JP4817749B2 - Method for producing 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 an application, since it is usually processed into a required shape by press molding, it is important to have excellent workability in addition to corrosion resistance.
The alloyed hot-dip galvanized steel sheet is manufactured by hot alloying after hot-dip Zn plating. For the heat alloying treatment, an alloying treatment furnace employing a burner heating method, a high frequency induction heating method, a heating method using both in combination, or the like is generally used.

In particular, alloyed hot-dip galvanized high-strength steel sheets that have come to be used frequently to reduce the weight of automobile bodies are made of high-tensile steel with low ductility. 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, by applying hot-dip Zn plating, and then alloying heat treatment, 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.

Similarly, the inventors of the present invention disclosed in Patent Document 2 that a Fe-based plating layer was formed as pre-plating before hot-dip Zn plating, thereby lowering the alloying treatment temperature after hot-zinc plating and steel. It was proposed to suppress the formation of pearlite and carbide in the steel, and to suppress the deterioration of the mechanical properties of the steel itself, especially the ductility.
JP 2001-279409 A JP 2004-285385 A

By the way, in order to prevent the formation and remaining of the ζ phase in the manufacturing method proposed in Patent Document 1, it is necessary to perform the alloying heat treatment at a high temperature of 530 ° C. or higher.
However, as shown in Patent Document 1, in order to form pearlite in the steel sheet by hot-dip alloy plating at a high temperature after hot-dip Zn plating on a steel sheet containing a large amount of Si and Mn, Strength and elongation are significantly reduced. Further, since the alloy heat treatment is performed at a high temperature, a Γ phase is easily formed in the plated alloy layer.
Further, even if the technique proposed in Patent Document 2 for forming a Fe-based plating layer and reducing the alloying treatment temperature as the pre-plating before the hot-dip Zn plating is applied, η− Zn phase and ζ phase are likely to appear.

When these Γ phase, η-Zn phase, and ζ phase appear in the plated alloy layer, powdering and flaking are caused and workability of the plated steel sheet is lowered.
The present invention has been devised to solve such problems. When a steel plate containing a large amount of Si and Mn is used as a base plate, pre-Fe plating, hot-dip Zn plating, and subsequent alloying heat treatment are performed. The pre-Fe plating layer formed on the surface of the plating base plate is adjusted with a specific relationship between the Zn plating adhesion amount and the Fe plating adhesion amount, and the Fe and Zn plating layers not containing Si and Mn are alloyed. An object of the present invention is to obtain an galvannealed steel sheet having high strength and excellent workability.

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: 0.05 to 0.25 mass%, Si: 0.6 to 2.0 mass. %, Mn: 0.8 to 3.0% by mass, the remainder: Fe-based plating is applied to a steel sheet having a composition of Fe and inevitable impurities, and then annealed at 700 to 900 ° C., and thereafter 2 to 200 It cools to 350-500 degreeC with the average cooling rate of (degreeC) / second, and does not hold | maintain for 0 to 20 minutes in the temperature range, but does hot Zn plating with the adhesion amount which satisfy | fills following formula (1), 460- It is held at a temperature range of 530 ° C. for 2 seconds to 2 minutes, and then cooled to 250 ° C. or lower at a cooling rate of 5 ° C./second or more to form a δ ₁ phase single phase alloyed plating layer.
0.08 ≦ [Fe adhesion amount] / ([Fe adhesion amount] + [Zn adhesion amount]) ≦ 0.15 (1)

  As the steel plate, at least one of Ti: 0.04 to 0.2% by mass, Nb: 0.003 to 0.2% by mass, V: 0.5% by mass or less, or B: 0 in steel. .01% by mass or less, Mo: 1.0% by mass or less, Cr: 1.0% by mass or less, Ni: 2.0% by mass or less, Co: 1.0% by mass or less good.

The high strength alloyed hot dip galvanized steel sheet provided by the present invention is the amount of Fe-based plating applied to a pre-plated Fe-based material when alloyed hot-dip Zn plating is performed on a steel sheet containing Mn and Si. Since the hot-dip Zn plating is performed while controlling the adhesion amount of Zn and the Zn plating in a predetermined relationship, the alloying heat treatment after plating can be performed in a low temperature range, and the galvanized phase, η-Zn phase or ζ A δ ₁ single-phase plating layer having no phase can be obtained.
For this reason, there can be obtained a hot-dip plated steel sheet that is free from strength and elongation, and that does not cause flaking or powdering during processing and has excellent workability.

