JP5092507B2 - High tensile alloyed hot dip galvanized steel sheet and its manufacturing method - Google Patents

Description

  The present invention relates to a high-strength galvannealed steel sheet and a method for producing the same. In particular, the present invention relates to a high-tensile alloyed hot-dip galvanized steel sheet having excellent plating adhesion suitable for applications such as press forming and bending, such as automobile bodies, and a method for producing the same.

  In recent years, in order to protect the global environment, there has been a demand for improvement in fuel efficiency of automobiles. In steel sheets for automobiles, tensile strength (hereinafter also referred to as “TS”) is 780 MPa or more in order to reduce the weight of the vehicle body and ensure safety. There is a growing need for high-strength steel sheets.

  However, it does not have to be just high strength. For example, high ductility and good bendability are required from the viewpoint of formability, and a high yield ratio is required from the viewpoint of impact performance, and a low yield ratio is required from the viewpoint of component accuracy. Further, regarding the component accuracy, it is required to further reduce fluctuations in TS and yield strength (hereinafter also referred to as “YS”). On the other hand, from the viewpoint of rust prevention, a steel sheet that has been subjected to hot dip galvanization is required. In particular, alloyed hot-dip galvanized steel sheets that are excellent in terms of economy, rust prevention function, and performance after coating are widely used.

This alloyed hot-dip galvanized steel sheet is usually produced as follows. The steel sheet is heated in a preheating furnace before hot dipping, annealed in a reducing atmosphere of H ₂ + N ₂ with a dew point adjusted to −20 ° C. or lower so as not to cause non-plating, then cooled to around the plating bath temperature, and then Is subjected to hot dip Zn plating. Then, an alloyed hot-dip galvanized steel sheet is manufactured by heating the hot-dip galvanized steel sheet at a material temperature of 480 to 600 ° C. for 3 to 30 seconds to form a Fe—Zn alloy plating phase in a heat treatment furnace. To do.

However, when press-working an alloyed hot-dip Zn-plated steel sheet, if there is a soft alloy phase (ζ phase) with a relatively low Fe content in the plating surface layer, the adhesion may occur between the plating surface layer and the mold surface. Since the slidability between the mold surface and the steel plate is inferior, plating peeling (flaking) or press cracking of the steel plate may occur. On the other hand, when the Fe content in the plating layer is high, a hard Γ, Γ ₁ , or δ _1c phase is formed in the vicinity of the interface between the steel plate and the plating layer. Therefore, powdering (powdering) of the plating layer is likely to occur. When this phenomenon occurs, a peeling piece adheres to the mold, and a pressing flaw occurs.

In order to solve such problems, for alloyed hot-dip galvanized steel sheets based on mild steel sheets, the coating layer of the coating may be made of a δ ₁ phase-based alloy phase with a relatively balanced hardness. Proposed.

For example, Patent Document 1 proposes an alloyed hot-dip galvanized steel sheet having a weight per unit area of 45 to 90 g / m ² / one side and excellent in powdering resistance and flaking resistance. Here, the Fe content in the plating layer is controlled to 8 to 12%, and the Al content is controlled to 0.05 to 0.25%. The Γ phase of the alloy layer at the interface between the material and the plating layer is 1.0 μm or less.

  In Patent Document 2, regarding the method for producing an galvannealed steel sheet that is alloyed so that the Fe content in the plating layer of the coating is 8 to 12%, the Al concentration in the plating bath is set to 0.13. In addition, the intrusion plate temperature of the base steel plate is increased as the Al concentration in the bath increases, and the plate temperature on the high-frequency induction heating furnace exit side is controlled within an appropriate range, thereby preventing It has been proposed to produce alloyed hot-dip galvanized steel sheets with excellent ring and flaking resistance.

