JPS6347357A - Production of black steel sheet having excellent workability - Google Patents

Abstract

PURPOSE:To obtain a black steel sheet which has a beautiful and strong film and has excellent workability by subjecting a cold rolled steel sheet contg. C, Si, Mn, and Al respectively at prescribed ratios to heating and soaking treatments in respectively specified temp. ranges then to cooling and further a soaking treatment. CONSTITUTION:The cold rolled steel sheet contg., by weight, <=0.10% C, <=0.10% Si, 0.05-2.0% Mn, and 0.005-0.10% Al is subjected to the heat treatment in the following manner. After the surface of said steel sheet is degreased, the steel sheet is heated to 700-850 deg.C in an oxidation atmosphere to form FeO on the surface. The steel sheet after the heating treatment is subjected to the soaking treatment for 5-60sec in the above-mentioned temp. region; thereafter, the steel sheet is cooled down to <=500 deg.C at >=20 deg.C/sec rate. The steel after the cooling is in succession subjected to the soaking treatment for >=200sec in a 350-550 deg.C temp. region, more preferably in a non-oxidation atmosphere, by which the objective black steel sheet having the excellent workability is obtd.

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial Application Field) The present invention relates to a method for manufacturing a surface-treated steel sheet that has a beautiful and strong coating on its surface and has excellent workability.

Recently, a so-called black steel plate with a blackened surface has been developed as one type of surface-treated steel plate. Black steel sheets have a more luxurious feel than other plated steel sheets, and are widely used because they can be used in office supplies, cameras, audio products, and the like.

(Prior art) Known methods for blackening the surface of a steel plate include (1) a method of forming a black resin film containing carbon black or the like as a main component on the surface of a steel plate or a plated steel plate (Japanese Unexamined Patent Publication No. 56-6
(2996 Publication) (2) A method of forming a film containing Ag on the surface of a steel sheet (3) A method of treating a steel sheet with a treatment solution in which an inorganic nitrate is added to an aqueous tannin iron solution (Japanese Unexamined Patent Publication No. 56
(4) First treatment of the Ni or Co plating layer with a treatment solution such as nitric acid or a mixed acid containing nitric acid as the main component;
There is a method of forming a blackening layer by secondary treatment with alkaline water such as KOH and Ca(Off)2).

However, in (1) above, the cost is high due to the use of silver, etc. In (2) above, the resin film whose main component is carbon black etc. has disadvantages such as poor adhesion and strength. Regarding No. 31 and (4), each had the disadvantage that the processing method and equipment were complicated.

(Problems to be Solved by the Invention) Therefore, a manufacturing method that advantageously achieves the formation of a beautiful and strong film on the surface of a steel plate and the improvement of the workability of the steel plate without using any chemicals or paints. It is an object of this invention to provide.

(Means for Solving the Problems) The inventors have discovered that the problem with using the scale that forms on the surface of a steel sheet during hot rolling as a film is that the scale is thick and easily peels off during processing such as bending. Therefore, various studies were conducted, especially regarding the adhesion of the scale.

That is, by the thermal cycle shown in FIG. 1, the thickness l mu
Scale was generated on the surface of the steel plate, and its bending adhesion was investigated using a bending peel test.

In the bending peel test, adhesive tape is pasted on the surface of the steel plate after scale formation, a bending test with a bending radius of 2 fins is performed, the tape is peeled off, and the adhesion is evaluated based on the amount of scale attached to the adhesive surface of the tape. It is evaluated on a five-point scale.

Note 1: Very poor (the scale has completely peeled off and the base metal is completely visible) 2: Poor (the powdery peeling is severe and the tape is black) 3: Somewhat poor (the scale has peeled off completely, but the base metal is completely visible) Iron is almost invisible) 4. Good (some powdery flaking occurs) 5. Very good (no powdery flaking at all) According to this experiment, 500℃
Alternatively, compared to the oxide film formed by a simple cycle of soaking at 750°C and then cooling, by holding the scale formed at 750°C at a temperature of 500°C for 200 seconds or more, the scale becomes a beautiful black color and also has better bending adhesion. As shown in FIG. 2, it was found that there was a significant improvement.

In other words, from Figure 2, soaking at 750℃ for 5 seconds or more is 60℃.
It can be seen that when the temperature is less than 2 seconds, it is necessary to hold the temperature at 500° C. in a non-oxidizing atmosphere for 200 seconds or more.

This invention is based on the above experiment.

That is, this invention has C: 0.10wtχ (hereinafter simply %
) Below, Si: 0.10% or less, Mn:
0.05-2.0% and Al: 0.005-0.
After degreasing the surface of a cold-rolled steel plate containing 10%
After heating to a temperature of 700 to 850°C in an oxidizing atmosphere and soaking in the temperature range for 5 seconds to 60 seconds,
Cooling at a rate of 20°C/second or more to a temperature of 00°C or less,
Continue to 200℃ in the temperature range of 350℃ or higher and 550℃ or lower.
This is a method for producing a black steel sheet with excellent workability, which is characterized by performing soaking treatment for more than a second.

Although it is not clear why the heat treatment according to the present invention forms a beautiful, strong film with excellent adhesion to the steel plate, FeO formed by short-time heating at 750°C is more effective when soaked at 500°C. Fe inside, 0. This is thought to be due to the formation of strong bonds at the interface of the base metal and the densification of the scale during transformation.

Furthermore, the heat treatment cycle according to the invention is a continuous annealing cycle for low carbon steel. Since it is a rapid cooling and aging cycle, if a steel plate that has undergone cold rolling is used, the base material has excellent workability and it is possible to produce a steel plate with a beautiful and strong black film on the surface.

