JP2002256387A - Galvannealed high tensile strength steel sheet and production method therefor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve the strength, workability, corrosion resistance and weldability of a galvannealed high tensile strength steel sheet. SOLUTION: A base metal steel sheet 3 has a composition containing 0.10 to 0.15% C, 0.05 to 0.15% Si, >=1.4% Mn, 0.01 to 0.03% P, <=0.02% Si, 0.005 to 0.10% dissolved Al, <=0.01% N, 0.020 to 0.045% Nb and 0.010 to 0.030% Ti and the balance Fe with inevitable impurities. The Si content [Si] and the Mn content [Mn] in the base metal sheet are set also so as to satisfy the inequality of 0.21[Si]+0.48[Mn]<1.0. The base metal steel sheet 3 is galvannealed and is then heated to form an alloy layer 4 containing Fe and Zn thereon.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a galvannealed high-strength steel sheet suitably used as a steel sheet for automobiles and a method for producing the same.

[0002]

2. Description of the Related Art In recent years, alloyed hot-dip galvanized high-strength steel sheets (hereinafter sometimes abbreviated as high-strength GA steel sheets) have attracted attention as materials having high strength, high workability and high corrosion resistance. In order to reduce the weight and to prevent rust, it is used for the suspension members of automobiles. Prior art relating to conventional typical high-tensile GA steel sheets is disclosed in
No. 31537.

[0003] This prior art includes C: 0.05 to 0.3.
%, Si: 2.0% or less, Mn: 2.0 to 3.5%, hot rolled and wound, pickled, cold rolled, and then subjected to iron-based pre-plating. Into the continuous hot-dip galvanizing equipment and heat-treated in the temperature range of Ac ₁ to Ac ₃ transformation point.
A method for manufacturing a high-tensile GA steel sheet, which is cooled to a temperature of not more than the Ms point at an average cooling rate of 2 ° C./sec or more, hot-dip galvanized, and then alloyed, is disclosed. This high tension GA
The steel sheet not only obtains high corrosion resistance by galvanization, but also has a high Si and Mn content in the steel, so that when heated to more than _one point of Ac, the austenite phase is transformed into a martensite phase even under a sufficiently low cooling rate. be able to.
Therefore, a high-strength, high-strength, high-tensile GA steel sheet can be obtained.

[0004]

SUMMARY OF THE INVENTION The above-mentioned Japanese Patent Application Laid-Open No. Hei 5-331
The high-tensile GA steel sheet disclosed in Japanese Patent No. 537 has the following problems.

(A) Since the strengthening mechanism is a transformation strengthening type in which an austenite phase is transformed into a martensite phase to obtain high strength, there is a large variation in strength such as tensile strength. (B) When joining by spot welding, the crystal grains of the heat-affected zone are coarsened and the rupture strength of the weld zone is reduced. (C) Since the contents of Si and Mn in the steel are high, plating wettability is poor, and non-plating is likely to occur. For this reason, it is necessary to perform Fe-based pre-plating before hot-dip galvanizing, which increases the number of manufacturing steps. (D) After being rolled into a coil by hot rolling, the inner diameter of the coil is likely to be crushed. Therefore, it is necessary to add a step for repairing the inner diameter of the coil, and the productivity is reduced.

SUMMARY OF THE INVENTION It is an object of the present invention to provide an alloy capable of performing hot-dip galvanizing without applying Fe-based pre-plating, having high strength, high workability, high corrosion resistance, and excellent weldability. An object of the present invention is to provide a hot-dip galvanized high-strength steel sheet and a method for producing the same.

[0007]

SUMMARY OF THE INVENTION The present invention provides a hot-dip galvanized high-strength steel sheet obtained by subjecting a base steel sheet having high strength to hot-dip galvanizing, and heating the resulting hot-dip galvanized high-tensile steel sheet to form an alloy plating layer containing Fe and Zn. In the alloyed hot-dip galvanized high-strength steel sheet having formed therein, the component of the base steel sheet is C: 0.10 to 0.10 by weight percentage.
0.15%, Si: 0.05 to 0.15%, Mn: 1.
4% or more, P: 0.01 to 0.03%, S: 0.02%
Hereinafter, dissolved Al: 0.005 to 0.10%, N: 0.0
1% or less, Nb: 0.020 to 0.045%, Ti:
0.010 to 0.030%, the balance being Fe and unavoidable impurities, and the Si content in the base steel sheet [S
i] and the Mn content [Mn] are inequality 0.21 [Si]
It is an alloyed hot-dip galvanized high-strength steel sheet satisfying +0.48 [Mn] <1.0.