On the other hand, regarding high-tensile steel sheets, several proposals have been made as follows for improving the powdering resistance of galvannealed steel sheets.
The alloyed hot-dip galvanized steel sheet proposed in Patent Document 3 has a chemical composition of a steel sheet as a base material in mass% of C: 0.05 to 0.20%, Si: 0.02 to 0.70%, Mn : 0.50 to 3.0%, P: 0.005 to 0.10%, S: 0.1% or less, sol.Al: 0.10 to 2.0%, N: 0.01% or less, And Si + Al: specified at 0.5% or more, reduction annealing at 750 to 870 ° C., and then retention at a low temperature of 350 to 550 ° C. for 20 seconds or more, followed by hot dip galvanization, and then a specific alloy It is obtained by performing an alloying treatment at an alloying temperature and a residence time. When the austenite (γ) phase content remains in the base steel sheet by 1% by volume or more, it becomes a base steel sheet. It has high strength as well as excellent local ductility. And while defining Fe content in the plating layer of a film to 8-15 mass%, the Γ phase average thickness in a plating layer is 2 micrometers or less, the maximum Γ ₁ phase length of the thickness direction is 1.5 micrometers or less, and By defining the maximum Γ ₁ phase length and Γ phase thickness ratio to 1 or less, the powdering resistance is improved.

  The alloyed hot-dip galvanized steel sheet proposed in Patent Document 4 has a chemical composition of a steel sheet as a base material in mass% of C: 0.05 to 0.20%, Si: 0.01 to 1.50%, Mn : 0.5-3.0%, P: 0.05% or less, S: 0.01% or less, Al: 0.01-2.0%, N: 0.01% or less, Si + Al: 0.5 %, And after reduction annealing at 780 to 870 ° C., the temperature range from 700 ° C. to 550 ° C. is cooled at an average cooling rate of 30 ° C./second or more, and then at a low temperature of 350 to 550 ° C. for 20 seconds or more. It is allowed to stay, and then cooled to room temperature. One or more of Ni, Cu and Co are attached to the obtained base material, and again, it is allowed to stay at 780 to 870 ° C. for 5 to 500 seconds to perform reduction annealing. And after cooling from the reached temperature to the vicinity of the plating bath temperature, plating is performed. It is obtained by performing an alloying treatment below. When the content of the austenite (γ) phase is 1% by volume or more in the steel plate as the base material, the tensile strength TS ( MPa) × elongation El (%) ≧ 20000 which satisfies the high strength and high ductility. Then, the Al content in the plating layer of the coating is regulated to 0.2 to 0.4 mass%, the Fe content is regulated to 8 to 15 mass%, and adhesion such as Ni, Cu and Co after the first annealing is performed. By increasing the amount and promoting alloying, the powdering resistance and flaking resistance are improved.

Patent Document 5 proposes a technique for improving plating adhesion by having an oxide at the grain boundary of the steel sheet and Patent Document 6 by having an oxide at the grain boundary and in the grain.
Patent Document 7 proposes a technique for improving plating adhesion by lowering the alloy component of the steel sheet surface layer than the central portion.
JP-A-1-68456 JP-A-4-276053 JP 2002-30403 A JP 2002-47535 A JP 09-176815 A JP 09-310163 A JP 2002-115039 A

  In recent years, an alloyed hot-dip galvanized steel sheet having a high strength base material has been strongly demanded and developed for weight reduction. However, alloyed hot-dip galvanized steel sheets that use high-strength steel sheets as the base material rapidly increase the surface pressure applied to the coating layer of the coating during press forming. The press factor has a greater effect on the film peeling behavior than the galvanized steel sheet. Therefore, an alloyed hot-dip galvanized steel sheet using a high-strength steel sheet as a base material has a problem that film damage during press forming becomes large.

  The alloyed hot dip galvanized steel sheet obtained by the alloyed hot dip galvanized steel sheet described in Patent Document 1 and the manufacturing method described in Patent Document 2 relates to the alloyed hot dip galvanized steel sheet using a mild steel plate as a base material. Although it defines the alloy phase of the coating layer of the coating, even when applied to an alloyed hot-dip galvanized steel sheet using a high-strength steel sheet as the base material, almost no effect of improving the powdering resistance during press forming is recognized. It turned out not to be.

  Next, in order to increase the adhesive strength at the interface between the base material and the plating layer, in the case of an ultra-low carbon steel plate, by increasing the Al concentration in the plating bath, the alloy in the grain and the grain boundary of the base steel plate The method of increasing the difference in the forming rate and increasing the unevenness at the interface between the steel sheet and the galvannealed layer could be adopted. In addition, a similar technique should be adopted for a high-tensile steel plate based on ultra-low carbon steel. However, in the case of a steel plate with a high C content such that the tensile strength is 780 MPa or more like the steel of the present invention, even if the Al concentration in the plating bath is increased by the same method, the steel plate and the alloyed hot dip galvanized layer It has been found that the adhesion strength is lower than that of the low-strength steel plate.

