JP2006193832A - Method for producing cold-rolled steel sheet - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for producing a cold-rolled steel sheet in which uniform strain is formed on the surface, to provide a method for producing a plated steel sheet utilizing the same, and, further concretely, to provide a method for annealing a cold rolled steel sheet in which uniform strain is formed on the surface at a low production cost (by single annealing). <P>SOLUTION: The temperature of a steel strip on and after a cooling zone composing a continuous annealing stage for a cold rolled steel sheet is controlled to the range of ≥100°C, and rolling is performed at a draft of 0.2 to 2.0%. Next, the steel strip is passed through as being contacted with two or more rolls having an outer diameter of 50 to 500 nm at a contact angle of ≥30°, respectively. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a method for annealing a cold-rolled steel sheet, and more specifically to a method for producing a cold-rolled steel sheet using the method.

Plated steel sheets such as hot dip galvanized and electrogalvanized are excellent in corrosion resistance, and are therefore widely used in automobiles, household electrical products, building materials, and the like. The use of hot-dip galvanized steel sheets and electrogalvanized steel sheets is also expanding for automobile exteriors, where appearance quality is particularly important, and improvements in the appearance quality of steel sheets are required. There are irregularities in the surface of the steel sheet and uneven plating on the electrogalvanized steel sheet. The unevenness of the surface and the unevenness of plating are not eliminated even by coating, and deteriorate the appearance quality of the product. The cause of surface irregularity defects is thought to be due to non-uniform reaction of the steel sheet and zinc due to non-uniform activity of the steel sheet surface after continuous annealing, resulting in non-uniform plating thickness. It appears as a visible defect later. On the other hand, the cause of uneven plating is thought to be because the orientation of the plating differs due to the non-uniformity of the activity of the steel sheet surface after continuous annealing.

  As a technique for preventing such surface defects, for example, Japanese Patent Application Laid-Open No. Hei 4-191354 discloses a region where the steel sheet temperature is 500 ° C. or higher in the soaking zone and / or the heating zone in the continuous annealing process before hot dip galvanizing. Thus, a method for producing an alloyed hot-dip galvanized steel sheet that passes through two or more rolls having an outer diameter of 50 mm or more and 500 mm or less at a contact angle of 30 ° or more is presented. In this method, the subsequent alloying reaction is made uniform as a uniform surface structure by applying strain with a small-diameter roll to prevent the occurrence of surface irregularities.

  JP-A-9-176814 discloses annealing at a recrystallization temperature or higher, then reducing at a reduction rate of 0.8 to 2.0%, and then annealing again to perform hot dip galvanizing and alloying treatment. A method for producing an alloyed hot dip galvanized steel sheet is presented.

  However, the method presented in Japanese Patent Application Laid-Open No. 4-191354 has a problem that rough skin is likely to occur during press molding in an automobile manufacturing process or the like.

  Further, the method presented in Japanese Patent Laid-Open No. 9-176814 has a problem that the annealing process is required twice and the manufacturing cost is increased.

  An object of the present invention is to solve the above-mentioned problems and to produce a cold-rolled steel sheet used in the manufacture of a plated steel sheet such as a hot-dip galvanized steel sheet or an electrogalvanized steel sheet having an excellent surface appearance shape, and a plated steel sheet using the same. It is in providing the manufacturing method of. Furthermore, specifically, annealing of cold-rolled steel sheets that prevents surface unevenness and surface roughness in alloyed hot-dip galvanized steel sheets and prevents plating adhesion unevenness in electrogalvanized steel sheets at a low manufacturing cost (one annealing). It is to provide a method.

  From the viewpoint of improving the activity of the steel sheet surface, the present inventors pay attention to bending and rolling performed in the continuous annealing process prior to the plating treatment. In the bending process, a large distortion is formed on the steel sheet surface, but the distortion is not. It became uniform and non-uniform work-hardened parts were formed, and in rolling, a uniform strain was formed on the surface of the steel sheet, but on the other hand, attention was paid to the fact that the strain reaches the inside of the steel sheet.