According to the present invention, since the upper limit of the Si content in the base steel sheet is set sufficiently low, the formation of an oxide film of Si on the surface of the base steel sheet is suppressed, and the decrease in plating wettability is prevented. Can be suppressed. This makes it possible to perform hot-dip galvanizing without performing Fe-based pre-plating. Further, since C, Mn, Ti, and Nb are sufficiently contained, high strength can be achieved by solid solution strengthening by C and Mn and precipitation strengthening by Ti and Nb.
In addition, since C, Ti, Nb, and N are sufficiently contained, Ti and Nb-based precipitates are sufficiently precipitated, and the pinning effect can prevent coarsening of crystal grains during spot welding. .

In the present invention, the tensile strength TS is preferably TS ≧ 59.
Was 0N / mm ^2, and the product of the tensile strength TS (N / mm ²⁾ and elongation at break El (%) is characterized in that it is a TS × El ≧ 13570.

According to the present invention, since the tensile strength is at a sufficient level, the weight of the vehicle body can be reduced. Further, since the product TS × El of the tensile strength TS and the elongation at break El is a sufficient level, the balance between the strength and ductility is good, and the occurrence of processing cracks during molding can be prevented.

Further, the present invention smelts a steel having the following components, and the components of the steel are expressed in terms of weight percentage as C: 0.10 to 0.1.
5%, Si: 0.05 to 0.15%, Mn: 1.4% or more, P: 0.01 to 0.03%, S: 0.02% or less,
Dissolved Al: 0.005 to 0.10%, N: 0.01% or less, Nb: 0.020 to 0.045%, Ti: 0.01
0-0.030%, the balance being Fe and unavoidable impurities, and the Si content [Si] and the Mn content [Mn] in the steel are inequality 0.21 [Si] +0.48 [Mn] < 1.0, and the steel is hot-rolled in a temperature range not lower than the Ar ₃ transformation point and wound at a winding temperature of 400 to 600 ° C., pickled, cold-rolled, and continuously hot-dip galvanized. After heating and holding at a temperature range of 810 to 880 ° C. for 75 to 130 seconds in the equipment, 500 at an average cooling rate of 10 ° C./sec or less.
This is a method for producing an alloyed hot-dip galvanized high-strength steel sheet, wherein the hot-dip galvanized steel sheet is cooled to a temperature of not more than 0 ° C. or less, and thereafter, the coating layer is heated and alloyed.

According to the present invention, the Si content [Si] and M
The value corresponding to the n content [Mn] (0.21 [Si] +
0.48 [Mn]) is set to a sufficiently low level, so that the austenite phase can be sufficiently transformed into a ferrite phase and then wound. Thereby, it is possible to prevent the coil inner diameter from being crushed after winding. In the continuous hot-dip galvanizing equipment, the base material steel sheet that has been hardened by cold rolling can be sufficiently recrystallized because it is heated and maintained in a sufficient temperature range for a sufficient time. Thereby, the breaking elongation El can be sufficiently improved,
The product of the tensile strength TS and the elongation at break El can be sufficiently improved. Therefore, the strength-ductility balance is improved, and workability can be improved.

[0013]

FIG. 1 is a cross-sectional view schematically showing the structure of a cross section perpendicular to the plate surface of an alloyed hot-dip galvanized high-strength steel sheet 1 according to an embodiment of the present invention. 2 is an enlarged image showing an optical microscope structure of a base steel sheet 3 shown in FIG. 1.
The high-strength GA steel sheet 1 is composed of a base steel sheet 3 having high strength and an alloy plating layer 4 formed on its surface and containing Fe and Zn. Base steel sheet 3 has a structure in which cementite and fine precipitates 5 are dispersed and precipitated in a ferrite phase, and does not include a martensite phase.

