JP2008056995A - Hot dip galvannealed steel sheet having excellent appearance quality and its production method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a hot dip galvannealed steel sheet having excellent appearance quality. <P>SOLUTION: Regarding the hot dip galvannealed steel sheet having excellent appearance quality in which the surface of a steel sheet comprising, by mass, 0.001 to 0.01% C, 0.001 to 0.2% Si, 0.01 to 2% Mn, 0.02 to 0.2% P, 0.001 to 0.03% S, 0.005 to 0.1% Al, 0.001 to 0.05% Ti, 0.001 to 0.05% Nb, and the balance Fe with inevitable impurities is provided with an iron-zinc alloy covering comprising ≥85% Zn, the surface layer part of matrix in a depth direction within 20 μm from the matrix surface is composed of ferrite grains satisfying the average crystal grain size of ≤15 μm and also the standard deviation of the crystal grain sizes of ≤8 μm in an observation field of 500μm×500μm. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to an galvannealed steel sheet excellent in appearance quality and a method for producing the same. More specifically, mainly in an alloyed hot-dip galvanized steel sheet based on a high-strength steel sheet, the appearance after plating is more uniform and beautiful than the conventional alloyed hot-dip galvanized steel sheet. The present invention relates to an alloyed hot-dip galvanized steel sheet excellent in appearance quality that can be used for automobiles and the like, and a method for producing the same.

Alloyed hot dip galvanized steel sheets are widely used in automobiles, home appliances, building materials and the like because of their excellent weldability, paintability, and post-paint corrosion resistance. This alloyed hot dip galvanized steel sheet is a hot dip galvanized steel sheet, and then heat-treated to diffuse Fe in the steel and Zn in the plating to cause an alloying reaction, thereby causing an iron-zinc alloy layer on the steel surface. Is formed. This alloying reaction occurs preferentially from the grain boundaries of the steel, but when the surface crystal grain size of the steel sheet is coarse, the total grain boundary length of the crystal is shorter than when the grain size is fine. A difference occurs in the alloying reaction rate of the steel sheet as a whole. That is, the alloying of the steel plate having coarse crystals is slower in alloying and the plating becomes thinner than the steel plate having fine crystals. Therefore, in a steel plate in which coarse grains and fine grains are mixed, unevenness of plating is generated due to a local alloying speed difference, resulting in poor appearance.
Moreover, when many elements which are easy to segregate are contained in grain boundaries, such as P, it has the function which further promotes the alloying reaction rate difference, and the external appearance defect of plating worsens further.

  For this reason, various measures for improving the appearance of plating by making the crystal grain size of the surface layer of the steel sheet uniform have been proposed. For example, in Patent Document 1, the ratio R (= d1 / d2) between the average ferrite particle diameter d1 on the outermost surface at a certain point on the steel sheet and the average ferrite particle diameter d2 on the outermost surface at a point 10 mm or more away from that point is However, even if R is 0.9 or more and 1.1 or less, the ferrite grain size is coarse (d1, d2). Is large), the plating appearance is not uniform.

  In Patent Document 2, the appearance is improved by setting the mixed crystal grain structure zone composed of equiaxed crystal grains and stretched crystal grains to 10% or less or 90% or more on the steel surface. However, even if the mixed grain structure zone is 10% or less or 90% or more, the distribution of equiaxed grains and stretched grains may be biased, or steel may contain alloying delay elements such as C and P. In the case of containing a large amount, the plating appearance does not improve.

  In Patent Document 3, the surface appearance of a hot rolled sheet as a plating original sheet is 70% or less in terms of area ratio in a field of view of 400 μm of ferrite crystals of 15 μm or less, and the plating appearance is improved. However, even if ferrite crystal grains of 15 μm or less occupy an area ratio of 70% or less, the appearance of plating is inferior when the distribution is uneven or when extremely coarse crystals exceeding 100 μm are present. .