(for production) The configuration of this invention will be described in more detail below.

C: 0.10% or less Cff1 greatly affects the adhesion of the film, but it needs to be 0.110% or less in order to obtain good workability in a rapid heating/quenching cycle.

Si: 0.10% or less Since Si is a harmful element to the adhesion of the film,
．． 10% or less.

Mn: 0.05-2.0% Mn needs to be contained at 0.05% or more to prevent S embrittlement during hot rolling, but a content exceeding 2.0% will lead to deterioration of workability. Therefore, it is set in the range of 0.05 to 2.0%.

Al: 0.005-0.10% Al needs to be contained at 0.005% or more to fix O and N, but if it exceeds 0.10%, the effect is saturated and the scale adhesion deteriorates. Also, the upper limit is set at 0.10%.

Since the purpose of this invention is to manufacture a beautiful graphite steel sheet, the raw material is a cold-rolled steel sheet having the above-mentioned composition. This is because the black film is as thin as several μm, and the properties of the material surface are reflected on the blackened surface.

Next, the cold rolled steel plate is heated to 700 to 850°C.

In this invention, FeO is generated and it is Pe304f+
In order to rapidly cool and transform the steel plate to the Jf range, first heat the steel plate to 700~
It is heated to 850° C. to generate FeO on its surface.

That is, if the heating temperature is less than 700°C, the processability, which is another object of the present invention, will be poor, and if the heating temperature exceeds 850°C, the oxidation rate will be too high, which is disadvantageous in terms of energy.

Regarding the holding time at the heating temperature, if the holding time is less than 5 seconds, a uniform film cannot be obtained and the corrosion resistance deteriorates, and if the holding time exceeds 60 seconds, the Fe0 layer becomes too thick, which is disadvantageous in terms of the adhesion of the oxide film.

Then, cooling is performed at a rate of 20° C./second or more.

If the cooling rate is less than 20°C/sec, Fe0-
= F e , O, transformation occurs, no transformation occurs at the scale base iron interface, and scale adhesion deteriorates.

The range of the soaking temperature in the subsequent soaking treatment is 350°C or more and 550°C or less. Heating above 550°C enters the FeO generation region, so Fe, 0. No metamorphosis progresses at 350℃
At a temperature lower than that, the diffusion rate of Fe and O decreases and transformation takes a long time.

Furthermore, the holding time in the above temperature range is 200 seconds or more. This is because if the time is less than 200 seconds, the transformation from FeO to Fe)0° at the scale substrate interface, which contributes to improving scale adhesion, becomes insufficient.

Note that there are no particular restrictions on the atmosphere during the soaking treatment at 350°C or higher and 550°C or lower, but if the holding time is long, a non-oxidizing atmosphere may be used to suppress new scale formation. desirable.

Further, since such a heat treatment cycle is similar to a continuous annealing heat cycle for cold-rolled steel sheets, there is an advantage that workability of the product can be ensured.

(Example) Example 1 A cold-rolled steel plate with a thickness of 1.0 mm having the composition shown in Table 1 was manufactured and subjected to heat treatment using a heat cycle as shown in FIG. Table 2 shows each heat treatment condition and the tensile properties and scale adhesion of the steel plate after heat treatment.

The tensile properties were measured using a JIS No. 5 test piece.

In addition, scale adhesion was evaluated using the bending peel test described above.

Table 1 (wtχ) From Table 2, the adapted example according to the present invention has mechanical properties and
It can be seen that the black steel sheet has good scale adhesion. However, when the soaking temperature (TI) is as high as 900°C, the scale thickness increases and scale adhesion deteriorates (i9), and when the soaking temperature (TI) is as low as 600°C, both the material and scale adhesion deteriorate. lI & llO)
, when the soaking time (1,) is long (11), when the heat retention temperature (T2) is high (l11h12), when the heat retention temperature (T2
) is low (Nn13) and when the heat retention time (t2) is short (lt14), scale adhesion also deteriorates. Also, if the cooling rate from soaking to heat retention is slow (
lt15), the material and scale adhesion deteriorate.

Example 2 Ultra-low carbon Al-killed steel with the composition shown in Table 3 was made into 1.0I
After cold rolling to II thickness, the heat treatment shown in FIG. 3 was performed under the conditions shown in Table 4. Similar to Example 1, the adapted example according to the present invention had good workability and scale adhesion.

Table 3 (wtχ) (Effects of the Invention) According to the present invention, a black steel plate having a beautiful and strong film and excellent workability can be manufactured.

This steel plate can be used unpainted for external parts, etc., and since the surface film has corrosion resistance, it can be further surface-treated. It can also be applied to blacken the surface of materials that have complex shapes after processing.

[Brief explanation of the drawing]

FIG. 1 is an explanatory diagram showing thermal cycles, FIG. 2 is a graph showing the relationship between scale adhesion and holding time at 500° C. and 750° C., and FIG. 3 is an explanatory diagram showing thermal cycles.

Claims (1)

[Claims] 1. For a cold rolled steel sheet containing C: 0.10 wt% or less Si: 0.10 wt% or less Mn: 0.05 to 2.0 wt% and Al: 0.005 to 0.10 wt% After degreasing the surface, heat it to a temperature of 700 to 850°C in an oxidizing atmosphere, perform soaking treatment in the temperature range for 5 seconds to 60 seconds, and then heat to 500°C.
A method for producing a black steel sheet with excellent workability, characterized by cooling at a rate of 20°C/second or more to the following temperature, and then subjecting it to a soaking treatment for 200 seconds or more in a temperature range of 350°C or higher and 550°C or lower. .