JP4592000B2 - Manufacturing method of high-strength galvannealed steel sheet with excellent workability - Google Patents

Description

  The present invention relates to a method for producing a high-strength steel sheet useful as a member for automobiles, buildings, electrical equipment, and the like, particularly a high-strength galvannealed steel sheet having excellent workability.

Alloyed hot-dip galvanized steel sheets are widely used as rust-proof steel sheets in various fields including automobile bodies and home appliances because they are excellent in corrosion resistance, paintability, adhesion after coating, and weldability. In such applications, since it is usually used after being processed into the required shape by press molding, it is also important that the processability is excellent in addition to the corrosion resistance.
Alloyed hot dip galvanized steel sheets are manufactured by hot galvanizing and then heat alloying. In general, an alloying furnace employing a burner heating method, a high-frequency induction heating method, a heating method using both in combination, or the like is used for the heating alloying treatment.

In particular, alloyed hot-dip galvanized high-strength steel sheets, which have come to be used frequently to reduce the weight of automobile bodies, use high-tensile steel with low ductility as the plating base plate, so plating that affects press formability The effect of the slidability of the layer surface is great, and if a large amount of ζ phase remains, not only peeling of the plating layer but also plate breakage may occur, and press molding may not be possible.
Therefore, the inventors of the present invention disclosed in Patent Document 1 in order to obtain an alloyed hot-dip galvanized steel sheet that does not leave the ζ phase at the time of alloying heat treatment and suppresses the growth of the Γ phase and has excellent workability. After forming a layer consisting essentially of Fe on the surface of the original plate, hot-dip galvanizing is performed, and then alloying heat treatment is performed to form a plating layer consisting of δ ₁ phase, Γ ₁ phase and Γ phase with a layer thickness of 1 μm or less. A method of manufacturing alloyed hot-dip galvanized high strength steel sheet was proposed.
JP 2001-279409 A

However, in the production method of Patent Document 1, it is necessary to perform the alloying heat treatment at a high temperature of 530 ° C. or higher in order to prevent the formation and remaining of the ζ phase.
Incidentally, in addition to workability, high strength is required for plated steel sheets used for automobile bodies, home appliances, and the like. In particular, in recent years, the weight of automobile bodies has been reduced due to the trend of reducing fuel consumption of automobiles. In terms of materials, higher strength is being promoted so that strength can be secured even if the thickness is reduced. Generally, in low carbon steel, Si and Mn which are elements effective for increasing the strength are added. Further, a large amount of Si and Mn is also contained in the galvanizing plate so as to increase the strength.
After hot dip galvanizing on a steel sheet containing a large amount of Si and Mn, when alloy heat treatment is performed at a high temperature, pearlite and carbides are formed in the steel sheet, so that the strength and elongation of the steel sheet itself are significantly reduced.

  The present invention has been devised to solve such a problem, and after forming a pre-Fe plating layer on the surface of a plating original plate before hot dip galvanizing, annealing is performed, and thus an alloy after plating is formed. A method of manufacturing an alloyed hot-dip galvanized steel sheet that is high in strength and excellent in workability by reducing the processing temperature to prevent deterioration of the mechanical properties of the original plate even if the heat treatment is omitted or performed. The purpose is to provide.

In order to achieve the object, the method for producing a high-strength galvannealed steel sheet excellent in workability according to the present invention has C: 0.04 to 0.25 mass%, Si: 0.2 to 2.0 mass. %, Mn: 0.5 to 3.0% by mass, and after forming an Fe-based plating layer on a steel sheet having a composition comprising Fe and inevitable impurities , the value represented by the following formula (1) is 1 After annealing at a heating temperature and time of 0.0 or more and cooling at an average rate of 2 to 200 ° C./second, hot dip galvanizing is performed immediately or at a temperature of 430 ° C. or more and less than 500 ° C. for 2 seconds to After holding for 2 minutes, it is cooled to 250 ° C. or lower at a cooling rate of 5 ° C./second or higher.
[{980-50 × ([Si] + [Mn] / 4)}-t / 4] / T (1)
However, [Si], [Mn]; Si, Mn concentration (mass%)
t: heating time (second), T: heating temperature (° C)

As a steel plate, at least one or two of Ti: 0.04 to 0.2% by mass, Nb: 0.003 to 0.2% by mass, or B: 0.01% by mass or less, and Mo in steel. : 1.0% by mass or less, Cr: 1.0% by mass or less, V: 0.5% by mass or less , Co: 1.0% by mass or less may be included.