  The inventors of the present invention, when using a steel plate containing a large amount of Si or Mn as a base plate, reduced the strength and ductility of the steel plate by hot-dip Zn plating and subsequent alloying heat treatment required a high alloying temperature. It has been found that the decrease in workability of the plated steel sheet after the heat treatment for alloying is that the Γ phase, η-Zn phase or ζ phase has appeared in the plating layer, and various measures have been studied for countermeasures. .

In the process of repeated examination, if an Fe-based plating layer is formed as pre-plating before applying hot-dip Zn plating, the alloying temperature after hot-dip Zn plating can be lowered, or alloying can be performed during hot-dip Zn plating. It has been found that the Γ phase is not generated, and that the mechanical properties of the steel material itself, in particular, the ductility can be suppressed.
Furthermore, if the Fe-based plating adhesion amount in performing Fe-based pre-plating is adjusted in relation to the Zn plating adhesion amount, there is no Mn or Si that suppresses alloying on the surface layer, and sufficient Fe is present. It has been found that an existing layer is formed, and even if the alloying temperature is low, alloying heat treatment that does not generate η-Zn phase or ζ phase and forms only δ ₁ phase is possible.

Details will be described below.
The plating base plate used in the present invention includes C: 0.05 to 0.25% by mass, Si: 0.6 to 2.0% by mass, Mn: 0.8 to 3.0% by mass, If necessary, at least one of Ti: 0.04 to 0.2 mass%, Nb: 0.003 to 0.2 mass%, V: 0.5 mass% or less, or B: 0.01 mass% Hereinafter, at least one of Mo: 1.0 mass% or less, Cr: 1.0 mass% or less, Ni: 2.0 mass% or less, Co: 1.0 mass% or less, with the balance being Fe and inevitable Those made of mechanical impurities are used.
The “%” display means “mass%” unless otherwise specified.

C: 0.05-0.25%
C is effective for increasing the strength. If it is less than 0.05%, 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: 0.6-2.0%
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 0.6%, the effect is not sufficiently exhibited. In addition, when the concentration exceeds 2.0%, the effect is saturated, and the Si diffusion phenomenon becomes remarkable during annealing, and even if Fe-based plating is performed, a Si oxide film layer is formed on the surface layer. Plating adhesion decreases.

Mn: 0.8 to 3.0%
Mn is an element that improves hardenability and is effective in increasing strength. If it is less than 0.8%, 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: 0.003-0.2%
Nb: 0.003 to 0.2%
V: 0.5% or less Ti, Nb, and V are effective for increasing the strength by refining the structure and improving the hole expandability of the steel sheet. In both cases of Ti and Nb, the effect is not exhibited at less than 0.003%. On the other hand, if it exceeds 0.2%, the effect is saturated and the industrial cost is only increased. If the concentration of V exceeds 0.5%, the effect is saturated and the industrial cost only increases.

B: 0.01% or less
Mo: 1.0% or less
Cr: 1.0% or less
Ni: 2.0% or less
Co: 1.0% or less These are effective elements for improving the hardenability and increasing the strength. 1 type (s) or 2 or more types can be added suitably. However, B: 0.01%, Mo: 1.0%, Cr: 1.0%, Ni: 2.0%, Co: exceeding 1.0%, on the contrary, the decrease in ductility increases. It only increases the industrial cost.

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 there are no particular restrictions on the type of electroplating solution, bath composition, plating conditions, etc., as long as the required adhesion amount is obtained. A predetermined amount of the Fe-based pre-plated layer is deposited by setting the above conditions. Fe-based pre-plating can be carried out in the electroplating line, but it is necessary to install an electroplating facility in front of the gas reduction annealing furnace in the hot-dip plating line to make Fe-based pre-plating and hot-dip Zn plating continuous. Is advantageous.