  In addition, the alloyed hot-dip galvanized steel sheet proposed in Patent Document 3 and Patent Document 4 as a method for improving powdering of the above-described high-strength steel sheet is a steel type in which the contents of Si and P in the steel are relatively high. Therefore, it is necessary to perform heat treatment with a complex reduction annealing heat pattern for the production. Furthermore, in order to obtain the alloyed hot-dip galvanized steel sheet proposed in these documents, a longer heat treatment time is required for alloying than conventional ones, so a heat treatment furnace having a long furnace length is required, and a new capital investment is required. There is a problem that is necessary.

  As an improvement in the adhesion of hot-dip galvanized high-strength steel sheets, methods utilizing oxides proposed in Patent Documents 5 and 6 have been proposed. These documents do not describe strength, but by analogy with the components of the examples, TS is at most 590 MPa class and not 780 MPa or more. According to the study by the present inventors, it has been found that with respect to a steel sheet having a TS of over 780 MPa, sufficient plating adhesion cannot be obtained by the methods proposed in Patent Documents 5 and 6.

  The present invention has been made for the purpose of solving such problems, and in particular, an alloyed hot-dip galvanized steel sheet excellent in pressability and improved in plating adhesion peculiar to a high-tensile steel sheet having a TS of 780 MPa or more. And a manufacturing method thereof.

The present inventors have studied a high-tensile steel plate excellent in workability.
As a result, by controlling C, Mn, N, Ti, Nb within a predetermined range, the average particle diameter of ferrite and hard second phase can be made 5.0 μm or less, and high strength of 780 MPa or more and excellent It has been found that it is possible to combine ductility and bendability.

  However, in the case of the fine grain steel plate as described above, the grain interface area on the steel plate surface is extremely increased. For this reason, it has been found that the technique for improving the plating adhesion, which is ensured by enlarging the difference between the grain boundary and the intragranular alloying rate, cannot be applied to a high-strength steel sheet having a grain size of 5 μm or less.

  As a result of studying a method for delaying the intragranular alloying rate, the present inventors have found that it is extremely effective to contain 0.02 to 0.60% Si. In the case of a conventional structure having a grain size of more than 5.0 μm, if Si is added, the intragranular area where the alloying speed is slow increases too much, so it takes a long time for alloying and non-plating also occurs. Production on a continuous hot dip galvanizing line was difficult. However, when 0.02 to 0.60% Si is contained in a fine high-strength steel sheet having a grain size of 5.0 μm or less as in the present invention, the alloying speed is high and the wettability is also good. It has been found that the grain boundary part and the inside of the grain with a moderately controlled alloying rate can be properly balanced, and the adhesion between the steel sheet and the galvannealed alloy can be improved.

  In order to further improve ductility, Si, Cu, and Ni may be included; in order to improve strength, Cr, Mo, V, and B may be included; in order to improve bendability, Ca and REM may be included. As a result, the present invention has been completed.

The present invention has been completed based on such new findings, and the gist thereof is as follows.
(1) In the steel plate provided with the alloyed hot-dip galvanized layer on the surface of the steel plate, the steel plate is in mass%, C: 0.03 to 0.25%, Si: 0.12 to 0.60%, Mn: 2.0 to 4.0%, P: 0.05% or less, S: 0.01% or less, sol. Al: 0.8% or less, N: 0.0020 to 0.015%, Ti: 0.500% or less and Nb: 0.500% or less A metal having a chemical composition consisting of Fe and impurities with the balance being Fe and impurities, and having an average crystal grain size of ferrite of 5.0 μm or less and an average grain size of hard second phase of 5.0 μm or less have a tissue, 540 ° C. or less of the high tensile galvannealed steel sheet tensile strength, characterized by comprising subjected the alloying treatment is not less than 780MPa in a temperature range.