  Further, in Patent Document 4, the finish rolling finish temperature of hot rolling is set to 1000 ° C. or (Ar3 transformation point + 20 ° C.), then scraped in the range of 700 to 820 ° C., and then cold rolled, annealed, melted It is said that a hot-dip galvanized steel sheet with excellent appearance quality can be manufactured by galvanizing. However, if the finish rolling temperature is in the above range, the appearance of plating due to the presence of non-recrystallized parts can be suppressed, but the appearance of plating due to non-uniform structure (mixed grain structure of coarse and fine grains) can be suppressed. It is difficult to eliminate defects.

  Furthermore, in patent document 5, it is supposed that the plating external appearance defect resulting from a fine structure can be suppressed by making the ferrite particle diameter of the steel surface in Ti containing steel into 8 micrometers or more. However, even if the surface ferrite grain size is 8 μm or more, the plating appearance does not improve if the grain size varies, such as when there are locally coarse crystals.

JP-A-7-228944 JP-A-8-20852 JP 2001-316663 A JP 2001-342522 A JP-A-2-38550

  As described above, various types of alloyed hot-dip galvanized steel sheets with excellent appearance quality and methods for producing the same have been proposed, but all of them are sufficient for the recent demands for strict appearance quality from customers. It cannot be answered. In addition, when a high-strength steel sheet containing P, Mn, or the like is used as a base, the crystal grain size after annealing is likely to be non-uniform, and as a result, the plating appearance is likely to deteriorate. The proposed method has little improvement effect.

  Therefore, the present invention has an improved effect compared to the conventionally proposed method, and can be applied to a high-strength steel sheet containing P, Mn, etc., which is likely to have a poor appearance, and has excellent appearance quality. It aims at providing a galvanized steel plate and its manufacturing method.

  In order to solve the above-mentioned problems, the inventors of the present invention are alloyed hot-dip galvanized steel sheets based on steel sheets containing 0.02% to 0.2% P. About the steel plate which nonuniformity did not generate | occur | produce, the crystal structure of the base iron which is a base material was investigated in detail. As a result, the surface iron surface of the steel sheet with uneven appearance has a lot of coarse ferrite grains, and the variation is large, while the surface iron surface of the steel sheet with no appearance unevenness is about 15 μm. It was found that the following ferrite grains accounted for the majority and almost no coarse grains were present. It was also found that a steel sheet containing a large amount of P and Mn was produced by a specific method, whereby the crystal structure after annealing became uniform and fine, and the appearance after plating and alloying became beautiful. That is, it has been found that the appearance can be controlled by controlling the ferrite grain size on the surface of the steel sheet after annealing. The gist of the present invention is as follows.

(1) In mass%,
C: 0.001% to 0.01%,
Si: 0.001% or more and 0.2% or less,
Mn: 0.01% or more and 2% or less,
P: 0.02% to 0.2%
S; 0.001% or more and 0.03% or less,
Al; 0.005% or more and 0.1% or less,
Ti: 0.001% or more and 0.05% or less,
Nb: 0.001% or more and 0.05% or less,
In the alloyed hot-dip galvanized steel sheet having an iron-zinc alloy coating containing 85% or more of Zn on the surface of the steel sheet, the balance of which is Fe and inevitable impurities, the surface layer portion within 20 μm in the depth direction from the surface of the base iron However, in an observation visual field of 500 μm × 500 μm, an alloyed hot-dip galvanized steel sheet having excellent appearance quality, comprising ferrite grains satisfying an average crystal grain size of 15 μm or less and a standard deviation of crystal grain size of 8 μm or less.

  (2) After hot rolling the low carbon steel slab comprising the components described in (1), pickling, and further subjecting to cold rolling, annealing, hot dip galvanizing, and heat alloying treatment, alloyed hot dip zinc In the method for producing an alloyed hot-dip galvanized steel sheet as a plated steel sheet, hot rolling is performed in a range where the finish rolling end temperature is not less than Ar3 point and less than (Ar3 point + 20 ° C.) in both the sheet width and the rolling direction. And then cooling to 750 ° C. at a rate of 25 ° C./second or more, a method for producing an alloyed hot-dip galvanized steel sheet having excellent appearance quality.