The chemical composition of the base steel sheet 3 is as follows: C: 0.10 to 0.15%, Si: 0.05 to 0.15 by weight percentage.
%, Mn: 1.4% or more, P: 0.01 to 0.03%,
S: 0.02% or less, dissolved Al: 0.005 to 0.10
%, N: 0.01% or less, Nb: 0.020 to 0.04
5%, Ti: 0.010-0.030%, balance F
e and unavoidable impurities and contained in the base steel sheet
The content [Si] and the Mn content [Mn] satisfy the inequality 0.21 [Si] +0.48 [Mn] <1.0. Thereafter, the Si content [Si] and the Mn content [Mn] in the base steel sheet and the steel were set to Si% and Mn, respectively.
Abbreviated as%. The same applies to other elements. The chemical components of the base steel sheet 3 are limited as described above for the following reason.

The reason why the lower limit of C is limited to 0.10% is that when C% is less than the lower limit, the base steel sheet cannot be sufficiently solid-solution strengthened. The upper limit of C is 0.15%
The reason for this is that C% exceeding the upper limit significantly reduces the weldability. The lower limit of Si is 0.05%
The reason for this is that if the content of Si is less than the lower limit, the base steel sheet cannot be sufficiently solid-solution strengthened. The reason why the upper limit of Si is limited to 0.15% is that, when the content of Si exceeds the upper limit, the oxide film of Si formed on the surface of the base steel sheet in the welding galvanizing process significantly reduces the plating wettability, and the Is likely to occur. That is, if the Si% exceeds the upper limit, Fe-based pre-plating is required in order to improve the plating wettability, as in the case of Japanese Patent Application Laid-Open No. 5-31537.

The reason why the lower limit of Mn is limited to 1.4% is that if the Mn% is less than the lower limit, the base steel sheet cannot be sufficiently solid-solution strengthened. Si% and Mn%
Will be further described later. The lower limit of P is limited to 0.01% because if the P% is less than the lower limit, the base steel sheet cannot be sufficiently solid-solution strengthened. The reason why the upper limit of P is limited to 0.03% is that if P% exceeds the upper limit, the workability is significantly reduced due to embrittlement of the base steel sheet. The upper limit of S is limited to 0.02% because S% exceeding the upper limit causes embrittlement of the base steel sheet. The lower limit of the dissolved Al is limited to 0.005% because if the dissolved Al% is less than the lower limit, deoxidation in the steel making process becomes insufficient. The upper limit of dissolved Al is 0.1
The reason why the content is limited to 0% is that even if dissolved Al exceeding the upper limit value is added, the deoxidizing effect of Al is saturated, and further improvement cannot be expected. The upper limit of N is limited to 0.01% because if N% exceeds the upper limit, the hardening of the base steel sheet becomes excessive.

The reason why the lower limit of Nb is limited to 0.020% is that if Nb% is lower than the lower limit, the amount of Nb-based fine precipitates decreases and the base steel sheet cannot be sufficiently strengthened by precipitation. It is. In addition, since the amount of Nb-based precipitates is small, the pinning effect due to the precipitates at the time of heat input during welding is reduced, and it is not possible to prevent the crystal grains from becoming coarse, and therefore, it is possible to prevent the embrittlement of the weld. Because it is gone. The reason why the upper limit of Nb is limited to 0.045% is that when Nb% exceeds the upper limit, the recrystallization temperature of the base steel sheet increases, so that the annealing temperature of the heat treatment must be increased, and energy consumption in the heat treatment is required. This is because the amount increases. Ti
The upper and lower limits of Nb are limited to 0.010% and 0.030%, respectively, for the same reason as the upper and lower limits of Nb. Ti is added together with Nb because the effect of increasing the recrystallization temperature of the Ti-based precipitate is smaller than that of the Nb-based precipitate, so the addition of Ti suppresses the amount of Nb added,
This is for preventing an excessive rise in the recrystallization temperature.

