JPH07284837A - Descaling method of wire made of austenitic stainless steel - Google Patents

Abstract

PURPOSE:To provide a descaling method of the wire made of austenitic stainless steel where the molten alkaline treatment or the complicated pickling treatment can be dispensed with, the cost is low, and the wire surface excellent in the wire drawability is obtained. CONSTITUTION:In a descaling method of the wire made of austenitic stainless steel, the scale of >=25% to <=80% in terms of the area ratio is removed by the strand shot blast of >=30kg/m<2> in the projection density, and the wire is recoiled, and the rest of the scale is successively removed by pickling the coil in the nitric-hydrofluoric acid solution containing the nitric acid of 150-200g/l and the hydrofluoric acid of 50-100g/l.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for removing scale from the surface of a wire rod coil produced in the manufacturing process of an austenitic stainless steel wire rod.

[0002]

2. Description of the Related Art Scales formed on the surface of an austenitic stainless steel wire rod during heat treatment such as hot rolling and annealing are chemically stable with Fe-based oxides.
It is characterized by the coexistence of r-based oxides and the formation of a Cr-deficient layer in the surface layer of the metal in contact with the scale. In addition, the features of the scale shape include a scale that is generated with a uniform thickness over the entire surface of the wire rod, a root-shaped scale that is generated in the recesses formed on the surface of the slab and the surface of the rolled material, and heating before rolling. In addition, the grain boundary oxide scale and the like generated by heating during heat treatment such as annealing are mixed.

As a means for removing scales having such characteristics, conventionally, a scale is removed by performing a scale modification by molten alkali treatment and then pickling with hydrochloric acid, hydrofluoric acid or the like. Although the method has been used, there are drawbacks that the productivity is low and that the chemical and fuel costs, especially the molten alkali treatment cost are high. In particular, if the pickling time is lengthened to remove the residue of root-like scale, the base material will be over-melted and lead to yield loss, increase in chemical cost and working cost, and also in quality such as defective dimension and shape of the wire. There is a risk of defects. Therefore, as a more efficient descaling method instead of the chemical method, a method of performing pickling after performing mechanical descaling such as shot blasting has been proposed.

That is, in Japanese Patent Application Laid-Open No. 57-177932, a shot density of 3000 kg / m is applied to a stainless steel wire.
A strand-type descaling method has been proposed in which most scales are completely removed by using ^two or more shot blasts, and then light pickling with hydrochloric acid or sulfuric acid is continuously performed. Further, in Japanese Unexamined Patent Publication No. 53-1640, as a method for removing surface scale of hot-worked materials such as stainless steel and stainless steel-coated steel, at least 80% or more of scale is removed by shot blasting, followed by immersion in hydrochloric acid for residual scale. And descaling methods have been proposed to remove the blasting material to improve the surface roughness of the material.
All of these have the common feature that most of the scale is removed by shot blasting, and the descaling burden due to pickling after shot blasting is minimized, but shot blasting that produces a projection density of 3000 kg / m ² or more. Alternatively, if shot blasting for removing scale of more than 80% is performed, the surface of the wire material is work hardened, and wire drawability and other secondary workability are deteriorated. Further, such a device is extremely large-scaled and requires a large equipment cost, which is not practical.

[0005] In shot blasting, the scale is impacted from the surface layer side to destroy and peel off the scale. Therefore, the scale upper layer side can be efficiently removed, but the scale is firmly attached to the base metal at the boundary with the base metal. It is difficult to peel and remove because it is in close contact with. In particular, since the root-shaped scale and the grain boundary oxidized scale are formed by invading the bare metal portion, it is difficult to remove them by peeling. In this way, complete descaling by shot blasting alone is not possible, but if the impact of the shot can crack the scale at the boundary with the ingot, the acid solution can easily penetrate to the surface layer of the ingot. ,
Descaling by pickling is promoted. In the area where the projection density is low, the present inventors have investigated the above-mentioned descaling promoting effect by shot blasting, and by combining this with nitric hydrofluoric acid-immersed pickling, by a relatively small-scale shot blasting equipment, We have found that high-quality descaling can be performed at low cost.

