JP3497201B2 - Manufacturing method of high strength hot-dip galvanized steel sheet for deep drawing with excellent surface properties - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a high-strength hot-dip galvanized steel sheet having excellent surface properties and deep drawability, which is used for an automobile inner plate or the like. 2. Description of the Related Art High-strength steel sheets have been widely used as automotive steel sheets because of their improved safety, reduced vehicle body weight, and reduced material usage. Among high-strength steel sheets for automobiles, it is required to further improve corrosion resistance in order to prevent corrosion caused by salts contained in a snow melting agent for roads, for example, in a cold season. The corrosion resistance of a steel sheet is improved by a method of adding alloy elements such as Cu and Cr to the steel, a method of applying metal plating to the surface of the steel sheet, and the like. However, the method of adding an alloy element cannot expect a given effect in a use environment exposed to a severe corrosive atmosphere such as salt damage. Therefore, in order to provide corrosion resistance to withstand such severe corrosive environment,
Metal plating, particularly hot-dip galvanizing, which can be easily and inexpensively thickened, is usually employed. [0003] In applications where coating film adhesion and weldability are required, it is generally employed to perform an alloying treatment after galvanizing. However, the alloyed hot-dip galvanized steel sheet is inferior in press workability, particularly deep drawability, and cannot be subjected to a high degree of forming work. In this regard, it is desired to establish a method for mass-producing plated steel sheets having excellent workability in addition to corrosion resistance. Further, a high-strength galvannealed steel sheet contains a large amount of strengthening elements such as Si, Mn, and P in order to ensure high strength as compared with a soft steel sheet. Increasing the amount of the strengthening element is effective for improving the strength, but adversely affects the hot-dip galvanizing property. In particular, in a steel sheet containing a solid solution strengthening element such as Si or Mn, plating defects such as pinholes and non-plating and defects such as surface roughness of the scale pattern are generated on the product surface, and the surface quality is easily deteriorated. . As a result, the rate of rejection due to poor appearance and poor corrosion resistance is high, and the current situation is that the yield of products cannot be said to be at a high level. The present inventors have conducted various investigations and researches on alloy components, hot rolling conditions, plating conditions, and the like in order to improve various surface properties of a high-strength galvannealed steel sheet. In the process, a solid solution strengthening element such as Mn, P, B, etc. is added to the ultra-low carbon Ti, Nb composite-added steel in a complex manner, subjected to hot rolling and annealing under controlled conditions, and then hot-dip galvanized. And 450-
It has been found that an alloyed hot-dip galvanized layer with few defects is formed when it is kept at a temperature of 600 ° C. for 1 second or more, and introduced as JP-A-2-163346. The resulting high-strength alloyed hot-dip galvanized steel sheet has excellent workability and does not cause secondary working cracks during press working. [0005] Further, in Japanese Patent Application Laid-Open No. 4-66653,
After hot rolling a steel sheet having a similar composition at a higher finishing temperature and a lower winding temperature, cold working at a high rolling rate of 70% or more, and then performing high-temperature reduction annealing in a hot-dip galvanizing tank, He introduced that workability can be improved. The present invention was devised in the course of continuous research and research on the workability and corrosion resistance of such high-strength hot-dip galvanized steel sheet. An object of the present invention is to form a hot-dip galvanized steel sheet having excellent workability and corrosion resistance by forming a hot-dip galvanized layer having excellent heat resistance on a steel sheet surface. According to the production method of the present invention, in order to achieve the object, C: 0.001 to 0.010% by weight, Si: 2.0% by weight or less, Mn: 0% or less. 0.5-3.0
% By weight, P: 0.03 to 0.30% by weight, S: 0.01
5 wt% or less, acid-soluble Al: 0.005 to 0.100 wt%, N: 0.007 wt% or less, B: 0.0002 to
0.0010% by weight, Ti: 0.10% by weight or less and N
b: contains 0.01 to 0.10% by weight, and contains Ti% /
The ultra-low carbon steel slab containing Ti-Nb composite satisfying 48 ≧ C% / 12 + N% / 14 + S% / 32 is 1150-12
After heating to 80 ° C., the hot rolling is completed at a temperature not lower than the Ar ₃ transformation point and not higher than 1000 ° C., wound up at 400 to 600 ° C., passed through a continuous hot-dip galvanizing line after cold rolling,
800 to 90 in an atmosphere having a H ₂ concentration of 40% by volume or more.