  In the present invention, when alloyed hot dip galvanizing is performed on a steel sheet containing Si and Mn, if hot dip galvanizing is performed on a pre-plated Fe-based pre-plat and then annealing under predetermined conditions, Even if the alloying heat treatment is omitted or performed, the temperature can be lowered. As a result, it was possible to prevent deterioration of the mechanical properties of the plating original sheet, and to produce an alloyed hot-dip galvanized steel sheet having high strength and excellent workability.

The inventors of the present invention previously proposed in Patent Document 1 in the method of manufacturing an alloyed hot-dip galvanized high-tensile steel sheet require a high temperature as an alloying heat treatment after hot-dip galvanizing. During the heat treatment, it was confirmed that pearlite and carbides were generated in the steel, and the strength and ductility of the steel sheet were reduced.
Therefore, in the process of further investigation, if a Fe-based plating layer is formed as pre-plating before hot-dip Zn plating and annealing is performed under predetermined conditions, alloying can be easily performed after hot-dip galvanizing. Therefore, it has been found that the alloying treatment temperature can be lowered or alloying can be performed at the time of hot dip galvanizing, and the mechanical properties of the steel material itself, in particular, ductility can be suppressed.

The reason why the alloying treatment temperature could be lowered was estimated as follows.
When the Si and Mn concentrations in steel are high, Si and Mn usually serve as a barrier, and in order to suppress mutual diffusion of Fe and Zn, alloying is not performed unless the alloy heat treatment temperature is increased. On the other hand, when the Fe pre-plated layer is formed, a layer that does not contain Si or Mn that suppresses alloying exists on the surface layer, and alloying treatment is possible even at a low alloying temperature. .

  By the way, even in an alloyed hot-dip galvanized steel sheet containing a large amount of Si, Mn, etc., annealing treatment is usually performed after Fe pre-plating and before galvanizing in order to develop stable high strength. By heating at this time, Si and Mn, which are easily oxidizable elements, diffuse into the Fe plating layer to form an oxide film again on the surface layer, and it is necessary to increase the alloying temperature after galvanization. When the alloying heating temperature becomes high, for example, when it exceeds 530 ° C., pearlite is generated, and problems as pointed out in the problems of the prior art occur. Even if it is 530 ° C. or lower, pearlite is not generated at an alloying heating temperature of 500 ° C. or higher, but bainite containing brittle Fe carbide is generated and sufficient workability cannot be obtained.

Therefore, when annealing is performed after Fe pre-plating and before galvanization, it is necessary to set annealing conditions so that Si and Mn do not diffuse on the Fe plating layer and appear on the surface.
In the present invention, if a heating temperature and a heating time such that the value of the formula (1) is 1.0 or more according to the contents of Si and Mn are adopted as the conditions, high Si and Mn It has been found that the alloying temperature can be made lower than 500 ° C. even with steel. Thereby, the appearance of brittle Fe carbide generated at a temperature of 500 ° C. or higher is suppressed, and an alloyed hot-dip galvanized steel sheet excellent in workability can be manufactured.

Below, the preferable form of this invention is demonstrated concretely.
The plating base plate used in the present invention includes C: 0.04 to 0.25% by mass, Si: 0.2 to 2.0% by mass, Mn: 0.5 to 3.0% by mass, If necessary, at least one or two of Ti: 0.04 to 0.2% by mass, Nb: 0.003 to 0.2% by mass, or B: 0.01% by mass or less, Mo: 1.0 Contains at least one or more of mass% or less, Cr: 1.0 mass% or less, V: 0.5 mass% or less, Ni: 2.0 mass% or less, Co: 1.0 mass% or less Can do.
Furthermore, you may use what was controlled to P: 0.015 mass% or less and S: 0.005 mass% or less as needed.
Or it may contain Cu: 0.02-0.15 mass% in the range of Cu / S> = 5 as needed.

The content of each component will be described below. In the description, “%” means “% by mass” unless otherwise indicated.
C: 0.04 to 0.25%
C is effective for increasing the strength. If it is less than 0.04%, the effect cannot be obtained. C is also an element having a great influence on weldability, and if it exceeds 0.25%, the spot weldability of the steel sheet is significantly lowered.

Si: 0.2-2.0%
In addition to being effective for increasing the strength, Si has an effect of suppressing the precipitation of cementite and is an element that has the effect of suppressing the formation of pearlite and the like in steel. If it is less than 0.2%, the effect is not exhibited. In addition, when the concentration exceeds 2.0%, the effect is saturated, and the Si diffusion phenomenon is remarkable during annealing, and even if pre-Fe plating is performed, a Si oxide film layer is formed on the surface layer. Plating adhesion decreases.