  Therefore, after the cooling zone of continuous annealing, a method of forming a uniform large strain on the steel sheet surface by forming an appropriate strain on the steel sheet by rolling and then forming a strain on the steel sheet surface by bending, The present invention has been completed.

The gist of the present invention is as follows (1).
(1) In a continuous annealing process consisting of a heating zone, a soaking zone, and a cooling zone of a cold-rolled steel sheet, rolling is performed at a reduction rate of 0.2% or more and 2.0% or less in the region after the cooling zone, and then the outer diameter is 50 mm or more. A method for producing a cold-rolled steel sheet, comprising passing through two or more bending rolls of 500 mm or less while contacting each other at a contact angle of 30 ° or more.

  According to the present invention, a hot-dip galvanized steel sheet and an electrogalvanized steel sheet excellent in appearance quality can be manufactured.

  FIG. 1 is a schematic explanatory view of a continuous annealing apparatus having a hot dip galvanizing apparatus for carrying out the present invention.

  In the figure, the steel plate 1 is sent to a continuous annealing apparatus 5 comprising a heating zone 2, a soaking zone 3 and a cooling zone 4, and after annealing, it is passed through a snout 6 and accommodated in a zinc pot 8 containing a hot dip galvanizing bath 7. Then, hot dip galvanization is performed, and then, it is sent to the heating device 9 for alloying treatment.

  Although the present invention is not necessarily limited thereto, if one production condition is indicated, the ultra low carbon steel is heated to 700 to 900 ° C. for 10 to 60 seconds in a non-oxidizing atmosphere in the continuous annealing process. After that, it is cooled to 450 to 500 ° C. in the cooling zone, and then plated in a hot dip galvanizing bath maintained at 450 to 465 ° C., and heated at 450 to 600 ° C. for 5 to 20 seconds to perform alloying treatment. It is carried out.

  According to the present invention, the steel sheet is rolled and bent after the cooling zone in the continuous annealing step. In the illustrated example, a pair of rolling rolls 10a and 10b are provided in the vicinity of the exit side of the cooling zone for rolling, and a pair of bending work is provided on the downstream side in the vicinity of the exit side of the cooling zone. Rolls 11a and 11b are provided to pass between them.

  The specific processing conditions at this time are rolling at a rolling reduction of 0.2% or more and 2.0% or less, and then using one or more opposing pairs of outer diameters of 50 mm or more and 500 mm or less, that is, two or more bending rolls. It is used, and it passes through it while contacting at a contact angle of 30 ° or more.

  If the rolling reduction ratio is less than 0.2%, the strain applied to the steel sheet surface becomes insufficient, the strain formed on the steel sheet surface after bending becomes non-uniform, and the uniformity of plating reaction and electrogalvanization The uniformity of the plating orientation is reduced. On the other hand, if it exceeds 2.0%, a large strain remains in the steel sheet, so that the elongation is lowered and the material properties are deteriorated. Therefore, rolling was performed at a rolling reduction of 0.2% to 2.0%.

  If the outer diameter of the bending roll is less than 50 mm, there is a problem that the roll strength is insufficient and equipment troubles occur, or equipment such as a backup roll is required, resulting in high equipment costs. Furthermore, since distortion remains in the steel plate, the material characteristics deteriorate. On the other hand, if it exceeds 500 mm, the effect of imparting strain to the steel sheet surface is reduced. Therefore, the outer diameter of the roll was set to 50 mm or more and 500 mm or less.

  When the contact angle is less than 30 °, the effect of imparting strain to the steel sheet surface is small. The upper limit of the contact angle is not particularly limited, but is preferably 180 ° or less from the viewpoint of equipment constraints. The reason why the number of bending rolls is set to two or more is that, when the sheet is passed by one roll, the plating reactivity is different between the front surface and the back surface of the steel sheet. In addition, since a material characteristic will deteriorate when the number of rolls increases, 3 pairs (6) or less are desirable.