The alloyed hot-dip galvanized steel sheet of the present invention and the alloyed hot-dip galvanized steel sheet that has undergone the production method of the present invention are excellent in appearance quality, slidability, and adhesion. For this reason, it can be used for automobiles, home appliances, building materials and the like, and its industrial value is extremely large.

  Hereinafter, the present invention will be described in detail.

  First, the reasons for limiting the steel components of the galvannealed steel sheet of the present invention will be described.

P: 0.02% or more and 0.2% or less P is added to the steel for the purpose of increasing the strength of the steel. Since P tends to be concentrated at the grain boundaries when the steel is heated, if the concentration is uneven, a difference in the alloying rate of plating occurs, resulting in poor appearance. If P is less than 0.02%, in the ordinary method for producing a galvannealed steel sheet, the unevenness of concentration is small and the difference in alloying speed hardly occurs. On the other hand, when P is added excessively, it becomes easy to become brittle. For this reason, P in this invention is mass%, and is 0.02% or more and 0.2% or less.

C: 0.001% to 0.01% C is an element necessary for strengthening steel. However, if the amount of C is less than 0.001%, the strength is insufficient, and if it exceeds 0.01%, embrittlement tends to occur. Therefore, the amount of C is made 0.001% to 0.01%.

Si: 0.001% or more and 0.2% or less Si is an element having the effect of strengthening and deoxidizing steel. However, if it is added excessively, it tends to become brittle. In addition, the wettability of the plating is hindered during hot dip galvanizing, and the plating adhesion is deteriorated. On the other hand, even if it reduces excessively, it only increases the manufacturing cost and does not bring any merit. For this reason, Si is made 0.001% or more and 0.2% or less.

Mn: 0.01% or more and 2% or less Mn is an element having the effect of strengthening and deoxidizing steel. However, if it is added excessively, it tends to become brittle. In addition, the wettability of the plating is hindered during hot dip galvanizing, and the plating adhesion is deteriorated. On the other hand, even if it reduces too much, it only increases the manufacturing cost. For this reason, Mn is made 0.01% or more and 2% or less.

S: 0.001% or more and 0.03% or less S is an impurity, and it is preferable to reduce the S content because it degrades workability and hot brittleness. However, excessive reduction causes an increase in manufacturing cost. For this reason, S is made 0.001% or more and 0.03% or less.

Al: 0.005% or more and 0.1% or less Al has a deoxidizing effect. Moreover, the affinity with N in steel is strong, and there exists an effect which fixes the solid solution N as a precipitate and improves workability. However, if the amount is too large, the workability is deteriorated. For this reason, Al is made 0.005% or more and 0.1% or less.

Ti: 0.001% or more and 0.05% or less Ti has the effect of fixing C and N and improving the workability of steel. However, if the amount is too large, the workability is deteriorated. For this reason, Ti is made 0.001% or more and 0.05% or less.

Nb: 0.001% or more and 0.05% or less Nb has the effect of fixing C and improving the workability of steel. However, if the amount is too large, the workability is deteriorated. For this reason, Nb shall be 0.001% or more and 0.05% or less.

Next, the iron-zinc alloy coating will be described. The Zn content in the iron-zinc alloy coating is 85% or more. If it is less than 85%, it is inferior to paintability and weldability. Further, the deviation of the adhesion amount of the iron-zinc alloy coating is desirably ± 7 g / m ² or less. When the standard deviation exceeds ± 7 g / m ² , the appearance unevenness (shading) is promoted, and the appearance is likely to be poor. The deviation of the adhesion amount may be such that, after plating, a wiping gas for controlling the adhesion amount of the plating uniformly hits the plate width and the longitudinal direction of the plate.