Mn: 0.5 to 3.0%
Mn is an element that improves hardenability and is effective in increasing strength. If it is less than 0.5%, the effect is not exhibited. On the other hand, if the concentration exceeds 3.0%, a large amount of martensite structure is formed, and the elongation is significantly reduced.

Ti: 0.04 to 0.2%
Nb: 0.003 to 0.2%
Ti and Nb are effective for increasing the strength by refining the structure and improving the hole expansibility of the steel sheet, and are added as necessary. If the Ti content is less than 0.04% or the Nb content is less than 0.003%, the effect is not exhibited. Further, in both cases, if it exceeds 0.2%, the effect is saturated and only the manufacturing cost is increased.

B: 0.01% or less
Mo: 1.0% or less
Cr: 1.0% or less
V: 0.5% or less
Co: 1.0% or less These are effective elements for improving the hardenability and increasing the strength. It is added as necessary. However, B: 0.01%, Mo: 1.0%, Cr: 1.0%, V: 0.5%, Co: Addition exceeding 1.0%, on the contrary, the decrease in ductility becomes large. It only increases the manufacturing cost.

P: 0.015% or less
S: 0.005% or less Since P and S are elements harmful to the weldability of the steel sheet, it is preferable to regulate P to 0.015% or less and S to 0.005% or less.

Cu: 0.02-0.15%, Cu / S ≧ 5
Since Cu fixes the solid solution S in the steel in the form of CuS, it has the effect of improving spot weldability and corrosion resistance, so it may be added as necessary. In order to obtain a sufficient effect, it is necessary that Cu / S ≧ 5 at 0.02% or more. However, even if added over 0.15%, the effect is saturated and only the manufacturing cost is increased.

The Fe-based pre-plating is preferably formed in the range of ^{3 to} 15 g / m ² of adhesion amount. If the amount of plating is less than 3 g / m ² , alloying does not proceed sufficiently only in the Fe-based pre-plated layer, so diffusion from the steel in which Mn and Si are present is required. Heat treatment is difficult. On the other hand, if it exceeds 15 g / m ² , an Fe plating layer that is not used for alloying is generated even if the Fe-based plating layer is increased, which only increases the manufacturing cost. An alloy layer can be formed without alloying by Fe-based plating of 5 g / m ² or more.

As the Fe-based pre-plated layer, a plated layer of Fe-B, Fe-C, Fe-P, Fe-N, Fe-O or the like can be used in addition to pure Fe. A trace amount of B, C, P, N, and O contained in the Fe-based pre-plated layer exhibits an action of suppressing concentration of Si and Mn.
The Fe-based pre-plated layer is formed by electroplating, but it imposes special restrictions on the type of electroplating solution, bath composition, plating conditions, etc. as long as an adhesion amount of 3 to 15 g / m ² per side can be obtained. is not. Fe-based pre-plating can be carried out in the electroplating line, but it is possible to increase the productivity and cost by attaching an electroplating facility in front of the gas reduction annealing furnace of the hot-dip plating line and making the Fe-based pre-plating and hot-dip galvanizing continuous. Is advantageous.

The mechanical properties of the alloyed hot-dip galvanized steel sheet also change depending on the annealing conditions of the pre-plated steel sheet before hot-dip galvanizing. It is necessary to set annealing conditions for obtaining Si and Mn contained in the raw steel sheet not to be diffused and exposed on the surface of the Fe plating layer, and to obtain high strength with higher ductility.
In the present invention, as described above, a heating temperature and a heating time are employed so that the value of the formula (1) becomes 1.0 or more according to the contents of Si and Mn. Under heating conditions of temperature and time where the value of the above formula (1) is less than 1.0, subsequent alloying at less than 500 ° C. becomes impossible.
Moreover, in order to obtain a steel plate having high ductility and high strength, it is preferable to employ a temperature in the range of 700 to 900 ° C. as an annealing temperature within a range satisfying the above conditions. If it is less than 700 ° C., recrystallization is not sufficiently performed.

The plating original plate subjected to gas reduction annealing is introduced into a hot dip galvanizing bath.
As the hot dip Zn plating bath, one having a bath temperature set to 420 or higher and lower than 490 ° C. is used. 420 ° C. is the freezing point of the plating bath, and if it exceeds 490 ° C., the bath containing the plating bath is eroded violently and requires frequent replacement, which is economically disadvantageous.
It is preferable to adjust the amount of plating deposition per one side of the hot-plated metal adhering to the original plating plate pulled up from the hot-dip galvanizing bath by gas wiping. If the plating adhesion amount is too large, the progress of the alloying reaction is slowed and it is not efficient. Therefore, it is preferable to set the plating adhesion amount to 90 g / m ² or less by gas wiping. In the gas wiping method adopted for adjusting the plating adhesion amount, the lower limit of squeezing is 30 g / m ² .