Further, as will be described later, the adhesion amount of the hot dip Zn adhering to the plating original plate pulled up from the hot dip Zn plating bath is adjusted to a required amount. The adjustment is preferably performed by a normal gas wiping method.
In the present invention, it is important to adjust the relationship between the Fe-based pre-plating adhesion amount and the subsequent hot-dip Zn plating adhesion amount. If the value of [Fe adhesion amount] / ([Fe adhesion amount] + [Zn adhesion amount]) is less than 0.08, the amount of Fe plating is insufficient, and ζ phase or η-Zn remains in the plating layer. Or a high alloying temperature is required. On the other hand, if the above value exceeds 0.15, the amount of Fe plating becomes excessive, and Fe that is not used for alloying remains, which only increases the industrial cost.

The mechanical properties of the galvannealed steel sheet also change depending on the annealing conditions of the pre-plated steel sheet before hot-dip Zn plating. The annealing conditions for obtaining higher strength with higher ductility will be described below.
The annealing temperature is in the range of 700 to 900 ° C. If it is less than 700 degreeC, recrystallization will not fully be performed but a mechanical property will fall. If it exceeds 900 ° C., Si and Mn diffuse into the Fe-based plating layer, and the effect of Fe pre-plating is attenuated.
The annealing atmosphere is a reducing atmosphere. A gas reducing atmosphere is preferable.
In the gas reducing atmosphere, even if the pre-plated Fe is partially oxidized, it is gas reduced to an active surface state, and the plating layer tends to adhere during subsequent hot-dip Zn plating. Furthermore, the subsequent alloying reaction rate also increases.

Next, the average cooling rate after soaking is set to 2 to 200 ° C./second. If the average cooling rate is less than 2 ° C./second, pearlite transformation occurs and the balance between strength and ductility is deteriorated. On the contrary, when the average cooling rate exceeds 200 ° C./second, the deviation in the width direction and the longitudinal direction of the steel sheet becomes large, and a uniform structure cannot be obtained.
Moreover, the end point temperature of cooling shall be 350-500 degreeC. When the end point of cooling exceeds 500 ° C., austenite is decomposed into carbides, pearlite is formed, and the balance between strength and ductility is deteriorated. If it is less than 350 ° C., a large amount of martensite is generated, so that the strength is improved, but the elongation is remarkably lowered, and the moldability and the like are deteriorated.
If the holding time is too long, cementite is produced, and the balance between strength and ductility is deteriorated. The holding time is preferably 20 minutes or less.
By such a manufacturing method, a high-strength galvannealed steel sheet having good workability can be obtained.

The plating original plate subjected to gas reduction annealing is introduced into a molten Zn plating 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.
The plating adhesion amount of the hot dip metal adhering to the plating original plate pulled up from the hot dip Zn plating bath is adjusted by gas wiping. The amount of adhesion required is as described above.

  After gas wiping, the steel sheet is heated to a temperature of 460 ° C. or higher and lower than 530 ° C. for 2 to 120 seconds to advance the alloying reaction. If the heating temperature is less than 460 ° C. or less than 2 seconds, alloying is insufficient and the η-Zn layer remains. When the temperature is 530 ° C. or higher, pearlite is generated in the steel, and the balance between strength and ductility is deteriorated, resulting in a decrease in workability. Even if the alloying temperature is less than 530 ° C., the higher the temperature, the more likely pearlite is generated in the steel. Therefore, the alloying temperature is preferably less than 490 ° C. By 120 seconds, alloying is sufficiently performed, and further heating is meaningless.

As long as the heating condition of 460 ° C. or higher and lower than 530 ° 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 of them, 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. An Fe pre-plated layer was formed on the surface of this cold-rolled steel sheet by electroplating under the plating conditions shown in Table 2.
The Fe-plated steel sheet was annealed under the conditions shown in Table 3.
Furthermore, hot-dip Zn plating was performed, and the hot-dip Zn plating layer having an adhesion amount shown in Table 4 was obtained by adjusting gas wiping.

Next, it was immersed in a molten Zn plating bath, and the amount of plating adhered was adjusted by gas wiping. The plating adhesion amount is shown in Table 4. Thereafter, alloying heat treatment was performed under the conditions shown in Table 4.
The obtained alloyed hot-dip galvanized steel sheet was subjected to a tensile test while observing the cross-sectional structure of the alloyed plated layer.
A case where there was no η-Zn phase, ζ phase, or Γ phase in the plated layer by cross-sectional observation was evaluated as ◯, and a case where η-Zn phase, ζ phase, or Γ phase 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 4.