(2) In the steel plate provided with the alloyed hot dip galvanized layer on the surface of the steel plate, the steel plate is in mass%, C: 0.03 to 0.25%, Si: 0.12 to 0.60 %, Mn: 2.0 to 4.0%, P: 0.05% or less, S: 0.01% or less, sol. Al: 0.8% or less, N: 0.0020 to 0.015%, Ti: 0.500% or less and Nb: 0.500% or less in a 0.050% or more on the free, and has a chemical composition the balance being Fe and impurities, the average crystal grain size of the ferrite is not more than 5.0μm average particle diameter of the hard second phase is less than or equal to 5.0μm It has a certain metal structure, the steel sheet and high tensile galvannealed tensile strength, wherein at least 780MPa that interfacial adhesion strength of the alloy allowance galvanized layers is not less than 20MPa is measured by the following tensile test Plated steel sheet.
Tensile test: An alloyed hot-dip galvanized steel sheet as a sample is cut into 20 mm × 100 mm so that the longitudinal direction is the rolling direction, and is a one-pack type epoxy structural adhesive made by Sunstar (trade name: E-6773) is used as an adhesive, and the stacking margin is 12.5 mm, the adhesive film thickness is 200 μm, the baking condition is 180 ° C. × 20 minutes, the tensile speed is 5 mm / minute, and the longitudinal direction is performed at room temperature. .

(3) The chemical composition is mass% instead of part of Fe, Cr: 1.0% or less, Mo: 1.0% or less, V: 1.0% or less, and B: 0.01% The high-tensile galvannealed steel sheet having a tensile strength of 780 MPa or more as described in (1) or (2) above, comprising one or more selected from the following group.

(4) The chemical composition may contain one or two selected from the group consisting of Ca: 0.050% or less and REM: 0.050% or less in mass% instead of part of Fe. A high-tensile galvannealed steel sheet having a tensile strength of 780 MPa or more according to any one of the above (1) to (3).

(5) The alloyed hot-dip galvanized layer contains, by mass%, Fe: 8 to 15% and Al: 0.15 to 0.50%, and the balance having a chemical composition composed of Zn and impurities. A high-tensile galvannealed steel sheet having a tensile strength of 780 MPa or more according to any one of the above (1) to (4).

(6) Hot rolling is performed on the slab having the chemical composition according to any one of (1) to (4) above, cooling is started within 4 seconds after completion of hot rolling, and within 10 seconds after hot rolling. The steel sheet is cooled to a temperature range of 700 ° C. or lower, wound up in a temperature range of 400 ° C. to 700 ° C. to form a hot rolled steel sheet, and the obtained hot rolled steel sheet is pickled and cooled at a reduction rate of 35 to 80%. Cold-rolled steel sheet is obtained by performing cold rolling, and the obtained cold-rolled steel sheet is allowed to stay in a temperature range of Ac _{3 to} 950 ° C for 5 to 200 seconds, and then cooled to a temperature range of 400 to 600 ° C. Any one of the above (1) to (5) , wherein the substrate is allowed to stay in a temperature range of 400 to 600 ° C. for 5 to 200 seconds and then subjected to a hot dip galvanizing treatment and an alloying treatment in a temperature range of 540 ° C. or less . High tensile alloying hot dip galvanizing with a tensile strength of 780 MPa or more Manufacturing method of steel sheet.

  ADVANTAGE OF THE INVENTION According to this invention, the high-tensile-alloyed hot-dip galvanized steel plate excellent in the bendability which has the outstanding powdering-proof property and the outstanding plating adhesiveness, and its manufacturing method are provided. According to the present invention, a high-tensile hot-dip galvanized steel sheet having a strength of 780 MPa or more and a yield ratio of 60 to 80% can be obtained. This high-tensile alloyed hot-dip galvanized steel sheet has excellent pressability and is suitable as a structural member in fields such as home appliances, building materials and automobiles.

The reason why the chemical composition of the steel sheet according to the present invention is defined as described above will be described. In the present specification, “%” defining the chemical composition is “mass%”.
(1) Chemical composition of steel plate used as base material C: 0.03 to 0.25%
C is an element effective for improving strength at low cost. If the C content is less than 0.03%, the effect of improving the strength is not sufficient, and it becomes difficult to ensure the target strength, so the C content is set to 0.03% or more. Preferably it is 0.06% or more. On the other hand, if the C content exceeds 0.25%, the weldability deteriorates. For this reason, C content is made into 0.25% or less. Preferably it is 0.16% or less.

Si: 0.02 to 0.60% (0.02 to 1.00%)
Si promotes the diffusion of Fe from the steel plate grain boundary into the coating layer of the coating during the alloying process, but suppresses the diffusion of Fe from within the grain into the plating layer. It is an important element that increases the interfacial adhesion strength between the base steel sheet and the plating layer by increasing the unevenness of the interface.