FIG. 3 is a graph showing the relationship between Si% and Mn%, the average cooling rate of the heat treatment, and the transformed and untransformed regions. The heat treatment was performed by setting the annealing temperature to 850 ° C., which is a two-phase temperature range of the Ac ₁ to Ac ₃ transformation point. The transformation zone is
Transformation from an austenite phase to a martensite phase and a bainite phase (hereinafter referred to as martensite transformation and bainite transformation) proceeds, and an untransformed region is a region where martensite transformation does not occur. The hatching in the figure represents the transformation region. The solid line in the figure represents the boundary between the transformed region and the untransformed region when the average cooling rate is 10 ° C./s, and the dotted line is the transformed region and the untransformed region when the average cooling rate is 2 ° C./s. Represents the boundary with The average cooling rate is calculated by dividing the amount of temperature drop due to cooling by the cooling time.

From FIG. 3, the transformation regions are Si% and Mn%
Is present in the high region, and the untransformed region is S
It can be seen that at least one of i% and Mn% exists in the low region. It can also be seen that as the average cooling rate of the heat treatment becomes lower, the transformation region moves in the direction of increasing Si% and Mn%. These are Si% and Mn
This is because martensitic transformation and bainite transformation are more likely to occur as the% becomes higher.

As described above, Si% and Mn% are selected so as to satisfy the inequality 0.21 [Si] +0.48 [Mn] <1.0. This is for the following reason. When Si% and Mn% in the base steel sheet deviate from the above inequalities, for example, when Si: 0.25% and Mn: 2.09%, they exist in the transformation region as shown at point A in FIG. When cooling is performed at an average cooling rate of 10 ° C./s, a martensite phase is formed in the base steel sheet. Therefore, the material of the base material steel plate, that is, the material of the high tensile strength GA steel plate becomes unstable, and the variation in tensile strength and breaking elongation of the high tensile strength GA steel plate both increase. On the other hand, when Si% and Mn% in the base steel sheet satisfy the above inequality, for example, Si: 0.
When it is 11% and Mn is 1.60%, since it exists in the untransformed region as shown at point B in FIG.
No martensite phase and no bainite phase are formed in the base steel sheet even when cooling is performed at a rate of ° C / s. Therefore, the mechanical properties of the base steel sheet, that is, the high-tensile GA steel sheet are stable, and the variation in the tensile strength and the breaking elongation of the high-tensile GA steel sheet are both reduced. As described above, the upper limit of Mn% is set so as to satisfy the inequality according to Si%. Also, Mn
The lower limit of% is preferably set to 1.0% or more in order to further specifically strengthen the high-tensile GA steel sheet.

The high tensile GA tensile strength TS of the steel sheet 1 of the present embodiment is a TS ≧ 590N / mm ^2, and the product of the tensile strength TS (N / mm ²⁾ and elongation at break El (%) Is T
It is preferable that S × El ≧ 13570. TS × E
l represents a balance between strength and ductility, and as this value increases, excellent ductility, that is, excellent workability is exhibited even at high strength. The lower limit of the tensile strength TS is 590 N / mm
^{The reason for} being limited to ² is that if the tensile strength is less than the lower limit, the strength level is insufficient and the weight of the vehicle body cannot be sufficiently reduced. The lower limit of the product TS × El is 13
It is limited to 570 because the strength-ductility balance is poor at the level of TS × El below the lower limit, and TS ≧ 590 N / mm
^{This is because, in} the case of ² , there is a possibility that a processing crack may occur during molding. On the other hand, TS ≧ 590
The high-tensile GA steel sheet having N / mm ² and TS × El ≧ 13570 has a good balance of tensile strength and strength-ductility balance, so that it is possible to reduce the weight of the vehicle body and to perform processing during molding. The occurrence of cracks can be prevented.