The adhesion amount of the iron-zinc alloy coating is not particularly specified, but is preferably ²⁰ to 100 g / m ² from the viewpoint of corrosion resistance and workability.

  Next, the steel structure will be described. The structure of the surface layer of the galvannealed steel sheet of the present invention has an average crystal grain size of 15 μm or less and a standard deviation of the crystal grain size of 8 μm within an observation field of 500 μm × 500 μm within 20 μm from the surface layer. It shall consist of ferrite grains satisfying the following. When the average crystal grain size exceeds 15 μm, even if the variation in grain size is small, the appearance of plating may be poor due to the difference in alloying speed between ferrite grain boundaries and ferrite grains. When the standard deviation of the crystal grain size exceeds 8 μm, the difference in alloying speed between the small part and the large part becomes remarkable, and the appearance after plating becomes poor. Therefore, it is necessary that the average crystal grain size is 15 μm or less and the standard deviation of the crystal grain size is 8 μm or less. The smaller the average crystal grain size and the standard deviation, the better the appearance, and in particular, no other adverse effects are caused. Therefore, the average crystal grain size and the lower limit of the standard deviation are not specified. Here, the average crystal grain size is an average value of the equivalent circle diameter of each crystal (the diameter of a circle having the same area as each crystal).

The alloyed hot-dip galvanized steel sheet of the present invention is manufactured by hot rolling a low carbon steel slab, pickling, and further performing cold rolling, annealing, hot-dip galvanizing, and heat alloying treatment. The slab heating conditions and hot rolling conditions are not particularly specified, and there is no problem as long as they are conditions for producing a general steel sheet. However, the finish rolling temperature during hot rolling is any of the sheet width and rolling direction. Also in the direction of, it is necessary to be at least Ar3 point and less than (Ar3 point + 20 ° C.). The Ar3 point referred to here is the γ → α transformation temperature in the cooling process and is obtained from the following equation.
Ar3 = 896-509 × (C%) + 26.7 × (Si%) − 63.5 × (Mn%) + 229 × (P%)

  Further, after finish rolling, it is necessary to cool to 750 ° C. at a rate of 25 ° C./second or more in both the sheet width and the rolling direction. By hot rolling under these conditions, it is possible to realize an alloyed hot-dip galvanized steel sheet having a fine and uniform ferrite grain size on the surface of the steel sheet of the present invention in combination with subsequent cold rolling and annealing conditions. . When the finish rolling temperature is lower than the Ar3 point, the ferrite grain size after annealing becomes nonuniform and remarkably coarse, and the appearance quality after hot dip galvanizing and heat alloying treatment is remarkably inferior. On the other hand, when the finish rolling temperature is (Ar3 point + 20 ° C.) or higher, the structure after annealing becomes coarse, and unevenness in appearance quality after hot dip galvanizing and heat alloying treatment tends to occur. A preferable range is (Ar3 + 5 ° C.) or more and (Ar3 point + 10 ° C.) or less.

  FIG. 1 shows the relationship between the finish rolling temperature, the average grain size, and the standard deviation. By setting the finish rolling temperature to not less than Ar3 point and less than (Ar3 point + 20 ° C.), the steel sheet surface structure is composed of ferrite grains satisfying an average crystal grain size of 15 μm or less and a standard deviation of crystal grain size of 8 μm or less. It can be seen that the galvannealed steel sheet of the present invention can be achieved.

  After the finish rolling, when the cooling rate to 750 ° C is less than 25 ° C / sec, the crystal structure of the steel sheet after hot rolling becomes uneven and coarse, and even in the subsequent cold rolling and annealing, the uniform structure is It is difficult to obtain. When cooled at a temperature of 25 ° C./second to a temperature lower than 750 ° C., the particle size becomes fine, but an element causing a plating appearance defect such as P tends to be concentrated on the steel plate surface, which is not preferable. A preferable cooling rate is 30 ° C./second or more. Note that if the cooling rate is extremely increased, the material becomes brittle, so the upper limit is set to 100 ° C./second.