[0006] Further, since the stainless steel wire rod is wire-drawn and used for various purposes, the wire drawability is an essential function. Therefore, it is required that the surface of the wire rod has good lubricant adhesion. With respect to this point, conventionally, for example, Japanese Laid-Open Patent Publication No.
As disclosed in JP-A-1-159215, there is a description example of the relationship between wire surface roughness and wire drawability by shot blasting or the like. Absent. Therefore, the present inventors investigated the effect of the surface of the wire material on the wire drawability due to shot blasting + nitric hydrofluoric acid immersion pickling, and found descaling conditions with good wire drawability.

[0007]

DISCLOSURE OF THE INVENTION The present invention is a low-cost method that does not require molten alkali treatment or complicated pickling treatment when descaling an austenitic stainless steel wire, and has excellent wire drawability. It is an object of the present invention to provide a descaling method capable of obtaining a surface of a wire rod.

[0008]

In order to solve the above-mentioned problems, the present invention combines a descaling promoting action by shot blasting in a region where the projection density is low and a descaling method by nitric hydrofluoric acid immersion pickling to obtain a low descaling method. The present inventors have found a descaling method that can obtain a wire rod surface having excellent wire drawability at a low cost, and as a result of earnest experiments, the optimum method and optimum condition range shown below have been clarified.

That is, the gist of the present invention is that
In the method for descaling an austenitic stainless steel wire rod, strand shot blasting with a projection density of 30 kg / m ² or more removes the scale of 25% or more and 80% or less in area ratio, and rewinds it into a coil. Nitric acid 150-200g / l, Hydrofluoric acid 50-10
A method for descaling an austenitic stainless steel wire rod is characterized in that it is removed by pickling a coil with a 0 g / l nitric hydrofluoric acid aqueous solution.

[0010]

In the following, with respect to the descaling method of the present invention, specific examples will be given and the reasons for setting conditions will be described. First, the reason why the strand method is specified as the shot blasting method is that the coil shot blasting causes descaling failure in the overlapping portion of the wire rods.

Next, the reason why the amount of scale to be peeled and removed by strand shot blasting with a projection density of 30 kg / m ² or more is defined to be 25% or more and 80% or less in area ratio will be described. FIG. 1 shows the results of experiments in which the strand shot blasting was carried out with various projection densities and the descaling property was evaluated by the subsequent nitric hydrofluoric acid pickling. The test material is an as-rolled SUS304 wire rod having a wire diameter of φ5.5 mm. The conditions of the strand shot blast were such that the shot particle diameter was 0.3 mm, the projection speed was 70 m / sec, and the projection density was changed in four stages of 25, 30, 60, and 90 kg / m ² . The pickling was carried out at a liquid temperature of 40 ° C. using an aqueous nitric hydrofluoric acid solution having a nitric acid concentration of 150 g / l and a hydrofluoric acid concentration of 50 g / l, which are the lower limits of the nitric acid hydrofluoric acid concentration range according to the present invention. The evaluation of the descaling condition was performed using a 20 × magnifying glass, and the scale remaining on the surface of the wire rod was classified according to the form as shown in FIG. It can be seen from FIG. 1 that at a shot projection density of 25 kg / m ² or less, the scale remains even after performing pickling for a long time. On the other hand, if the shot projection density is 30 kg / m ² or more, the pickling time is 1
Complete descaling is possible within 20 seconds.