As described above, in the present embodiment, the strength of the high-strength GA steel sheet is enhanced by the solid solution strengthening of the base steel sheet by C, Mn and the like and the precipitation strengthening of the base steel sheet by Ti, Nb and the like. ing. Therefore, variation in strength can be reduced as compared with the prior art in which high strength is obtained mainly by using martensitic transformation and bainite transformation. In addition, since fine precipitates of Ti and Nb are dispersed in the base steel sheet, even if the temperature of the base steel sheet near the weld increases due to heat input during welding, the crystal is formed by the pinning effect of the fine precipitate. Grain coarsening can be prevented, and embrittlement of the welded portion can be prevented. In addition, Fe-
Since the Zn alloy layer is formed, the corrosion resistance can be improved. Thereby, the high-tensile GA steel sheet of the present embodiment can have high strength, high workability, high corrosion resistance, and good weldability. In addition, since the upper limits of Ti and Nb% are set so that the recrystallization temperature of the base steel sheet does not rise excessively, the base steel sheet can be sufficiently recrystallized during heat treatment, and the elongation at break is improved. Can be done.

FIG. 4 is a view showing a manufacturing process of the galvannealed high-strength steel sheet shown in FIG. A method for manufacturing the high-tensile GA steel sheet shown in FIG. 1 will be described with reference to FIG. First
In the process, the molten steel is produced using a converter so that the molten steel has the above-mentioned chemical composition. In the second step, continuous casting is performed to produce a slab.

In the third step, the slab is hot-rolled. The hot rolling is performed at a finishing temperature of not less than the Ar ₃ transformation point and a winding temperature of 400 to 600 ° C. Ar finishing temperature
_The reason for setting the transformation point to ₃ or more is to homogenize the structure of the hot-rolled steel sheet by performing hot rolling in the austenite phase temperature range. The Ar ₃ transformation point is about 830 ° C.
The reason why the winding temperature is set to 400 ° C. or higher is that if the winding temperature is lower than 400 ° C., the hot-rolled steel sheet has an excessively high strength and may not reach the target thickness accuracy in the subsequent cold rolling step. Because there is. The reason why the winding temperature is set to 600 ° C. or less is that, at a winding temperature exceeding 600 ° C., the oxide scale on the surface of the hot-rolled steel sheet becomes thick, which hinders descaling in the subsequent pickling step.

In the fourth step, pickling is performed, and oxide scale on the surface of the hot-rolled steel sheet is removed by mechanical grinding with a brush and pickling with hydrochloric acid. In the fifth step, cold rolling is performed. The cold rolling reduction is set to 35% to 75%.
The cold-rolled steel sheet is introduced into a continuous hot-dip galvanizing facility while being cold-rolled. Therefore, the steps after the sixth step are performed in the continuous hot-dip galvanizing equipment. The continuous hot-dip galvanizing equipment includes a degreasing device, a heat treatment furnace, a hot-dip galvanizing device, and a temper rolling mill in this order from the upstream side to the downstream side. The hot-dip galvanizing apparatus includes a storage tank for storing a hot-dip galvanizing bath and an alloying furnace. Sixth
In the process, degreasing is performed, and the rolling oil adhering to the cold-rolled steel sheet is removed by the alkaline degreasing agent. In the seventh step, the heat treatment of the cold-rolled steel sheet is performed in the heat treatment furnace. The heat treatment of the cold-rolled steel sheet is continuously performed in a hydrogen-nitrogen mixed gas atmosphere, and the cold-rolled steel sheet work-hardened by the cold rolling is recrystallized and annealed, and the steel sheet surface is reduced and cleaned.

In the heat treatment of the present embodiment, the cold-rolled steel sheet is heated and held in a temperature range of 810 to 880 ° C. for 75 to 135 seconds, and then cooled to 500 ° C. or less at an average cooling rate of 10 ° C./sec or less. Done. The heat treatment conditions are limited as described above for the following reasons.