After gas wiping, the steel sheet is heated to a temperature of 430 ° C. or higher and lower than 500 ° C. for 2 to 120 seconds to advance the alloying reaction. If the heating temperature is less than 430 ° C. or less than 2 seconds, alloying is insufficient and the η-Zn layer remains. Above 500 ° C., pearlite is generated in the steel and the amount of retained austenite is reduced. Further, even if pearlite is not generated, bainite containing brittle Fe carbide is generated and the ductility is lowered and it becomes difficult to process. By 120 seconds, alloying is sufficiently performed, and further heating is meaningless. Further, when the amount of Fe-based plating is large, an alloy layer can be formed without heating after plating.
As long as the heating condition of 430 ° C. or more and less than 500 ° C. × 2 to 120 seconds is satisfied, the heating method is not particularly limited, and a burner heating method, a high frequency induction heating method, a heating method using both in combination, etc. are adopted. An alloying furnace is used.
The alloyed steel sheet is cooled at a cooling rate of 5 ° C./second or more until the plate temperature reaches 250 ° C.

Example 1:
A low carbon steel having the composition shown in Table 1 was melted, and a cold rolled steel sheet having a sheet thickness of 1.0 mm and a sheet width of 1000 mm was manufactured through hot rolling, pickling and cold rolling processes. On the surface of this cold-rolled steel sheet, a hot-dip galvanized original sheet was prepared in which an Fe-B pre-plated layer having a B content of 20 ppm was formed at an adhesion amount of 5.5 g / m ² under the electroplating conditions shown in Table 2 below.

Next, after annealing under the conditions shown in Table 3, hot dip galvanizing with a zinc adhesion amount of 45 g / m ² was performed. Thereafter, alloying heat treatment was performed under the conditions shown in Table 3.
About the obtained galvannealed steel plate, while observing the alloying state of a plating layer, the tension test was done.
As for the alloyed state of the plating layer, the case where there was no η-Zn layer in the plating layer by cross-sectional observation was evaluated as ◯, and the case where the η-Zn layer was observed was determined as x.
In the tensile test, a JIS-5 test piece was sampled perpendicularly to the rolling direction and subjected to a tensile test.
The evaluation results are also shown in Table 3.

As can be seen from the results shown in Table 3, if the annealing treatment conditions after pre-plating are made more appropriate, the alloyed state after the alloying heat treatment is good, and the plated steel sheet has a good balance between tensile strength and elongation. Is obtained.
On the other hand, test No. of the comparative example which performed the annealing process of the conditions where the value of Formula (1) is less than 1.0. In No. 13, the alloying temperature of the plating layer was sufficiently increased since the subsequent alloying treatment temperature was raised to 530 ° C., but the mechanical properties were lowered. Similarly, test no. In the case of 14 to 16, alloying could not be performed at a temperature lower than 500 ° C. thereafter.

Claims (3)

C: 0.04 to 0.25% by mass, Si: 0.2 to 2.0% by mass, Mn: 0.5 to 3.0% by mass, with the balance being composed of Fe and inevitable impurities After forming the Fe-based plating layer on the steel sheet, after annealing at a temperature and time of heating conditions such that the value represented by the following formula (1) is 1.0 or more, and cooling at an average rate of 2 to 200 ° C./second The workability is characterized by applying hot dip galvanizing and immediately or holding at a temperature of 430 ° C. or higher and lower than 500 ° C. for 2 seconds to 2 minutes and then cooling to 250 ° C. or lower at a cooling rate of 5 ° C./second or higher. A method for producing excellent high-strength galvannealed steel sheets.
[{980-50 × ([Si] + [Mn] / 4)}-t / 4] / T (1)
However, [Si], [Mn]; Si, Mn concentration (mass%)
t: heating time (second), T: heating temperature (° C)   The high strength excellent in workability according to claim 1, wherein the steel sheet further contains at least one or two of Ti: 0.04 to 0.2 mass% and Nb: 0.003 to 0.2 mass%. A method for producing a galvannealed steel sheet. The steel plate is at least B: 0.01% by mass or less, Mo: 1.0% by mass or less, Cr: 1.0% by mass or less, V: 0.5% by mass or less , Co: 1.0% by mass or less The method for producing a high-strength galvannealed steel sheet excellent in workability according to claim 1 or 2, wherein the process comprises one or more kinds.