FIG. 2 shows shot projection densities of 25, 30, and 6.
The measurement result of the scale removal rate by the strand shot when changing in four steps of 0 and 90 kg / m ² is shown.
The scale removal rate is 100 as the scale of the as-rolled wire.
%, And the percentage (%) of the amount of scale removed therefrom. The scale removal rate was obtained by enlarging the surface of the wire rod with a microscope and measuring the scale area by image analysis. It can be seen from FIG. 2 that when the shot projection density is 30 kg / m ² or more, the scale of 25% or more in area ratio is peeled and removed. Therefore, if the scale is destroyed to this extent, not only the scale that forms a uniform thickness on the entire surface of the wire rod, but also the root-shaped scale that has formed in the recess will be cracked. It is thought that it easily penetrated to dissolve the metal and the scale was removed. If less than 25% of the scale is removed, the effect of accelerating the permeation of the acid solution due to scale destruction is low, so that root-like scale remains or pickling requires a long time, which is not practical. Further, as described above, if a strong shot is applied, adverse effects such as surface hardening are brought about. Therefore, it is necessary to set the scale removal rate to 80% or less, preferably 50% or less.

Next, as means for removing the remaining scale,
The reason for using the nitric hydrofluoric acid aqueous solution containing 150 to 200 g / l of nitric acid and 50 to 100 g / l of hydrofluoric acid will be described. The role of the acid solution in descaling is to uniformly and efficiently dissolve the metal directly under the scale to remove the scale, and it is desirable that the metal part after the scale removal has little dissolution. In the austenitic stainless steel wire rod, a Cr-deficient layer is formed in the surface layer of the base metal just below the scale. Therefore, a nitric hydrofluoric acid aqueous solution having a property of selectively dissolving the Cr-deficient layer was used as the pickling aqueous solution.

In order to optimize the concentration range of the aqueous solution of nitric hydrofluoric acid, an acid solution prepared by combining various concentrations of nitric acid and hydrofluoric acid is prepared, and a pickling treatment is performed after the strand shot blasting treatment to remove the scale. Was evaluated. The shot projection density was constant under the condition of 30 kg / m ² . Pickling
Nitric acid concentration: 50, 100, 150, 200, 250 g /
l 5 conditions, hydrofluoric acid concentration: 25, 50, 100, 150,
All 25 kinds of acid solutions in which 5 conditions of 200 g / l were combined were immersed for 120 seconds at a liquid temperature of 40 ° C. The results are shown in Fig. 3. The evaluation of the descaling condition was performed using a 20 × magnifying glass, and the scale remaining on the surface of the wire was classified according to the form as shown in FIG. From FIG. 3, the descaling property tends to be better in the high concentration range of 150 to 200 g / l than in the low concentration range of 50 to 100 g / l, and to the descaling property at the nitric acid concentration of 250 g / l. On the contrary, it is inferior. It is considered that this is because nitric acid having a concentration of 150 to 200 g / l was retained in the active dissolution region of the Cr deficient layer. When the nitric acid concentration becomes 250 g / l, the descaling property is deteriorated. It is considered that the oxidizing property of the solution is too strong and the Cr-deficient layer is moved to the passive state region. Regarding hydrofluoric acid, when the concentration is 25 g / l, the descaling property is poor, but 50,100
Descaling tends to improve as the g / l and hydrofluoric acid concentrations increase. This is because the dissolution rate increased as the concentration of hydrofluoric acid increased.

Next, in the test of FIG. 3, the results of evaluation of the wire drawability using the material which has been completely descaled by pickling are shown in FIG. For wire drawing, a lubricant was applied to the wire material, continuous wire drawing was carried out at a wire drawing reduction rate of 70%, and the quality of wire drawing was judged by observing the surface properties after wire drawing. Those in which seizure flaws had occurred were considered to be inferior in wire drawability, and were marked with X in FIG. 3, and those with good surface properties were marked with ◯. From FIG. 3, the nitric acid concentration is 150 in the acid liquid concentration range in which the wire drawability is good.
It can be seen that the hydrofluoric acid concentration is 50 to 100 g / l at ˜200 g / l.