FIG. 5 is a graph showing the relationship between the annealing temperature of the heat treatment and the mechanical properties of the cold-rolled steel sheet after the heat treatment. The components of the cold-rolled steel sheet to be heat-treated are components that satisfy the component range of the present embodiment, and are C: 0.11%, S
i: 0.11%, Mn: 1.60%, Ti: 0.030
%, Nb: 0.020%. The plate thickness is 1.2mm
Met. From FIG. 5, when the annealing temperature is 810 ° C. or higher,
It can be seen that recrystallization is completed and stable and good mechanical properties are obtained. That is, as shown in FIGS. 5 (1) and 5 (2), the yield strength and the tensile strength soften as the annealing temperature increases, and therefore, as the recrystallization proceeds, and become almost constant at the annealing temperature of 810 ° C. or higher. It turns out that it reaches. The tensile strength of the cold-rolled steel sheet at an annealing temperature of 810 ° C. or higher is about 600 N / mm ² . The elongation at break is shown in FIG.
As shown in (3), as the annealing temperature increases, the temperature increases with the progress of recrystallization.
From the above, it can be seen that a satisfactory level is reached. As described above, the lower limit of the annealing temperature of the heat treatment is limited to 810 ° C.
This is for this reason. The reason why the upper limit of the annealing temperature in the heat treatment is limited to 880 ° C. is that even if the material is heated to an annealing temperature exceeding the upper limit, no further improvement in mechanical properties can be expected, and the energy loss increases. Things.

FIG. 6 is a graph showing the relationship between the soaking time of the heat treatment and the mechanical properties of the cold-rolled steel sheet after the heat treatment.
In the figure, the symbol □ indicates that the annealing temperature is 815 ° C., and the symbol • indicates that the annealing temperature is 850 ° C. The components of the cold-rolled steel sheet to be heat-treated are components that satisfy the component range of the present embodiment, and are C: 0.13%,
Si: 0.10%, Mn: 1.60%, Ti: 0.02
%, Nb: 0.035%. The plate thickness is 1.2mm
Met.

FIG. 6 shows that stable and good mechanical properties can be obtained when the soaking time is 75 seconds or longer. That is, as shown in FIG. 6A, the tensile strength softens as the soaking time becomes longer regardless of the annealing temperature, and the soaking time becomes 75%.
It can be seen that the value reaches almost a constant value in seconds or more. Soaking time 75
The tensile strength in seconds and above is about 600 N / mm ² .
Also, as shown in FIG. 6 (2), the elongation at break increases as the soaking time increases, and reaches a favorable level when the soaking time is 75 seconds or more. As described above, the lower limit of the soaking time of the heat treatment is limited to 75 seconds for this reason. Further, the upper limit of the soaking time of the heat treatment is limited to 130 seconds, as in the case of the annealing temperature shown in FIG. Cannot be expected, and therefore the energy loss increases.

The average cooling rate after heating and holding in the heat treatment is 10 ° C./sec or less, and the temperature is cooled to 500 ° C. or less when the average cooling rate exceeds 10 ° C./sec, as shown in FIG. This is because site transformation and bainite transformation are likely to occur, and a martensite phase may appear. Also, it is cooled to below 500 ° C
If the temperature is 500 ° C. or less, the effect of the cooling rate can be ignored.

Referring again to FIG. 4, in the eighth step, hot-dip galvanizing is performed. Hot-dip galvanizing is performed by continuously immersing a cold-rolled steel sheet which becomes the heat-treated base steel sheet in a hot-dip galvanizing bath. In the present embodiment, since the Si% in the base steel sheet is sufficiently reduced as described above, a decrease in plating wettability due to an oxide film of Si can be suppressed, and the occurrence of non-plating can be prevented. be able to. In the ninth step, an alloying process is performed. The alloying treatment is performed by continuously introducing a hot-dip galvanized steel sheet into an alloying treatment furnace and heating it to alloy the galvanized layer and Fe in the base steel sheet. by this,
An alloy plating layer containing Fe and Zn is formed on the surface of the galvanized steel sheet. In the tenth step, temper rolling is performed, and the production of the high-tensile GA steel sheet is completed.