  Further, when winding the coil, the coil may be wound at a general temperature, for example, a temperature of 550 ° C. or more and less than 750 ° C. After winding, the surface temperature of the coil outermost layer is 300 ° C. or less. It is preferable to cool at a moderate rate of 0.8 ° C./second or less until In this way, by slowly cooling after scraping, elements that cause poor appearance such as P are taken into the scale, and the concentration of the steel sheet surface is suppressed, combined with the effect of making the underlying structure uniform and fine, Can significantly improve the appearance after plating of a high-strength steel sheet containing P, which has been difficult.

  After hot rolling, pickling is performed, and the scale generated during hot rolling is removed. What is necessary is just to implement the pickling conditions by the method currently performed conventionally, for example, a steel plate is immersed in 50 degreeC or more hydrochloric acid. Although it cold-rolls after pickling, it is preferable that the rolling reduction shall be 80% or more. If it is less than 80%, the ferrite grain size after annealing tends to be coarser than 15 μm. After cold rolling, it is preferable to heat at a soaking temperature of 750 ° C. or higher and 850 ° C. or lower for 30 seconds or more and 150 seconds or less. If it is out of this range, recrystallization may be incomplete and the particle size may be uneven and coarse. After annealing, hot dip galvanization and heat alloying treatment are performed. The temperature of the galvanizing bath is preferably 440 ° C to 500 ° C, and the heating alloying temperature is preferably 480 to 560 ° C.

  By producing under the above conditions, a hot dip galvanized steel sheet excellent in appearance quality of the present invention can be realized.

Steel having the composition shown in Table 1 was melted in a converter and continuously cast into a slab. The slab was heated at 1200 ° C. for 1 hour and then hot-rolled to obtain a hot-rolled steel plate having a thickness of 4 mm. The finishing temperature at the time of hot rolling was set to three levels of less than Ar3 point, less than (Ar3 point to Ar3 point + 20 ° C.), and Ar3 point + 20 ° C. or more. Moreover, the cooling rate until it reaches 750 ° C. after finish rolling was set to two levels of less than 25 ° C./second and 25 ° C./second or more. The scraping temperature was 700 ° C., and cooling was performed at a rate of about 1 ° C./second until 300 ° C. was reached. The obtained hot-rolled steel sheet was pickled in 10% hydrochloric acid and then cold-rolled to obtain a cold-rolled steel sheet having a thickness of 1 mm. The cold-rolled steel sheet was annealed using a continuous hot dipping plating equipment with a soaking temperature of 800 ° C. and a soaking time of 120 seconds as standard conditions, cooled to 465 ° C. at a cooling rate of 20 ° C./second, and then a bath temperature of 460 ° C. Immerse in a Zn-0.13% Al plating bath for 3 seconds, adjust by wiping to an adhesion amount of 45 g / m ^2, and then heat alloying at a temperature according to the steel type, alloying hot dip galvanization A steel plate was produced.

The produced galvannealed steel sheet was evaluated as follows.
(1) Appearance Visual observation was performed, and grades 1 to 5 were classified into 5 levels in increments of 1 depending on the degree of appearance unevenness. A score of 3 or less is acceptable.
(2) Observation of ferrite particle size The plating was dissolved with 10% hydrochloric acid, the surface layer of the steel plate was observed with SEM, and the average crystal grain size in the outermost layer in the 500 × 500 μm field of view and the standard deviation of the crystal grains were determined. The SEM used is S-2460N manufactured by Hitachi.
(3) Adhesion amount of iron-zinc alloy coating layer The adhesion amount of iron-zinc alloy coating was determined, and its deviation and composition were measured. For the measurement, several places were selected from arbitrary positions in the plate width direction and the longitudinal direction, and the iron-zinc alloy coating was dissolved with 10% hydrochloric acid and analyzed by ICP.

  Each evaluation result is shown in Table 2.