The reason why the drawability is good when the nitric acid concentration is 150 to 200 g / l is considered as follows. That is, in this nitric acid concentration range, since it is held in the active dissolution region of the Cr-deficient layer, the acid solution dissolves the Cr-deficient layer immediately below the scale at a high rate, and the dissolution rate of the metal is slower than this. Therefore, the root-shaped scale and the grain boundary oxide scale formed in the concave portions on the surface of the wire rod are selectively removed by the dissolution of the Cr-deficient layer around the scale, so that the scale removal marks immediately after the descaling are completed. Are formed in a pit shape. It is presumed that the reason why the wire drawability is good is that the lubricant permeates and is retained in the pits that are present innumerably on the surface of the wire as shown in FIG. 4, and the adhesiveness of the lubricant is increased.

The drawability is inferior when the concentration of hydrofluoric acid is 150 g / l or more. When the concentration of hydrofluoric acid is increased, there is a tendency that there is no difference between the dissolution rate of the Cr-deficient layer and the dissolution rate of the bare metal portion. It is considered that removal of root scales and intergranular oxide scales by dissolution of the layers and dissolution of the bare metal portion other than the Cr-deficient layer proceeded at the same time, and as a result, pit formation was hindered and the surface became smooth. Further, as the concentration of hydrofluoric acid increases, the amount of sludge produced increases, and the treatment work is frequently required. Therefore, the concentration of hydrofluoric acid must be suppressed to 100 g / l or less.

From the results shown in FIG. 1 and FIG. 3, the nitric acid concentration is 150 to 200 g / as an acid solution condition for obtaining a surface of a wire material having a good descaling property and an excellent wire drawing property.
1, the concentration of hydrofluoric acid aqueous solution having a hydrofluoric acid concentration of 50 to 100 g / l was defined. Regarding the liquid temperature, in general, the higher the temperature,
The dissolution rate increases and high-speed pickling becomes possible, but at the same time NO
_Since the problem of _X gas treatment becomes remarkable, it is desirable that the temperature is as low as possible at 40 ° C or higher.

[0019]

EXAMPLES Tables 1 and 2 (continued from Table 1) show the evaluation results of the descaling property and the wire drawing property of the examples of the present invention and the comparative examples. The evaluation material is SU rolled to a wire diameter of φ5.5 mm.
A S304 wire rod coil was used. FIG. 5 shows the descaling method according to the present invention. Strand shot blast is φ
Shot particles of 0.3 mm were carried out at a projection speed of 70 m / sec and a projection density of 25, 30, 40 and 60 kg / m ² . The pickling was performed by immersing the coil in an actual pickling tank. The pickling was carried out at an acid solution temperature of 40 to 45 ° C. The pickling time was set to 120 seconds, which is the conventional nitric hydrofluoric acid immersion time. The descaling property was evaluated by visually inspecting the surface of the wire using a 20 × magnifying glass, and as shown in FIG. 6, the scale was classified according to the residual form and evaluated. The wire drawability was evaluated by applying a lubricant to the wire material and continuously drawing wire at a wire drawing reduction rate of 70%, observing the surface properties after wire drawing, and confirming whether or not seizure flaws have occurred. The quality was judged.

[0020]

[Table 1]

[0021]

[Table 2]

The superiority of the present invention will be described below with reference to Tables 1 and 2 (continued from Table 1). Test No. No. 1 is a comparative example in which the pickling with hydrofluoric acid was carried out alone when the strand shot blasting was not carried out. The concentration of the applied aqueous solution of nitric hydrofluoric acid is 150 g / l of nitric acid and 50 g / l of hydrofluoric acid, which is the lower limit of the concentration range of nitric hydrofluoric acid according to the present invention, but scale remains on the entire surface of the wire after pickling. However, it was difficult to descale by pickling with nitric hydrofluoric acid alone.

