JP2007247017A - Galvannealed steel sheet having excellent appearance grade and method for manufacturing the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a galvannealed steel sheet having an excellent appearance grade. <P>SOLUTION: The galvannealed steel having the excellent appearance grade is the galvannealed steel sheet having an iron-zinc alloy coating containing ≤85% Zn on the surface of a steel consisting, by mass%, of ≤0.01 C, ≤0.2% Si, ≤2% Mn, 0.02 to 0.2% P, ≤0.03% S, 0.005 to 0.1% Al and the balance Fe with inevitable impurities, in which the matrix surface layer part within a depth direction 40 μm from the matrix surface is ≤10% in area rate of ferrite grains of a grain size ≥40 μm in an observation visual field of 500 μm × 400 μm. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to an alloyed hot-dip galvanized steel excellent in appearance quality and a method for producing the same. More specifically, mainly in alloyed hot-dip galvanized steel based on high-strength steel sheets, the appearance after plating is more uniform and beautiful than conventional alloyed hot-dip galvanized steel sheets. The present invention relates to an alloyed hot-dip galvanized steel 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 is widely used in automobiles, home appliances, building materials and the like because of its excellent weldability, paintability, and post-paint corrosion resistance. This alloyed hot-dip galvanized steel is heat-treated after hot-dip galvanizing of the steel, diffuses Fe in the steel and Zn in the plating, and causes an alloying reaction to produce an iron-zinc alloy layer on the steel surface. Is formed. This alloying reaction is said to occur preferentially from the grain boundaries of the steel, but if the grain boundaries contain many elements that are easily segregated, the diffusion of Fe and Zn is locally inhibited, so the alloying reaction Becomes non-uniform and the appearance is uneven. Although this unevenness does not affect the mechanical properties or weldability, many consumers reject it as a poor appearance. In particular, in recent years, the strength of steel has increased and steel containing a large amount of P tends to cause unevenness, which is a problem. This is because P is unevenly concentrated on the steel surface and grain boundaries when the steel is heated, thereby inhibiting the diffusion of Fe and Zn during plating alloying, and the difference in local Fe and Zn alloying reaction rate. This is considered to cause a plating thickness difference.

  For this reason, various galvannealed steels having excellent appearance quality have been studied. For example, (Patent Document 1) discloses a method of forming a metal coating layer of Fe, Ni, Co, Cu or the like before plating an annealed steel sheet. However, this method is accompanied by an increase in equipment, which increases costs and processes, and is not easy to put into practical use. (Patent Document 2) discloses a method of defining the Al concentration in the plating bath by the concentrations of P and Ti in the steel. However, since the concentrations of P and Ti in the steel differ depending on the steel type, it is difficult to control the Al concentration in the bath according to the steel type, and the feasibility is extremely low. In (Patent Document 3), the diffraction X-ray intensity ratio from the {200} plane and {222} plane from the X-ray diffraction of the steel sheet surface after cold rolling annealing, I (200) / I (222) is 0.17. Although steel to be less than is disclosed, it is not easy to control the crystal orientation, and since the P content of the target steel material is 0.025% or less, high strength containing a large amount of P in recent years Not applicable to steel plates. In (Patent Document 4), a method of including Cu in steel is disclosed, but there is a concern about the influence on the material, which is not a general-purpose method. In (Patent Document 5), by controlling the surface layer structure of the hot-rolled steel sheet so that the ferrite grains of 15 μm or less are 70 area% or less, the appearance is reduced when cold rolling or alloying hot dip galvanizing is performed thereafter. It is possible to achieve uniform and beautiful plating without defects. However, this method defines the structure of the hot-rolled steel sheet, and the appearance becomes non-uniform depending on cold rolling, annealing, and galvannealed alloying conditions.

JP-A-6-88187 JP-A-5-132784 JP-A-10-18011 JP-A-6-17216 JP 2001-316663 A

  As described above, various types of alloyed hot-dip galvanized steels with excellent appearance quality have been proposed, but they are difficult to realize because of problems such as equipment introduction and increased process, and high-strength steels containing a large amount of P It is also not applicable to. Therefore, the present invention provides an alloyed hot-dip galvanized steel having excellent appearance quality even when steel containing a large amount of P is solved, and a method for producing the same, and solves such problems of equipment increase and process increase. It is aimed.