It is characterized by performing reduction heating in a temperature range of 0 ° C., controlling the temperature of a sheet immersed in a galvanizing bath in the subsequent cooling process to a temperature range of 500 to 650 ° C., and then performing hot dip galvanizing. Next, various conditions specified in the present invention will be described. [Components of Material Steel] C: C contained in the steel material used in the present invention is T
It is fixed as a carbonitride such as iC or NbC. The lower the C content, the more advantageous the deep drawability and ductility, and the lower the content of Ti, Nb, etc. used as the fixing element. In this regard, the upper limit of the C content is set to 0.010% by weight, thereby suppressing an increase in the consumption of Ti, Nb, etc. according to the C content, and avoiding an increase in steel material cost. However, 0.00
If the C content is less than 1% by weight, excessive decarburization scouring is required in the steel making process, which causes an increase in production cost. Therefore, in the present invention, 0.001-0.
The C content was determined in the range of 010% by weight. [0008] Si: an alloy element effective for improving the strength of a steel sheet. However, if the content exceeds 2.0% by weight, the adhesion of the plating layer is extremely reduced.
The upper limit of the i content was set to 2.0% by weight. Mn: Like Si, it is an effective alloying element for improving the strength of a steel sheet. When the content is 0.5% by weight or more, the effect of Mn becomes remarkable. However, a large amount of Mn exceeding 3.0% by weight
Is contained, the A ₃ transformation point is greatly reduced, and the α → γ transformation of the metal structure is promoted during annealing. As a result, the texture of the {111} orientation formed during the recrystallization process is damaged,
Decrease the Rankford value (hereinafter referred to as r value).
Therefore, in order to secure required workability, the upper limit of the Mn content is set to 3.0% by weight. P: An alloying element that effectively works to improve the strength of the steel sheet. The effect of P becomes remarkable at 0.03% by weight or more. However, when a large amount of P exceeding 0.30% by weight is contained, secondary working cracks are remarkably generated during press working, and alloying in a continuous hot-dip galvanizing apparatus becomes difficult. S: A harmful element that forms nonmetallic inclusions by combining with Mn and generates defects such as work cracks during press working.
In addition, since TiS is formed, T required for fixing C is required.
It also increases the content of i and increases the cost of steel. Therefore, in the present invention, the upper limit of the S content is set to 0.015% by weight. Al: an element added as a deoxidizing agent, 0.00
An addition of 5% by weight or more is required. However, 0.10
When a large amount of Al is contained, the Al ₂ O ₃
Inclusions increase the workability and surface quality. N: An impurity element inevitably contained in steel, and the r value is improved by being fixed as TiN. When the N content exceeds 0.007% by weight, the amount of Ti necessary for stabilizing N increases, and the cost of steel material increases. Therefore, in the present invention, the upper limit of the N content is set to 0.007% by weight. B: Segregates preferentially at crystal grain boundaries over P to prevent press formability from deteriorating due to grain boundary embrittlement caused by P. In order to suppress grain boundary segregation of P and secure good press-formability, it is necessary to contain 0.0002% by weight or more of B. However, when a large amount of B is contained in excess of 0.0010% by weight, the growth of crystal grains is hindered, and the r value and ductility of the steel sheet decrease. Ti: While fixing S, N and C,
It is an effective alloying element for developing a {111} orientation recrystallization texture. In order to obtain this effect, the equivalents of S, N and C contained in the steel material, that is, (48/1
2) x C% + (48/14) x N% + (48/32) x
It is necessary to contain Ti in an amount of S% or more. However, if the content is 0.10% by weight or more, the effect corresponding to the increase of Ti cannot be obtained, which may rather increase the cost of steel material. Nb: Stable Ti carbonitride is used as a nucleus, and coarse composite carbide of Ti and Nb is generated during hot rolling. By the formation of the composite carbide, the recrystallization texture of the {111} orientation is more easily developed, and the drawability and the in-plane anisotropy of the r value are improved. The effect of such Nb is that the content is 0.01% by weight.