  FIG. 2 exemplifies the sheet passing form of the steel sheet during rolling and bending in the present invention, and FIG. 2 (a) shows two bending processes paired downstream of a pair of rolling rolls. FIG. 2B shows a case where two pairs of bending rolls are provided. In the figure, the same elements as those in FIG. 1 are denoted by the same reference numerals. Symbol α is a contact angle.

  Even if such a roll for rolling and a roll for bending are provided in a heating zone or a soaking zone, the strain energy introduced into the steel sheet by rolling or bending is released, and the effect of improving the uniformity of the plating reaction or electrogalvanizing The effect of improving the plating orientation is reduced. Therefore, the rolling and bending processes in the present invention are performed after the cooling zone, but here, “after the cooling zone” is from the vicinity of the cooling zone exit side in the case of a hot dip galvanizing line directly connected to the continuous annealing apparatus. In the case of a continuous annealing apparatus for electrogalvanized steel sheets until just before the plating bath, it is particularly limited as long as it is from the outlet side of the primary cooling zone to just before the final cooling zone provided for cooling to room temperature. However, in the former case, the vicinity of the cooling zone outlet side is preferable, and in the latter case, the outlet side of the primary cooling zone is preferable.

  On the other hand, even if a rolling and bending mechanism is provided after the cooling zone, if the steel sheet temperature is less than 100 ° C., a large strain remains in the steel sheet, so that the elongation is greatly reduced and the material properties such as press formability deteriorate. Therefore, the steel sheet temperature is set in the region of 100 ° C. or higher after the cooling zone. The upper limit of the temperature is not particularly limited, but is generally about 600 ° C. or less because it is after the cooling zone.

  In FIG. 1, reference numeral 12 denotes a turn roll that changes the traveling direction of the steel sheet. Usually, a roll having a diameter of about 800 mm is used, but the roll is changed to a predetermined dimension and a predetermined contact angle is obtained. It may be arranged as described above and used as a bending roll of the present invention.

  Here, although this invention was demonstrated to the example of the continuous annealing process in manufacture of a hot dip galvanized steel plate, it is the same also in the continuous annealing process which manufactures the raw material of an electrogalvanized steel plate. In the production of an electrogalvanized steel sheet, the plating process and the continuous annealing process are generally not directly connected, and the present invention is not necessarily limited to this. In the continuous annealing process, after heating in a non-oxidizing atmosphere at 700 to 900 ° C. for 10 to 60 seconds, it was cooled to 350 to 550 ° C. in the primary cooling zone, and then held at 350 to 450 ° C. for several minutes. Thereafter, it is cooled to 150 to 250 ° C. in the secondary cooling zone, and then cooled to room temperature in the final cooling zone. Thereafter, the cooled coil is conveyed to an electrogalvanizing production line and subjected to a plating process. The present invention can be applied to this continuous annealing process.

Example 1
Using a hot dip galvanizing manufacturing facility with the structure shown in FIG. 1, using an ultra-low carbon steel plate with a thickness of 0.8 mm and a width of 1200 mm, in a continuous annealing process, it is cooled to 50 to 500 ° C. after heating at 870 ° C. for 30 seconds. Then, rolling is performed at a rolling reduction of 0.0 (no rolling) to 3.4% using a rolling roll having an outer diameter of 450 mm, and subsequently bent at a contact angle of 60 ° using a pair of rolls having an outer diameter of 30 mm or 600 mm. Then, it was sent to a hot dip galvanizing bath at 460 ° C., and an alloying treatment was performed at 500 ° C. after the completion of plating.