In order to solve the above-mentioned problems, the inventors of the present invention are alloyed hot-dip galvanized steel based on steel containing 0.02% to 0.2% P, and steel with an uneven appearance. The steel in which unevenness did not occur was examined in detail for the crystal structure of the base iron as a base material. As a result, the steel ground surface with uneven appearance has many coarse ferrite grains, whereas the steel ground surface with non-uniform appearance has mostly ferrite grains of about 40 μm or less. It was found to be a uniform tissue. That is, it was found that the appearance can be controlled by controlling the ferrite grain size on the steel surface after annealing.

  The gist of the present invention is as follows.

  (1) By mass%, C; 0.01% or less, Si; 0.2% or less, Mn; 2% or less, P; 0.02 to 0.2%, S; 0.03% or less, Al; In an alloyed hot-dip galvanized steel having an iron-zinc alloy coating containing 85% or more of Zn on the steel surface of 0.005 to 0.1%, the balance being Fe and unavoidable impurities, An alloyed hot-dip galvanized steel excellent in appearance quality, wherein the surface layer portion within 40 μm in the longitudinal direction has an area ratio of 10% or less of ferrite grains having a grain size of 40 μm or more in an observation field of view of 500 μm × 500 μm .

  (2) The steel is further characterized by containing one or two of Ti; 0.001 to 0.05%, Nb; 0.001 to 0.05% in mass% (1). Alloyed hot-dip galvanized steel with excellent appearance quality as described in 1.

(3) The alloyed hot-dip galvanized steel having excellent appearance quality according to (1) or (2), wherein the deviation of the adhesion amount of the iron-zinc alloy coating is ± 7 g / m ² or less. .

  (4) After hot-rolling the low carbon steel slab comprising the components described in (1) or (2), pickling, and further performing cold rolling, annealing, hot dip galvanizing, and heat alloying treatment, In the manufacturing method of alloyed hot-dip galvanized steel to be alloyed hot-dip galvanized steel, the coiling temperature of the coil after hot rolling is over 790 ° C, and then the surface temperature of the outermost layer portion of the coil that has been wound is A method for producing an alloyed hot-dip galvanized steel excellent in appearance quality, characterized by cooling at a rate of 0.8 ° C./min or less until reaching 300 ° C. or less.

  The alloyed hot-dip galvanized steel of the present invention and the alloyed hot-dip galvanized steel subjected to the production method of the present invention are excellent in appearance quality, and are excellent in 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 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 0.02% or less, in the ordinary method for producing galvannealed steel, the unevenness of the concentration is small and the difference in alloying speed does not easily occur. 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: 0.01% or less C is an element necessary for strengthening steel. However, if the C content exceeds 0.01%, embrittlement tends to occur, so the C content is set to 0.01% or less.

Si: 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. For this reason, Si is made 0.2% or less.

Mn: 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 galvanization, and the plating adhesion is deteriorated. For this reason, Mn is made 2% or less.

S: 0.03% or less S is an impurity, and it is desirable that the content of S be as small as possible because it deteriorates workability and hot brittleness. For this reason, S is made 0.03% or less.

Al; 0.005 to 0.1%
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 to 0.1%.

Ti; 0.001 to 0.05%
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 to 0.05%.

Nb; 0.001 to 0.05%
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 is made 0.001 to 0.05%.

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 , unevenness in appearance (shading) is promoted and the appearance tends 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 alloyed hot-dip galvanized steel of the present invention is 10% or less of ferrite grains having a grain size of 40 μm or more in an observation field of 500 μm × 500 μm. When ferrite grains of 40 μm or more are present in excess of 10%, a local delay occurs in the alloying speed during alloying heating after plating, resulting in poor appearance. In addition, when extremely fine ferrite grains are present, the difference in the alloying speed is increased due to the difference in the grain interface area with the surrounding crystals, which tends to cause a poor appearance. For this reason, it is preferable to make a crystal grain of 10 micrometers or less into 10% or less. In addition, the surface iron surface layer said here is defined as a place within a depth of 40 μm from the surface of the surface iron.