  No. 1 to No. 6, and no. 13 to No. 16 is the case where the finishing temperature, the average crystal grain size, and the standard deviation of the hot rolling of the steel A are changed. When the finishing temperature is below the range of the present invention (No. 1 to No. 6), since extremely coarse crystal grains are present on the surface layer of the steel, both the average crystal grains and the standard deviation are out of the scope of the present invention, and after plating The appearance is inferior. In addition, when the finish rolling temperature exceeds the range of the present invention (No. 13 to No. 16), the crystal growth is promoted, resulting in coarse grains and out of the range of the present invention. Inferior. When the finishing temperature, average crystal grain size, and standard deviation are within the scope of the present invention (No. 7 to No. 12), the appearance after plating is uniform and beautiful. In particular, the finish rolling temperature is in the range of (Ar3 + 5 ° C.) to (Ar3 point + 10 ° C.). 8-No. No. 10 has a particularly good appearance.

  No. 17 to No. 19 is a case where the finishing temperature of hot rolling of steel B, the average crystal grain size, and the standard deviation are changed. When the finish rolling temperature, the crystal grain size, and the standard deviation are outside the scope of the present invention (No. 17 and No. 19), the appearance after plating becomes non-uniform. On the other hand, when the finish rolling temperature, grain size, and standard deviation are within the scope of the present invention (No. 18), the appearance after plating is uniform and beautiful.

  No. 20 to No. 22 is a case where the finishing temperature, the average crystal grain size, and the standard deviation of the hot rolling of the steel C are changed. When the finish rolling temperature, crystal grain size, and standard deviation are out of the scope of the present invention (No. 20 and No. 22), the appearance after plating becomes non-uniform. On the other hand, when the finish rolling temperature, crystal grain size, and standard deviation are within the scope of the present invention (No. 21), the appearance after plating is uniform and beautiful.

  No. Nos. 23 to 25 are cases where the cooling rate after finish rolling at the time of hot rolling of the steel A is 15 ° C./second. At any finish rolling temperature, the crystal grain size and standard deviation are outside the scope of the present invention, and the appearance after plating is inferior.

  No. 26 is Steel A, No. 26. 7 manufactured under the same conditions. However, when winding the coil after hot rolling, the coil was wound at 650 ° C. and cooled at a rate of 0.8 ° C./second until it became 300 ° C. or lower. In addition to the base structure becoming uniform and fine, the P concentration on the surface of the steel sheet was reduced. The appearance was better than 7.

The relationship between the finish rolling temperature, the average grain size, and the standard deviation.

Claims (2)

% By mass
C: 0.001% to 0.01%,
Si: 0.001% or more and 0.2% or less,
Mn: 0.01% or more and 2% or less,
P: 0.02% to 0.2%,
S; 0.001% or more and 0.03% or less,
Al; 0.005% or more and 0.1% or less,
Ti: 0.001% or more and 0.05% or less,
Nb: 0.001% or more and 0.05% or less,
In the alloyed hot-dip galvanized steel sheet having an iron-zinc alloy coating containing 85% or more of Zn on the surface of the steel sheet, the balance of which is Fe and inevitable impurities, the surface layer portion within 20 μm in the depth direction from the surface of the base iron However, in an observation visual field of 500 μm × 500 μm, an alloyed hot-dip galvanized steel sheet having excellent appearance quality, comprising ferrite grains satisfying an average crystal grain size of 15 μm or less and a standard deviation of crystal grain size of 8 μm or less.   After hot rolling the low carbon steel slab comprising the component according to claim 1, pickling, further cold rolling, annealing, hot dip galvanizing, heat alloying treatment, In the manufacturing method of the alloyed hot-dip galvanized steel sheet, the hot rolling is finished in a range where the finish rolling end temperature is not less than Ar3 point and less than (Ar3 point + 20 ° C.) in both the sheet width and the rolling direction, and thereafter A method for producing an alloyed hot-dip galvanized steel sheet excellent in appearance quality, characterized by cooling to 750 ° C. at a rate of 25 ° C./second or more.

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