Test No. Nos. 2 to 4 are shot projection densities changed to 25, 60, and 30 kg / m ² .
The concentration of nitric hydrofluoric acid aqueous solution is 150 g / l nitric acid, 50 g / hydrofluoric acid
1 and the lower limit within the nitric hydrofluoric acid concentration range according to the invention was applied. Test No. No. 2 is a comparative example in which strand shot blasting with a projection density of 25 kg / m ² was carried out, but the scale removal rate after strand shot blasting was 21%.
The scale remained as a strip on the surface of the wire after the pickling with hydrofluoric acid. On the other hand, the test No. Nos. 3 and 4 are examples of the present invention in which a shot density of 60 kg / m ² and a shot density of 30 kg / m ² are applied, respectively, but the scale removal rates after strand shot blasting are 40% and 25%, respectively. Also, no scale remained after the nitric hydrofluoric acid pickling, and good descaling was possible.

As described above, the test No. From the results of 1 to 4, in order to complete descaling using the aqueous solution of nitric hydrofluoric acid according to the present invention, it is necessary to perform strand shot blasting with a projection density of 30 kg / m ² or more and remove 25% or more of the scale. It is clear that there is. Test No. 4 to 8 are
After carrying out strand shot blasting with a projection density of 30 kg / m ² or 40 kg / m ² , pickling was performed using an aqueous solution of hydrofluoric acid in which each concentration of nitric acid and hydrofluoric acid was changed within the concentration range specified in the present invention. It is an example of the present invention performed. The concentration of each aqueous solution of nitric hydrofluoric acid is as shown in Test No. No. 4 is nitric acid 150 g / l / hydrofluoric acid 50 g / l, test No. 4 No. 5 is 200 g / l nitric acid / 50 g / l hydrofluoric acid, and the test No. 6 is nitric acid 150 g / l / hydrofluoric acid 1
00g / l, test No. 7 is 200 g nitric acid / l / hydrofluoric acid 1
00g / l, test No. 8 is nitric acid 175 g / l / hydrofluoric acid 7
It is 5 g / l. In all cases, no scale remained after pickling, and the descaling property was good. Further, the surface quality after wire drawing was also free from seizure flaws and the wire drawability was good.

Test No. 9 to 12 have a projection density of 30 kg
/ M ² or 40 kg / m ² strand shot blasting followed by pickling using a nitric hydrofluoric acid aqueous solution in which a nitric acid concentration within the range of the present invention and a hydrofluoric acid concentration exceeding the upper limit of the present range were combined. Here is an example. The concentration of each aqueous solution of nitric hydrofluoric acid is as shown in Test No. 9 is 150g nitric acid / l / 150g hydrofluoric acid
/ L, test No. 10 is nitric acid 200 g / l / hydrofluoric acid 150
g / l, test No. 11 is nitric acid 150 g / l / hydrofluoric acid 20
0 g / l, test No. 12 is nitric acid 200 g / l / hydrofluoric acid 2
It is 00 g / l. In all cases, no scale remained after pickling and the descaling property was good, but the pits on the surface of the wire became shallow, and seizure flaws occurred on the surface of the wire after wire drawing. It was confirmed that the degree of seizure flaws was higher when the concentration of hydrofluoric acid was higher, and that the drawability was impaired by increasing the concentration of hydrofluoric acid.

Test No. 13 to 20 are comparative examples carried out to determine the superiority within the concentration range of the nitric hydrofluoric acid aqueous solution specified in the present invention. That is, the projection density is 30 kg /
After performing m ² strand shot blasting, pickling was performed using a nitric hydrofluoric acid aqueous solution having the following concentration. Test N
o. 13 is a nitric acid concentration less than the lower limit of the present invention of 125 g /
1 and a nitric hydrofluoric acid aqueous solution having a hydrofluoric acid concentration of 37 g / l, which is less than the lower limit of the range of the present invention, the pickling was carried out, but after the pickling, the scale remained in a band shape.

Test No. 14 is a nitric acid concentration of 175 g / l within the range of the present invention and a hydrofluoric acid concentration of 37 g below the lower limit of the range of the present invention.
Although pickling was carried out using an aqueous solution of nitric hydrofluoric acid of 1 / l, the scale remained as a band after pickling. Test No. No. 15 was pickled using a nitric acid hydrofluoric acid aqueous solution having a nitric acid concentration of 225 g / l exceeding the upper limit of the present invention and a hydrofluoric acid concentration of 37 g / l lower than the lower limit of the present invention. It remained.