As described above, in the present embodiment, since C, Mn, Nb and Ti are sufficiently contained in the steel, that is, in the base steel sheet, the solid solution strengthening by C and Mn and the Nb and Ti
A sufficiently high strength can be achieved by a synergistic effect with precipitation strengthening by the system precipitate. Further, since the Si% in the base steel sheet is sufficiently suppressed, the plating property of hot-dip galvanizing is good, and it is not necessary to perform Fe-based pre-plating before hot-dip galvanizing. Further, since Si and Mn% are adjusted so as not to cause martensitic transformation, it is possible to prevent the formation of a martensite phase and to reduce the variation in tensile strength. Also, Ti, N
Since b% is adjusted so as not to excessively raise the recrystallization temperature, the base steel sheet can be sufficiently recrystallized during the heat treatment. Further, since the annealing temperature and the soaking time of the heat treatment are properly set based on experiments, desired mechanical properties can be reliably obtained. In addition, Fe
-Since the Zn alloy layer is formed, the corrosion resistance is good.

Next, Example 1 in which the steel component satisfies the entire component range of the present invention and Comparative Example 1 in which the steel component is out of the component range of the present invention.
2 and a high tensile strength GA steel sheet were manufactured, and the characteristic values were compared. Table 1 shows the chemical components of Example 1, Comparative Examples 1 and 2. Comparative Example 1 was composed of Si%, Ti%, Nb%, (0.21
Si + 0.48Mn) is out of the component range of the present invention, and in Comparative Example 2, C%, Si%, Ti%, and Nb% are out of the component range of the present invention.

[0034]

[Table 1]

A steel having the chemical components shown in Table 1 was melted,
A high-tensile GA steel sheet was manufactured according to the manufacturing process shown in FIG. Table 2 shows the manufacturing conditions. However, Comparative Example 1 was subjected to Fe plating after cold rolling and introduced into a continuous galvanizing facility. Manufacturing conditions other than those shown in Table 2 are common, soaking time of heat treatment: 78 seconds, average cooling rate from annealing temperature to 500 ° C .: 7 ° C./sec, zinc coating amount of hot-dip galvanized: 10 g / side mm ² , Alloying condition: 550 ° C ×
30 seconds. After producing each high-tensile GA steel sheet, plating quality, mechanical properties and weldability were evaluated. Regarding the plating quality, the presence or absence of non-plating was visually determined. as a result,
No plating was not observed in any of Example 1, Comparative Examples 1 and 2. Table 2 shows the mechanical properties and weldability of each high-tensile GA steel sheet.

[0036]

[Table 2]

The weldability shown in Table 2 was evaluated by a cross-shaped tensile test. FIG. 7 is a schematic diagram of a cross-shaped tensile test. In the cross-shaped tensile test, two strip-shaped test pieces 8 are overlapped in a cross shape and spot-welded, and two test pieces 8 are formed.
In the direction of the thickness of the sheet so that the spot weld 9
And breaking strength N / mm ² is measured. The size of the strip-shaped test piece is 50 m in width.
m, length: 150 mm.

From Table 2, it can be seen that Example 1 has good mechanical properties and weldability, and has a tensile strength TS ≧ 590 N / m.
m ² , TS × El ≧ 13570 is satisfied. Comparative Example 1 has excellent mechanical properties as in Example 1, but has a low breaking strength of the spot weld 9 and poor weldability. In Comparative Example 2, TS × El, which is low in elongation at break and strength-ductility balance, is poor. Therefore, according to the present invention, an alloyed hot-dip galvanized high-strength steel sheet excellent in balance between weldability, plating property and strength ductility can be manufactured without performing Fe-based pre-plating.

[0039]

As described above, according to the first aspect of the present invention, since the upper limit of the Si content is set sufficiently low, a decrease in plating wettability can be suppressed, and the Fe-based Hot-dip galvanizing can be performed without plating. In addition, since C, Mn, Ti, and Nb are sufficiently contained, high strength can be achieved by solid solution strengthening by C and Mn and precipitation strengthening by Ti and Nb.
In addition, since C, N, Ti, and Nb are sufficiently contained, Ti and Nb-based precipitates are sufficiently precipitated, and it is possible to prevent crystal grains from becoming coarse during spot welding.

According to the present invention, since the tensile strength is at a sufficient level, the weight of the vehicle body can be reduced. Further, since the product of the tensile strength TS and the elongation at break El is at a sufficient level, the strength-ductility balance is good, and the occurrence of working cracks during molding can be prevented.

