JP2006274288A - High strength hot dip galvanized steel sheet having excellent surface appearance - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a high strength hot dip galvanized steel sheet having excellent surface appearance by preventing the generation of the defect in plating properties considered to be caused by Mn in steel added for solid solution strengthening. <P>SOLUTION: The high strength hot dip galvanized steel sheet having excellent surface appearance is characterized in that a steel sheet having a composition comprising, by mass, 0.0040 to 0.010% C, ≤0.05% Si, 1.0 to 2.5% Mn, 0.02 to 0.1% P, ≤0.02% S, 0.01 to 0.1% sol.Al, ≤0.0100% N, 0.0055 to 0.0160% Sb, 0.036 to 0.14% Nb and ≤0.0015% B, and the balance substantially Fe with inevitable impurities is galvanized. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a high-strength hot-dip galvanized steel sheet having an excellent surface appearance.

  In response to the need for higher strength and lighter weight of automobiles, plated steel sheets applied to automobile structural members such as outer panels or members of automobiles and reinforcements require strength, formability, and particularly good surface properties. It has been. The r value is important for the formability as well as the elongation. In recent years, there is a high demand for improving formability even for high-strength steel sheets, and high r-values are required. In order to obtain a high r value, it is effective to add carbonitride-forming elements such as Ti and Nb based on an ultra-low carbon steel with a C concentration of about 30 ppm or less. Generally, Interstitial Free steel (hereinafter referred to as IF) Widely used as abbreviated steel). In addition, high strength steel sheets with solid solution strengthening elements such as Mn, P, and Si have been developed based on this steel. However, since the solid solution strengthening element is expensive, there is a problem that the cost of the steel sheet increases. Moreover, it is difficult to ensure both high r value and rough skin resistance during press molding. In other words, high-temperature annealing is indispensable for improving the r value, but IF steel with high crystal grain growth property obtains a high r value. When high-temperature annealing is performed, crystal grains become coarse and rough skin is likely to occur.

In order to reduce solid solution elements such as Mn, P, Si, etc., a technique of adding 30 to 100 ppm, which is slightly higher as an ultra-low carbon steel, and precipitation strengthening with TiC is disclosed (Patent Document 1, Patent) Reference 2). Although TiC contribution to precipitation hardening to a large size of the precipitates to produce at a high temperature of at least _three points A, miniaturization of the hot-rolled sheet structure, miniaturization of the cold-rolled sheet structure, the effect of the precipitation strengthening is small . Further, when Ti and Nb are added in combination, C precipitates as TiC first at high temperature during cooling, so that NbC fine precipitates are not generated. Therefore, a steel sheet excellent in surface properties and press formability due to the effect of fine precipitation of NbC unique to the case of adding Nb alone could not be produced.

As a technique for solving the problems of Patent Document 1 and Patent Document 2, Patent Document 3 discloses that C is controlled within a limited range of 0.0040 to 0.010% and Nb is balanced with C optimally. By adding, NbC is finely precipitated, and the effect of this fine precipitate has refined the crystal grain of the hot-rolled sheet, and therefore a technique for significantly improving the r value of the product after cold rolling annealing is disclosed.
Japanese Patent Laid-Open No. 10-46289 Japanese Patent Laid-Open No. 5-195080 Japanese Patent Laid-Open No. 2001-131692

  According to Patent Document 3, a high-strength steel sheet that is excellent in high formability and skin roughness resistance and that is generally free of surface defects after pressing and pressing is obtained. In a hot-dip galvanized steel sheet, steel added for solid solution strengthening is obtained. Plating property defects that may be caused by Mn inside may occur.

  An object of the present invention is to provide a high-strength hot-dip galvanized steel sheet that prevents the occurrence of poor plating properties that may be attributed to Mn in steel added for solid solution strengthening and has an excellent surface appearance. is there.

  The gist of the present invention for solving the above problems is as follows.