Test No. 16 is a nitric acid concentration of less than the lower limit of the present invention of 125 g / l and a hydrofluoric acid concentration of 75 g of the present invention.
Although pickling was carried out using an aqueous solution of nitric hydrofluoric acid of 1 / l, the scale remained as a band after pickling. Test No. No. 17 was pickled using a nitric acid hydrofluoric acid aqueous solution having a nitric acid concentration of 225 g / l exceeding the upper limit of the present invention and a hydrofluoric acid concentration of 75 g / l within the present invention, but the scale remained strip-shaped after pickling. .

Test No. No. 18 was pickled using a nitric hydrofluoric acid aqueous solution having a nitric acid concentration of 125 g / l below the lower limit of the present invention and a hydrofluoric acid concentration of 125 g / l above the upper limit of the present invention. It remained. Test No.
No. 19 was pickled using an aqueous nitric hydrofluoric acid solution having a nitric acid concentration of 175 g / l within the range of the present invention and a hydrofluoric acid concentration of 125 g / l exceeding the upper limit of the range of the present invention. There was no scale residue after pickling, and the descaling property was good, but a slight seizure flaw occurred on the surface of the wire rod after wire drawing, and wire drawability was poor.

Test No. No. 20 was pickled using a nitric acid hydrofluoric acid aqueous solution having a nitric acid concentration of 225 g / l exceeding the upper limit of the present invention and a hydrofluoric acid concentration of 125 g / l exceeding the upper limit of the present invention. It remained. From the above, the test No. out of the concentration range of the nitric hydrofluoric acid aqueous solution defined in the present invention was obtained. The results of Nos. 13 to 20 are all poor in descaling property or wire drawing property. Therefore, the concentration range of the nitric hydrofluoric acid aqueous solution defined by the present invention, that is, the nitric acid concentration of 15
The superiority of 0 to 200 g / l and hydrofluoric acid concentration of 50 to 100 g / l was confirmed.

Test No. 21 is a conventional method, that is, a method of performing pickling after the molten alkali treatment. After performing molten alkali treatment and hydrochloric acid pickling, nitric acid 200 g /
l, hydrofluoric acid was pickled at a concentration of 50 g / l. Both descaling property and wire drawing property are good, but there is a drawback that the cost of molten alkali treatment is high.

[0032]

INDUSTRIAL APPLICABILITY The present invention is capable of producing a wire surface having excellent wire drawability at a low cost which does not require molten alkali treatment or complicated pickling treatment when descaling an austenitic stainless steel wire rod. It provides a scale method, and has a great industrial effect.

[Brief description of drawings]

FIG. 1 is a diagram showing a relationship between shot projection density and descaling property by shot + nitric hydrofluoric acid pickling.

FIG. 2 is a diagram showing a relationship between shot projection density and scale removal rate by shots.

FIG. 3 is a diagram showing an optimum condition range of a nitric hydrofluoric acid aqueous solution concentration.

FIG. 4 is a diagram showing a state of adhesion of a pit and a lubricant after scale removal.

FIG. 5 is a diagram showing a descaling method of the present invention.

FIG. 6 is an explanatory diagram of a scale residual form.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Yutaka Sadano 3434 Shimada, Hikari City, Yamaguchi Prefecture Nippon Steel Corporation Hikari Steel Works Ltd.

Claims (1)

[Claims] 1. A method of descaling an austenitic stainless steel wire rod, wherein a scale of 25% to 80% in area ratio is removed by strand shot blasting with a projection density of 30 kg / m ² or more, and rewinding into a coil, The remaining scale is 150-200 g of nitric acid /
1. A method for descaling an austenitic stainless steel wire rod, which comprises removing by performing coil pickling with a nitric hydrofluoric acid aqueous solution containing 50 to 100 g / l of hydrofluoric acid.