  About the obtained galvannealed steel sheet, surface observation was performed using a scanning electron microscope to investigate the uniformity of the plating thickness, and a tensile test (JIS No. 5 test) was conducted to investigate the workability. The uniformity of plating thickness was evaluated by classifying as follows, assuming that the variation rate of plating thickness (plating thickness variation / average plating thickness) was less than 10%. In addition, the workability was evaluated by classifying as follows, assuming that the elongation was 48% or more. Table 1 shows the results.

・ Uniformity of plating thickness (rate of variation of plating thickness):
○: Less than 10%;
Δ: 10% or more and less than 20%;
X: 20% or more.

・ Processability (elongation):
○: 48% or more;
Δ: Less than 48% 45% or more;
X: Less than 45%.

  As shown in Comparative Examples 1, 4, and 5 in Table 1, the workability was poor when the steel plate temperature was less than 100 ° C., the rolling reduction was over 2.0%, or the bending roll diameter was less than 50 mm. . Further, as shown in Comparative Examples 2, 3, and 6, when the rolling reduction is less than 0.2% and the diameter of the bending roll is more than 500 mm, the plating thickness is uneven and the surface appearance quality is poor. there were.

  On the other hand, Examples 1 and 2 of the present invention had uniform plating thickness, good surface appearance, and good workability. In the press molding test using the steel plates of Invention Examples 1 and 2, surface defects such as surface roughness did not occur and were satisfactory.

(Example 2)
Using actual equipment of continuous annealing, using ultra-low carbon steel plate with a thickness of 0.8mm and a width of 1200mm, heating at 850 ° C for 40 seconds, cooling to 50-400 ° C, then using a roll for rolling with an outer diameter of 450mm Rolled at a reduction ratio of 0.0 (no reduction) to 3.4%, and subsequently bent at a contact angle of 60 ° using a pair of bending rolls having an outer diameter of 30 mm or 600 mm. Manufactured. Subsequently, it sent to the electrogalvanization equipment and the plating process was performed.

  About the obtained electrogalvanized steel sheet, the plating adhesion unevenness was investigated and a tensile test (JIS No. 5 test) was conducted to investigate the workability. The plating adhesion unevenness was visually observed on the surface of the steel sheet, and was evaluated in three stages: ◯: no adhesion unevenness; Δ: adhesion unevenness small; x: adhesion unevenness large; The workability was evaluated by the same method as described in Example 1. Table 2 shows the results.

  As shown in Comparative Examples 1, 4 and 5 in Table 2, the workability was poor when the steel plate temperature was less than 100 ° C., the rolling reduction was over 2.0%, or the bending roll diameter was less than 50 mm. . Further, as shown in Comparative Examples 2, 3, and 6, when the rolling reduction is less than 0.2% and the diameter of the bending roll exceeds 500 mm, uneven adhesion of the plating occurs and the steel sheet appearance quality is poor. It was.

  In Invention Examples 1 and 2, there was no uneven plating adhesion and good workability.

It is a schematic explanatory drawing of the continuous annealing apparatus which has the hot dip galvanizing apparatus which implements this invention. FIG. 2 (a) illustrates two plate-forming rolls that are paired downstream of a pair of rolling rolls. FIG. 2B shows a case where two pairs of bending rolls are provided.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Steel plate 2 Heating zone 3 Soaking zone 4 Cooling zone 5 Continuous annealing device 6 Snout 7 Hot dip galvanizing bath 8 Zinc pot 9 Heating device 10a, 10b Roll for rolling 11a, 11b, 11c, 11d Bending roll 12 Turn roll α Contact Corner

Claims (1)

In a continuous annealing process consisting of a heating zone, a soaking zone, and a cooling zone for a cold-rolled steel sheet, rolling is performed at a reduction rate of 0.2% to 2.0% in the region after the cooling zone, and then the outer diameter is 50 mm to 500 mm A method for producing a cold-rolled steel sheet, comprising passing through two or more bending rolls while contacting each other at a contact angle of 30 ° or more.

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