It becomes remarkable above, and saturates at 0.10% by weight. [Manufacturing conditions] In a converter, an electric furnace or the like, a steel material whose content of each alloy element is adjusted is continuously cast into a slab. The slab is directly sent without cooling, or is subjected to hot rolling after re-heating the once cooled slab. The hot-rolled material becomes a thin steel sheet through a cold rolling process. Hot rolling conditions: A steel material having the composition specified in the present invention becomes a steel sheet exhibiting extremely excellent deep drawability by controlling the slab heating conditions in the hot rolling process. That is, 1
When reheating to a temperature below 280 ° C and hot rolling,
The precipitate in the hot-rolled material and the precipitate during the subsequent cold rolling annealing are adjusted, and the formation of a texture advantageous for deep drawability is promoted. The lower limit of the hot rolling start temperature is set to 1150 ° C. in order to secure a required hot rolling finishing temperature. In the hot rolling, the hot rolling finishing temperature is set to be equal to or higher than the Ar ₃ transformation point in order to grow a {111} orientation recrystallization texture. When the hot-rolling finishing temperature is lower than the Ar ₃ transformation point, a hot-rolled texture that adversely affects the {111} orientation recrystallization texture is likely to be generated. The upper limit of the hot rolling finishing temperature is set to 1000 ° C. When the hot-rolling finishing temperature exceeds 1000 ° C., the crystal grain size of the hot-rolled material increases, and the nucleation rate of the recrystallized texture having a {111} orientation decreases during cold rolling. As a result, the r value after annealing decreases. The hot-rolled sheet material is wound up in a temperature range of 400 to 600 ° C. When the winding temperature is higher than 600 ° C., the pickling property of the scale formed on the surface of the hot-rolled material is reduced. Conversely, if the winding temperature is lower than 400 ° C., defects such as poor shape are likely to occur in the wound steel sheet. Hot-dip galvanizing conditions: The hot-rolled material is cold-rolled after the surface of the steel sheet is cleaned by ordinary pickling. The obtained cold rolled sheet is introduced into an in-line annealing type continuous hot-dip galvanizing apparatus. The atmosphere in the line is maintained as a reducing atmosphere having an H ₂ concentration of 30% by volume or more. The H ₂ concentration of 30% by volume or more in the reducing atmosphere is an important factor in preventing the occurrence of surface property defects such as pinholes and non-plating in the hot-dip galvanized layer formed on the surface of the steel sheet. When the H ₂ concentration is less than 30% by volume, as shown in FIG. 1, even if the conditions such as the reduction heating temperature and the temperature at which the steel sheet is immersed in the galvanizing bath (hereinafter referred to as the inlet temperature) are changed, Inevitably, defects such as non-plating occur. In the present application, it is described in claim of H ₂ concentration of 40% by volume or more of the atmosphere as an invention specific requirements. [0015] The introduced cold rolled sheet is placed in a reducing atmosphere for 8 hours.
