion in a 5% hydrochloric acid solution at 85°C.

The acid treatment time at this time was within 2 seconds.

The steel strip after temper rolling of Section 1 was phosphate treated and painted, and paint adhesion and paint corrosion resistance were evaluated by a salt spray test.

As a result, the surface properties of the cold-rolled steel strip produced in this manner were as shown in Table 1 (compared to the conventional method).

Example 2 A steel strip cold-rolled under the same conditions as Example 1 was rolled at 700°C in N2.
The steel strip was box annealed for 5 hours at a temperature of 300°C and de-stressed.

Thereafter, the surface properties after te acid treatment and temper rolling under the conditions of Example 1 and painting were evaluated.

The acid treatment time at this time was a maximum of 2 seconds, and the surface properties were comparable to those of the conventional method, as shown in Table 1.

Although the above description has been made regarding cold-rolled steel sheets, the method of the present invention can also be applied to the production of various types of plated and surface-treated steel sheets, as well as silicon steel sheets and stainless steel sheets.

Claims (1)

[Claims] 1. A hot-rolled steel plate with scale still attached is cold-rolled to produce a cold-rolled steel plate with scale still attached, and then this cold-rolled steel plate is annealed without performing a cleaning treatment. A method for producing a cold rolled steel sheet, which is characterized in that scale is removed by acid treatment. 2. The method for producing a cold-rolled steel sheet according to claim 1, characterized in that continuous annealing including a direct flame heating process is performed.