  The alloyed hot-dip galvanized steel 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 general steel. However, when winding the steel sheet after hot rolling into a coil, It is necessary to set the temperature to be higher than 790 ° C. and further to cool at a rate of 0.8 ° C./min or less until the plate temperature at the center of the coil and at the center of the plate width is 300 ° C. By winding up under these conditions, it is possible to realize an alloyed hot-dip galvanized steel of the present invention with a combination of subsequent cold rolling and annealing conditions. When the coiling temperature is less than 790 ° C., the ferrite grain size after annealing is non-uniform and coarse grains of 40 μm or more, and the appearance quality after hot dip galvanizing and heat alloying treatment is inferior. On the other hand, if the coiling temperature is too high, the scale grows thick and a load is applied to the subsequent pickling process. For this reason, the upper limit of the coiling temperature is desirably 820 ° C.

  In addition, even if the coiling temperature is over 790 ° C, after that, if the condition of cooling at 0.8 ° C / min or less is not satisfied until the plate temperature at the coil center portion and the plate width center portion is 300 ° C or less, The ferrite grain size after annealing is non-uniform and coarse grains of 40 μm or more, resulting in poor appearance quality after hot dip galvanizing and heat alloying treatment. However, when the cooling rate is extremely low, it becomes a factor of reducing productivity. A preferable cooling rate is 0.5 ° C./min or more and 0.8 ° C./min or less.

  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, steel 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 40 μm or more. After cold rolling, annealing is preferably performed at 800 ° C. or more for 120 seconds or more. When the temperature is not lower than 800 ° C. and not longer than 120 seconds, recrystallization may be incomplete and the particle size may be nonuniform. After annealing, hot dip galvanization and heat alloying treatment are performed. The temperature of the galvanizing bath is desirably 445 ° C. to 500 ° C., and the heating alloying temperature is desirably 480 to 540 ° C.

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 sheet having a thickness of 5 mm. Finishing temperature during hot rolling is 900 ° C or higher, winding temperature is 700 ° C or 800 ° C, up to 300 ° C at a rate of 0.4 ° C / min, 0.8 ° C / min, or 1.2 ° C / min After cooling, it was cooled with water. 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 at a soaking temperature of 820 ° C. and a soaking time of 120 seconds using a continuous hot dipping equipment, cooled to 465 ° C. at a cooling rate of 20 ° C./sec, and then Zn-0 having a bath temperature of 460 ° C. .Immerse in a 13% Al plating bath for 3 seconds, adjust by wiping so that the adhesion amount is 45 g / m ^2, and then heat alloying at a temperature of 520 ° C. according to the steel type, alloyed hot dip galvanized steel Manufactured.
The produced galvannealed steel was evaluated as follows.

(1) Appearance Visual observation was performed, and grades 1 to 6 were classified into 11 levels in 0.5 increments according to the degree of appearance irregularity. 3.5 or more pass.

(2) Observation of grain size of ferrite particles Dissolve the plating with 10% hydrochloric acid, observe the surface layer of the iron matrix with SEM, and the number of ferrite grains having a grain size of 40 μm or more and a grain size of 10 μm or less in a 500 × 500 μm field and their grains. The diameter was measured. 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 3 are cases where the winding temperature of steel A and the cooling rate after winding are changed. When the coiling temperature and the cooling rate after coiling are outside the scope of the present invention (No. 1), there are 10% or more ferrite grains having a grain size of 40 μm or more on the surface layer of the steel, and the appearance quality after plating and alloying Inferior to On the other hand, when the coiling temperature and the cooling rate after coiling are within the scope of the present invention (No. 2 and 3), the ferrite grains having a grain size of 40 μm or more in the surface iron surface layer are 10% or less, plating, alloying Later appearance is good.

  No. 4 to 6 are cases where the winding temperature of steel A and the cooling rate after winding were changed. When the coiling temperature and the cooling rate after coiling are out of the scope of the present invention (No. 4), 10% or more of ferrite grains having a grain size of 40 μm or more exist on the surface layer of the ground iron, and the appearance quality after plating and alloying Inferior to On the other hand, when the coiling temperature and the cooling rate after coiling are within the scope of the present invention (Nos. 5 and 6), the ferrite grains having a grain size of 40 μm or more in the surface iron surface layer are 10% or less, plating, alloying Later appearance is good.