Heat annealing is performed in a temperature range of 00 to 900 ° C. When the reduction heating temperature is lower than 800 ° C., when the cold-rolled sheet is introduced into the hot-dip galvanizing bath, the wet adhesion of the molten zinc to the steel sheet surface decreases, and defects such as pinholes and non-plating cannot be eliminated. The decrease in wet adhesion of molten zinc is caused by the solid solution strengthening element S
It is presumed that the oxide film of i and Mn is formed thickly on the surface of the steel sheet, and the thick oxide film remains during the hot-dip galvanizing treatment. On the other hand, at a reduction heating temperature exceeding 900 ° C., the recrystallization texture of the {111} orientation changes from α → γ
It disappears with the transformation, and the r value deteriorates. For this reason, it is necessary to perform the reduction heating annealing in a temperature range of 800 to 900 ° C. The cold-rolled sheet needs to be maintained at an inlet temperature of 500 to 650 ° C just before being introduced into the hot-dip galvanizing bath. At an inlet temperature of less than 500 ° C., as shown in FIG. 2, even if the reduction heating temperature is changed, the molten zinc wet adhesion decreases, and defects such as pinholes and non-plating are prevented from being formed on the steel sheet surface. Absent. At an inlet temperature exceeding 650 ° C., surface flaws are apt to occur in the continuous hot-dip galvanizing line, and the diffusion of zinc fume and the like accompanying the rise in the temperature of the galvanizing bath becomes active, making operation difficult. EXAMPLE A slab was obtained by continuously casting molten steel adjusted to a predetermined composition. Table 1 is a steel having the composition according to the present invention, and Table 2 is a steel for comparison. Various steel materials were subjected to hot rolling under the conditions shown in Tables 3 and 4, respectively, to produce hot-rolled sheets having a thickness of 3.2 mm. The hot-rolled sheet is subjected to a pickling step and a cold rolling step to form a 0.8 mm-thick cold-rolled steel sheet, and then subjected to reduction annealing in a continuous galvanizing line under the conditions shown in Tables 3 and 4, Hot-dip galvanizing was applied at a basis weight of 60 g / m ² per side. [Table 1] [Table 2] [Table 3] [Table 4] The galvanized steel sheet was subjected to in-line alloying treatment with a plating layer, and then subjected to skin pass rolling at an elongation of 0.8%. The resulting hot-dip galvanized steel sheet had the tensile properties, secondary work brittleness resistance and surface properties shown in Tables 5 and 6. For the investigation of tensile properties, JI
No. 5 test piece specified in SZ2201 was used. The secondary work brittleness resistance was investigated by the procedure shown in FIG. That is, using a blank punched to a diameter of 90 mm, a three-stage drawing with a drawing ratio of 2.73 was used to form a flat-bottomed cylindrical cup with a diameter of 33 mm. The flat-bottomed cylindrical cup was cut at a position 35 mm from the bottom of the cup, and a punch having a tip angle of 60 ° was gradually pushed in from the top of the cylindrically formed cup to crush the cup.
Then, the lowest temperature at which no brittle cracking occurred was measured as the vertical cracking limit temperature, and the secondary working brittleness resistance was evaluated based on this temperature. Regarding the surface properties of the plated steel sheet, the steel sheet surface was observed with respect to surface roughness, occurrence of defects, and the like, and the observation results were relatively evaluated. [Table 5] [Table 6] As shown in Table 6, test numbers B1 to B
The hot-dip galvanized steel sheet No. 4 satisfies the requirements for the components of the raw material specified in the present invention, has a high strength of 390 N / mm ² class or higher, and has an r value as high as 1.3 or higher. However, since the hot-dip galvanizing conditions were out of the range specified in the present invention, defects such as surface roughening, pinholes, and non-plating occurred, resulting in poor surface properties. Test number B
In the hot-dip galvanized steel sheets of Nos. 5 to B7, any one of the Si, C and Mn contents is out of the range specified in the present invention, and the hot-dip galvanizing condition is also out of the range specified in the present invention.