  1st invention is the mass%, C: 0.0040-0.010%, Si: 0.05% or less, Mn: 1.0-2.5%, P: 0.02-0.1%, S: 0.02% or less, sol.Al:0.01-0.1% , N: 0.0100% or less, Sb: 0.0055-0.0160%, Nb: 0.036-0.14%, B: 0.0015% or less, with the remainder substantially zinc and galvanized plating made of unavoidable impurities This is a high-strength hot-dip galvanized steel sheet having an excellent surface appearance.

  According to the present invention, the occurrence of poor plating properties due to Mn is prevented, and a hot-dip galvanized steel sheet having a good appearance can be stably produced.

  The reason for limiting the composition of the additive element of the present invention will be described. In addition,% shows the mass%.

C: 0.0040 to 0.010%
C combines with Nb to form a fine carbide of NbC, which is a feature of the present invention. Optimizing the C concentration is essential in order to precipitate fine NbC at an appropriate volume ratio, and control of the C concentration is one of the most important constituent elements of the present invention. Finely precipitated NbC has the effect of refining the hot rolled plate crystal grain size and improving the r value after cold rolling annealing. In addition, since NbC can be precipitated very finely, a great effect of precipitation strengthening can be obtained, and the strength can be increased without requiring the addition of a large amount of solid solution elements such as Mn, P, and Si. For this reason, uneven color on the plating surface due to the solid solution element is reduced, and the surface properties are improved. If the C concentration is less than 0.0040%, the volume fraction of NbC precipitates is not sufficient to obtain the effect of the present invention, and if it exceeds 0.010%, the effect of suppressing the grain growth of the cold-rolled annealed sheet by NbC becomes too large, and the r value Deteriorate. Moreover, since NbC produces | generates excessively and elongation deteriorates, it is 0.0040 to 0.010%. Further, C is preferably 0.0050% or more for improving surface properties, and 0.0080% or less for improving moldability. Furthermore, in order to improve moldability, it is desirable that it is 0.0074% or less.

Si: 0.05% or less
Si can be added because it can be strengthened at a low cost by addition, but if added over 0.05%, the surface of Si will be concentrated during annealing, and the plating properties will deteriorate. Therefore, Si must be 0.05% or less.

Mn: 1.0-2.5%
Mn is added to increase the strength by solid solution strengthening. If it is less than 1.0%, the effect of solid solution strengthening is not sufficient. If it is 2.5% or more, the surface properties and elongation of the steel sheet are remarkably deteriorated, so Mn must be 1.0 to 2.5%.

P: 0.02-0.1%
P is added to increase the strength by solid solution strengthening. Addition of 0.02% or more is indispensable in order to achieve both surface properties and high strength. On the other hand, if the P concentration exceeds 0.1%, slab cracking may occur due to segregation of the slab center. P needs to be 0.02 to 0.1%. Further, when the P concentration exceeds 0.08%, when alloying the zinc-based plating layer, it takes a long time for the alloying treatment.

S: 0.02% or less
S exists as an impurity in steel, but it must be removed to 0.02% or less in order to significantly deteriorate the appearance of the plate surface.

sol.Al: 0.01-0.1%
Al is added as a deoxidizing material. If the solid solution Al concentration is less than 0.01%, deoxidation is not sufficient, and if it exceeds 0.1%, the steel sheet is strengthened by solid solution strengthening of Al, and the ductility deteriorates.

N: 0.0100% or less
N forms a solid solution in the steel sheet structure, and if it is excessively contained, it causes surface defects such as stretcher strain marks. Therefore, N must be 0.0100% or less.

Sb: 0.0055-0.0160%
Sb is an indispensable component in order to prevent poor plating properties that may be caused by Mn. If Sb is less than 0.0055%, the effect of preventing the occurrence of poor plating properties due to Mn is not sufficient, and if it exceeds 0.0160%, the production cost becomes uneconomical. The poor plating properties due to Mn are thought to be due to the formation of Mn oxide on the steel sheet surface during annealing. When Sb is added, Sb is concentrated on the surface of the steel sheet during annealing and covers the Mn oxide generated on the surface of the steel sheet, so that it is considered that the occurrence of poor plating properties due to Mn can be prevented.