From the results shown in Table 4, test Nos. Using steel types A to K having the alloy composition specified in the claims. In Nos. 1 to 11, an alloyed hot-dip galvanized steel sheet in which the alloying state after the alloying heat treatment is good and the balance between tensile strength and elongation is good is obtained.
On the other hand, in the comparative example No. 12 in which the formula (1) was less than 0.08 and the alloying heat treatment temperature was raised, the Γ phase was generated although it could be alloyed. It is expected that the powdering resistance at the time of molding will decrease. Similarly, in Comparative Examples No. 13 and 14 in which the formula (1) is less than 0.08 and the alloying heat treatment temperature is the same as that of the present invention, sufficient alloying is not possible, and a η-Zn phase is observed. It was. It is expected that the flaking resistance during molding will be reduced.

Example 2:
Among the steel types shown in Table 1, a steel sheet A was used as a raw material, and an Fe pre-plated layer was formed at an Fe deposition amount of 6 g / m ² by the same method as in Example 1 to prepare a molten Zn-plated original sheet.
This pre-plated steel sheet was subjected to an annealing treatment under conditions shown in Table 5, a Zn adhesion amount of 45 g / m ² hot-dip Zn plating, and an alloying heat treatment. At this time, the equation (1) is 0.11.
The obtained alloyed hot-dip galvanized steel sheet was subjected to a tensile test in the same manner as in Example 1 while observing the cross-sectional structure of the alloyed plated layer.
The evaluation results are also shown in Table 5.

From the results shown in Table 5, when pre-plated steel sheet is annealed under the conditions specified in the claims, alloying heat treatment is performed on the hot-dip Zn-plated steel sheets under the conditions specified in the claims. An alloyed hot-dip galvanized steel sheet having a good balance between tensile strength and elongation is obtained.
On the other hand, in Comparative Example No. 25 where the annealing temperature applied to the pre-plated steel sheet was low, recrystallization did not proceed sufficiently and the mechanical properties were insufficient. On the other hand, in Comparative Example No. 26 where the annealing temperature was too high, diffusion of Si and Mn occurred in the Fe-based plating layer and the effect of Fe-based pre-plating was lost, and a η-Zn phase was generated by alloying heat treatment. It was. It is expected that the flaking resistance during molding will be reduced.
Further, in Comparative Example No. 27 in which the cooling rate during annealing was too slow, the balance between strength and elongation was poor. It is expected that the powdering resistance at the time of molding will decrease. Further, Comparative Example No. 28 in which the cooling end point temperature during annealing was too low and Comparative Example No. 29 in which the cooling end point temperature was too high also had a poor strength-elongation balance. Furthermore, Comparative Example No. 30, in which the temperature during the alloying heat treatment was too high, also had a poor strength-elongation balance. In any case, the moldability is expected to be lowered.

Claims (3)

C: 0.05 to 0.25% by mass, Si: 0.6 to 2.0% by mass, Mn: 0.8 to 3.0% by mass, balance: Fe and steel with inevitable impurities After applying Fe-based plating, annealing is performed at 700 to 900 ° C., followed by cooling to 350 to 500 ° C. at an average cooling rate of 2 to 200 ° C./second, and holding or holding in that temperature range for 0 to 20 minutes. Without applying, the hot-dip Zn plating is performed with an amount satisfying the following formula (1), held at a temperature range of 460 to 530 ° C. for 2 seconds to 2 minutes, and then cooled to 250 ° C. or less at a cooling rate of 5 ° C./second or more. Then, a method for producing a high-strength galvannealed steel sheet excellent in workability, characterized by forming a δ _1- phase single-phase alloyed plating layer .
0.08 ≦ [Fe adhesion amount] / ([Fe adhesion amount] + [Zn adhesion amount]) ≦ 0.15 (1) The steel sheet further contains at least one or more of Ti: 0.04 to 0.2 mass%, Nb: 0.003 to 0.2 mass%, and V: 0.5 mass% or less. A method for producing a high-strength galvannealed steel sheet with excellent workability. The steel plate is further at least B: 0.01% by mass or less, Mo: 1.0% by mass or less, Cr: 1.0% by mass or less, Ni: 2.0% by mass or less, Co: 1.0% by mass or less The method for producing a high-strength alloyed hot-dip galvanized steel sheet having excellent workability according to claim 1 or 2, comprising at least one kind.