Inferior in r value and toughness, there were problems in surface properties such as surface roughness, pinholes, and non-plating. Test No. 8 with an excessive P content and Test No. B9 with an excessive B content were galvanized steel sheets of 390 N
/ Mm ² or more and an r value of 1.3 or more, but poor in secondary work brittleness resistance. Further, although the hot-dip galvanized steel sheet of Test No. 10 satisfies the requirements specified in the present invention in both hot rolling conditions and plating conditions, since it does not contain B, the vertical crack limit temperature is significantly higher, It was inferior in secondary work brittleness. A similar trend is P
In Test No. 8 containing an excess of On the other hand, the hot-dip galvanized steel sheet of Group A according to the present invention is shown in Table 5
No significant decrease in ductility as shown in FIG.
It shows high strength of N / mm ² or more and r value of 1.3 or more. In addition, the secondary work brittleness resistance is -4 at the vertical crack limit temperature.
It was as good as 0 ° C. or less, and had good surface quality without defects such as surface roughness, pinholes, and non-plating. As is clear from the comparison between Table 5 and Table 6,
In order to have both excellent workability and surface quality, it is necessary to maintain the components of the material, the manufacturing conditions and the hot-dip galvanizing conditions within the ranges specified in the present invention. If any of the raw material components and the hot-dip galvanizing conditions do not satisfy the conditions, a plated steel sheet having poor workability and / or surface quality is obtained. In addition, when a region having a good surface property was examined in relation to the H ₂ concentration in the reducing atmosphere and the reduction heating temperature, the relationship shown in FIG. 1 and the relationship between the inlet temperature and the reduction heating temperature were examined. The relationships shown in FIG. From FIG. 1 and FIG. 2, in order to form a hot-dip galvanized layer having good surface quality on the steel sheet surface,
₂ It can be seen that there is a specific relationship between the concentration, the reduction heating temperature, and the inlet temperature. As described above, in the present invention, by combining the composition of the material, the manufacturing conditions and the hot-dip galvanizing conditions under specified conditions, the hot-dip galvanized steel having a good surface quality is obtained. Manufactures high-strength hot-dip galvanized steel sheets with layers and excellent workability and corrosion resistance. The obtained high-strength hot-dip galvanized steel sheet is used in a wide range of fields, such as a steel sheet for automobiles, which has taken advantage of its excellent properties to achieve safety and weight reduction.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a graph showing the effect of the H ₂ concentration in the reducing atmosphere and the reduction heating temperature on the surface properties of a hot-dip galvanized steel sheet. Graph showing the effect on the surface properties of galvanized steel sheet [Figure 3] Diagram showing the test method for secondary work brittleness resistance

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁷ Identification code FI C22C 38/14 C22C 38/14 (72) Inventor Teruo Tanaka 11-1 Showa-cho, Kure-shi, Hiroshima Pref. Nisshin Steel Corporation (56) References JP-A-4-66653 (JP, A) JP-A-3-191047 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) C21D 9/46 C23C 2/06 C23C 2/40

Claims (1)

(57) [Claims 1] C: 0.001 to 0.010% by weight, S
i: 2.0% by weight or less, Mn: 0.5 to 3.0% by weight,
P: 0.03 to 0.30% by weight, S: 0.015% by weight
Hereinafter, acid-soluble Al: 0.005 to 0.100% by weight,
N: 0.007% by weight or less, B: 0.0002 to 0.0
010% by weight, Ti: 0.10% by weight or less and Nb:
0.01 to 0.10% by weight, and Ti% / 48
Ti satisfying ≧ C% / 12 + N% / 14 + S% / 32
-Nb composite-containing ultra-low carbon steel slab is 1150-1280
After heating to ° C., the hot rolling ends at Ar ₃ transformation point or higher 1000 ° C. or less of the temperature, taken up at 400 to 600 ° C.,
After cold rolling to Tsuban a continuous galvanizing line, H ₂
800-900 ° C in an atmosphere with a concentration of 40% by volume or more
Deep drawing with excellent surface properties characterized by reducing and heating at a temperature in the range described above, controlling the temperature of the sheet immersed in the galvanizing bath in the subsequent cooling process to a temperature in the range of 500 to 650 ° C., and then performing galvanizing. For manufacturing high-strength hot-dip galvanized steel sheet.