Nb: 0.036 to 0.14%
Nb combines with C to form a fine precipitate of NbC essential for the present invention, which is added to refine the structure and improve the surface properties and mechanical properties. Nb in the carbide-forming elements is the most preferred element in that very fine precipitates is obtained to precipitate just below ₃ points A. On the other hand, precipitates Ti is the same carbide-forming elements for precipitation at a high temperature of at least _three points A, precipitates in order to obtain the effect of the present invention is too coarse. Therefore, it is an essential constituent requirement of the present invention to add Nb appropriately. If it is less than 0.036%, the precipitation amount of NbC is insufficient and the effect of forming precipitates cannot be obtained. If it exceeds 0.14%, the volume fraction of NbC becomes too high and the strength is remarkably increased and the moldability is deteriorated. Further, in order to enhance the effect of NbC precipitation by adding Nb, it is desirable to exceed 0.08%.

  In addition, when 12 Nb / 93C is 1.1 or less, solid solution C remains in a non-equilibrium state, surface defects such as stretcher strain are likely to occur, and the assembly has a (111) surface suitable for deep drawing in the plate surface direction. Since it has the effect of preventing the generation of tissue, it is preferable to exceed 1.1. On the other hand, if it is 2.5 or more, Nb is excessively contained in Fe, so that not only ductility is deteriorated but also hot workability is deteriorated and yield is reduced. Furthermore, in order to enhance the effect of fine precipitation of NbC, it is more preferable that 12Nb / 93C exceeds 1.5. In order to further enhance the effect, it is more preferably 1.7 or more.

B: 0.0015% or less
B may be added to prevent secondary processing embrittlement. However, if added over 0.0015%, the r value and elongation deteriorate significantly, so when added, it is necessary to make it 0.0015% or less.

  The metal structure preferably has an average crystal grain size of 10 μm or less. The crystal grain size is important for improving surface properties after molding such as pressing. If it exceeds 10 μm, the surface properties after the molding process deteriorate, so the crystal grain size is preferably 10 μm or less.

  Next, the manufacturing method of the steel plate of this invention is demonstrated.

The steel having the component composition described in the present invention is subjected to hot rolling directly or after reheating to 1100 to 1250 ° C. after continuous casting. This is because if the temperature is lower than 1150 ° C, the rolling load is high and the work efficiency deteriorates, and if it exceeds 1250 ° C, the surface properties of the product deteriorate. The hot rolling is performed at a final temperature of 10% to 40% and a finishing temperature of Ar _{3 to} 920 ° C. in the final two stages. In order to exhibit the characteristics of the steel of the present invention, it is important to control the rolling reduction here. If it is less than 10%, the hot rolled structure becomes coarse and the effect of improving the r value cannot be obtained. On the other hand, if it exceeds 40%, it becomes partially non-recrystallized austenite after hot rolling, resulting in a non-uniform hot rolled structure, and the r value and surface properties deteriorate. On the other hand, if the finishing temperature is less than Ar ₃ , the surface layer becomes ferritic and the hot rolled structure becomes coarse, so the r value decreases. On the other hand, even when the temperature exceeds 920 ° C., the growth of austenite grains during cooling is fast, making it difficult to refine the hot-rolled structure, and the r value cannot be improved.

  The cooling rate after hot rolling must be 15 ° C / sec or more and 700 ° C or less. If it is less than 15 ° C./sec, crystal grain growth occurs during cooling, the hot rolled structure becomes coarse, and an r value of 1.5 or more cannot be obtained. Since this effect becomes remarkable above 700 ° C., it is important to cool to this temperature or lower.

  The rolling rate of cold rolling must be 50% or more. Although the rolling rate of cold rolling is appropriately controlled depending on the steel component, an r value of 1.5 or more cannot be obtained with less than 50% of the steel of the present invention.