  No. 7 to 9 are cases in which the winding temperature of steel A and the cooling rate after winding are changed. When the coiling temperature and the cooling rate after coiling are out of the scope of the present invention (No. 7), 10% or more of ferrite grains having a grain size of 40 μm or more exist on the surface layer of the ground iron, and the appearance quality after plating and alloying Inferior to On the other hand, when the coiling temperature and the cooling rate after coiling are within the scope of the present invention (No. 8 and 9), the ferrite grains having a grain size of 40 μm or more on the surface layer of the ground iron are 10% or less, and plating and alloying are performed. Later appearance is good.

  No. 10 to 12 are cases where the steel D was wound at 700 ° C. In either case, 10% or more of ferrite grains having a particle size of 40 μm or more in the surface layer of the ground iron are present, and the appearance quality after plating and alloying is inferior. No. 13-15 is the case where steel D is wound up at 800 degreeC. When the cooling rate after winding is 0.8 ° C./min or less (Nos. 13 and 14), the ferrite grains having a grain size of 40 μm or more on the surface layer of the ground iron are 10% or less, and the appearance after plating and alloying is good However, when it exceeds 0.8 ° C./min (No. 15), the ferrite grains having a particle size of 40 μm or more in the surface layer of the base iron exceed 10%, and the appearance quality after plating and alloying is inferior.

  No. 16-18 are the cases where the steel E is wound up at 700 degreeC. In either case, 10% or more of ferrite grains having a particle size of 40 μm or more in the surface layer of the ground iron are present, and the appearance quality after plating and alloying is inferior. No. 19-21 are the cases where the steel E is wound up at 800 degreeC. When the cooling rate after winding is 0.8 ° C./min or less (Nos. 19 and 20), the ferrite grains having a grain size of 40 μm or more on the surface layer of the base iron are 10% or less, and the appearance after plating and alloying is good However, when it exceeds 0.8 ° C./min (No. 21), ferrite grains having a particle size of 40 μm or more on the surface layer of the base iron exceed 10%, and the appearance quality after plating and alloying is inferior.

  No. 22-24 is the case where the steel F is wound up at 700 degreeC. In either case, 10% or more of ferrite grains having a particle size of 40 μm or more in the surface layer of the ground iron are present, and the appearance quality after plating and alloying is inferior. No. 25-27 is the case where the steel F is wound up at 800 ° C. When the cooling rate after winding is 0.8 ° C./min or less (Nos. 25 and 26), the ferrite grains having a grain size of 40 μm or more on the surface layer of the base iron are 10% or less, and the appearance after plating and alloying is good However, when it exceeds 0.8 ° C./min (No. 27), the ferrite grains having a particle diameter of 40 μm or more in the surface layer of the base iron exceed 10%, and the appearance quality after plating and alloying is inferior.

Claims (4)

% By mass
C: 0.01% or less,
Si: 0.2% or less,
Mn: 2% or less,
P: 0.02 to 0.2%,
S; 0.03% or less,
Al; 0.005 to 0.1%,
In an alloyed hot-dip galvanized steel having an iron-zinc alloy coating containing 85% or more of Zn on the steel surface, the balance of which is Fe and inevitable impurities, the surface layer portion of the steel surface within 40 μm in the depth direction from the surface of the ground iron However, in an observation field of view of 500 μm × 500 μm, an alloyed hot-dip galvanized steel excellent in appearance quality characterized in that ferrite grains having a particle size of 40 μm or more are 10% or less in area ratio. Said steel is further mass%,
Ti; 0.001 to 0.05%,
Nb; 0.001 to 0.05%,
The alloyed hot-dip galvanized steel having excellent appearance quality according to claim 1, comprising one or two of the following. The alloyed hot-dip galvanized steel excellent in appearance quality according to claim 1 or 2, wherein the deviation of the adhesion amount of the iron-zinc alloy coating is ± 7 g / m ² or less.   The low carbon steel slab comprising the component according to claim 1 or 2 is hot-rolled, then pickled, and further subjected to cold rolling, annealing, hot dip galvanizing, heat alloying treatment, and alloyed hot dip galvanizing. In the manufacturing method of alloyed hot-dip galvanized steel used as steel, the coiling temperature of the coil after hot rolling is set to exceed 790 ° C., and then the surface temperature of the outermost layer portion of the wound coil becomes 300 ° C. or less. A method for producing a galvannealed steel having excellent appearance quality, characterized by cooling at a rate of 0.8 ° C./min or less.