According to the third aspect of the present invention, since the winding temperature is set at a sufficiently low level, it is possible to prevent the inner diameter of the coil after winding from being crushed. In the continuous hot-dip galvanizing equipment, since it is heated and held in a sufficient temperature range for a sufficient time, the elongation at break can be sufficiently improved. Therefore, the strength-ductility balance is improved, and workability can be improved.

[Brief description of the drawings]

FIG. 1 is a cross-sectional view schematically illustrating the structure of a cross section perpendicular to the plate surface of a galvannealed high-strength steel sheet 1 according to an embodiment of the present invention.

FIG. 2 is an enlarged image showing an optical microscope structure of the base steel sheet 3 shown in FIG.

FIG. 3 is a graph showing a relationship between Si% and Mn%, an average cooling rate of heat treatment, and a transformed region and an untransformed region.

FIG. 4 is a view showing a manufacturing process of the galvannealed high-strength steel sheet shown in FIG.

FIG. 5 is a graph showing the relationship between the annealing temperature of the heat treatment and the mechanical properties of the cold-rolled steel sheet after the heat treatment.

FIG. 6 is a graph showing the relationship between the soaking time of heat treatment and the mechanical properties of a cold-rolled steel sheet after heat treatment.

FIG. 7 is a schematic view of a cross-shaped tensile test.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Galvannealed high-strength steel plate 3 Base steel plate 4 Alloy plating layer 5 Fine precipitation part 9 Spot welding part

 ──────────────────────────────────────────────────続 き Continued on the front page F term (reference) 4K037 EA01 EA06 EA15 EA18 EA19 EA23 EA25 EA27 EA31 EB06 EB08 FC07 FE01 FE02 FH01 FJ05 FJ06 FK02 FM02 FM04 GA05 HA03 JA06

Claims (3)

[Claims] An alloyed hot-dip galvanizing method in which a hot-dip galvanized high-strength steel sheet is heated to form an alloy plating layer containing Fe and Zn. In the high-strength steel sheet, the components of the base steel sheet are expressed in terms of percentage by weight as C: 0.10 to 0.15.
%, Si: 0.05 to 0.15%, Mn: 1.4% or more, P: 0.01 to 0.03%, S: 0.02% or less,
Dissolved Al: 0.005 to 0.10%, N: 0.01% or less, Nb: 0.020 to 0.045%, Ti: 0.01
0 to 0.030%, the balance being Fe and unavoidable impurities, and the Si content [Si] and Mn in the base steel sheet
A galvannealed high-strength steel sheet wherein the content [Mn] satisfies the inequality 0.21 [Si] +0.48 [Mn] <1.0. 2. The tensile strength TS is TS ≧ 590 N / mm ²
And tensile strength TS (N / mm ² ) and elongation at break E
The alloyed hot-dip galvanized high-strength steel sheet according to claim 1, wherein the product of l (%) is TS x El ≥ 13570. 3. A steel having the following components is melted, and the components of the steel are as follows: C: 0.10 to 0.15% by weight, Si:
0.05-0.15%, Mn: 1.4% or more, P: 0.
01-0.03%, S: 0.02% or less, dissolved Al:
0.005 to 0.10%, N: 0.01% or less, Nb:
0.020-0.045%, Ti: 0.010-0.0
30%, the balance being Fe and unavoidable impurities,
And Si content in the steel [Si] and the Mn content [Mn] and satisfies the inequality 0.21 [Si] +0.48 [Mn] <1.0, further the steel, Ar ₃ transformation point or more of While hot rolling in the temperature range, winding at a winding temperature of 400 to 600 ° C., pickling, cold rolling, and heating and holding at a temperature range of 810 to 880 ° C. for 75 to 130 seconds in a continuous galvanizing equipment. Then, at an average cooling rate of 10 ° C./second or less, 500
A method for producing an alloyed hot-dip galvanized high-strength steel sheet, which is characterized in that the hot-dip galvanized steel sheet is cooled to a temperature of not more than 0 ° C. or less, and thereafter, the coating layer is alloyed by heating.