Next, after cold rolling, annealing is performed using a continuous hot dipping line equipped with an annealing step, followed by hot dipping. In order to achieve both an r value of 1.5 or more and a crystal grain size of 10 μm or less, continuous annealing is performed at 780 ° C. to Ac ₃ . When the annealing temperature is less than 780 ° C., an r value of 1.5 or more cannot be obtained. When Ac ₃ is exceeded, not only equiaxed crystal grains of 10 μm or less can be obtained due to austenitization, but also the r value becomes less than 1.5. C to Ac ₃ . Furthermore, in the steel of the present invention, by setting the annealing temperature to 860 ° C. or higher, the r value can be further improved at a crystal grain size of 10 μm. Therefore, the annealing temperature is preferably set to 860 ° C. or higher. The reason for continuous annealing is that the heating rate is 20 ° C./sec or more, and if it is less than 20 ° C./sec, a texture that is advantageous for deep drawing is not formed and the r value is less than 1.5.

  The hot dipping conditions are not particularly limited. Temper rolling is performed after plating or after alloying. In temper rolling, the rolling rate is 0.4 to 1.0%. The control of the temper rolling ratio is important in the invention. If it is less than 0.4%, the influence of N aging may not be sufficiently suppressed. Deteriorate the sex.

  When the plating layer is alloyed, it is desirable to use an induction heating type alloying furnace in order to shorten the alloying time.

Steels Nos. 1 to 5 shown in Table 1 were continuously cast, reheated at 1200 ° C., and then hot rolled to a thickness of 3.4 mm. The hot rolling finish temperature was 840 ° C. At that time, the total of the final two stages of rolling reduction was set to 15%. Then, after cooling at an average cooling rate of 20 ° C./sec, it was wound up at 560 ° C. Further cold-rolled to 1.015 mm (cold rolling rate: 70.1%), held at 800 ° C for about 90 seconds using a hot-dip galvanizing line equipped with continuous annealing equipment, then hot-dip galvanized, alloyed coating layer, A test material was obtained by temper rolling with a rolling rate of 0.7%. In the hot dip galvanizing, a steel sheet at 480 ° C. was immersed in a hot dip zinc bath (containing Al: 0.15%) at a bath temperature of 460 ° C. and then pulled up from the hot dip zinc bath to adjust the galvanizing amount to 48 g / m ² . Next, using an induction heating type alloying furnace, alloying was performed so that the Fe content in the plating film was 10.5%, and temper rolling was further performed at a rolling rate of 0.5%.

The surface properties of the alloyed hot-dip galvanized steel sheet produced as described above were visually observed to evaluate the presence or absence of poor plating properties due to Mn. The mechanical properties were also investigated. The r-value measurement is carried out after pickling with hydrochloric acid in order to remove the influence of the plating layer, and the average r-value from the r-value measurement results in three directions: mean-r = (r ₀ + 2 * r ₄₅ + r ₉₀ ) / 4 Was calculated. Here, r _{0 is} the r value in the direction parallel to the rolling direction, r _{45 is} the r value in the rolling direction and the 45 ° direction, and r ₉₀ is the r value in the direction perpendicular to the rolling direction. The surface properties were evaluated by visually observing the appearance of the plated steel sheet and evaluated as “O”: no occurrence and “X”: occurrence depending on the presence or absence of plating property failure caused by Mn. The survey results are shown in Table 2.

  In the comparative steel, plating property defects due to Mn occurred, but in the steel according to the present invention, plating property defects due to Mn did not occur. In addition, all the steels of the present invention have high strength, and an r value of 1.5 or more is excellent in press formability.

  The hot dip galvanized steel sheet of the present invention is excellent in surface appearance, and has an r value of 1.5 or more and excellent formability. Therefore, automotive exterior panels such as hoods, doors, fenders and side panels that require surface appearance and formability are required. Or it can utilize as a steel plate used for automobile structural members, such as a member and reinforcement.

Claims (1)

  In mass%, C: 0.0040 to 0.010%, Si: 0.05% or less, Mn: 1.0 to 2.5%, P: 0.02 to 0.1%, S: 0.02% or less, sol.Al: 0.01 to 0.1%, N: 0.0100% In the following, a steel sheet containing Sb: 0.0055 to 0.0160%, Nb: 0.036 to 0.14%, B: 0.0015% or less, and the balance being substantially composed of Fe and inevitable impurities is characterized by being subjected to zinc-based plating. High strength hot-dip galvanized steel sheet with excellent surface appearance.