  If the Si content is less than 0.02%, the effect of improving the interfacial adhesion strength is not sufficient. Preferably it is 0.04% or more. On the other hand, when the Si content exceeds 0.60%, the alloying speed is remarkably reduced, so that it is necessary to lengthen the alloying treatment time, resulting in a decrease in productivity and an increase in equipment length. Increasing the alloying treatment temperature in order to shorten the alloying treatment time results in a decrease in operability or a decrease in the interfacial adhesion strength. For this reason, Si content shall be 0.60% or less. Preferably it is 0.30% or less.

  However, if one or two of Cu and Ni are added in a total of 0.050% or more, preferably 0.10% or more, the plating processability is improved, so the upper limit of the Si content is 1.00%. Can be relaxed.

Mn: 2.0-4.0%
Mn is an element effective for increasing the strength of steel. It also has the effect of lowering the Ac ₃ point of steel and expanding the preferred annealing temperature range. Therefore, the Mn content is set to 2.0% or more. On the other hand, excessive content degrades the balance between strength and ductility, so the Mn content is 4.0% or less. Desirably, it is 3.0% or less.

P: 0.05% or less P is an element contained as an impurity, but is also an element effective for increasing the strength, and can be contained as necessary. However, if it is contained excessively, the alloying speed is lowered, and it is necessary to lengthen the alloying treatment time, resulting in a reduction in productivity and an increase in equipment length. When the alloying treatment temperature is increased in order to shorten the alloying treatment time, the operability is lowered or the interfacial adhesion strength is lowered. For this reason, the content of P is set to 0.05% or less. The preferred content is 0.025% or less. In order to more reliably obtain the effect of increasing the strength due to P, the P content is preferably set to 0.02% or more.

In addition, even if it does not contain P, it is not necessary to contain P positively, when sufficiently high intensity | strength is achieved by another alloy component. In this case, the lower limit of P is not limited.
S: 0.01% or less S is an element contained as an impurity, and the content thereof is preferably low. If the S content exceeds 0.01%, the precipitation of MnS becomes prominent and the stretch flangeability and toughness of the steel sheet deteriorate. For this reason, S content shall be 0.01% or less. In particular, for applications that require low-temperature toughness, the S content is preferably 0.002% or less. However, if one or two of Ca and REM are added in a total of 0.0005% or more, preferably 0.0010% or more, the precipitation form of sulfide is changed, and stretch flangeability and toughness are improved. Even for such applications, the upper limit of the S content can be relaxed to 0.005%.

sol.Al: 0.8% or less Al can be contained for deoxidation of steel. However, even if it is contained excessively, the effect is saturated and the cost is increased, so the sol.Al content is set to 0.8% or less. In order to obtain the deoxidation effect more reliably, the sol.Al content is preferably set to 0.01% or more.

N: 0.0019 to 0.015%
N is an impurity element that is inevitably contained in general, but in the present invention, a Ti-based, Nb-based, or Ti-Nb composite-based nitride or carbonitride is formed in the steel sheet, Since it is effective for increasing the strength of the steel sheet, the N content is set to 0.0010% or more. Preferably it is 0.0033% or more. On the other hand, excessive content forms coarse TiN and causes toughness deterioration, so the N content is set to 0.015% or less. Preferably it is 0.0006% or less.

Ti: 0.50% or less, Nb: 0.50% or less, Ti + Nb ≧ 0.050%
Ti and Nb are contained in one or two kinds, and are elements effective in increasing the strength of the steel sheet by forming carbides, nitrides, or carbonitrides. Further, it has an effect of suppressing recrystallization of ferrite during annealing, promotes transformation to austenite, and significantly promotes ferrite transformation during cooling after annealing. In addition, the crystal grain size is extremely refined. In order to exhibit such an effect, the total content of at least one of Ti and Nb is 0.050% or more. Moreover, even if it contains excessively, since an effect will be saturated and a cost increase will be caused, each content shall be 0.500% or less. Preferably each is 0.300% or less.

Cu, Ni: 1.5% or less The steel sheet of the present invention is provided with corrosion resistance by hot dip galvanizing. Cu and / or Ni to be included as necessary in the steel composition is concentrated on the surface to suppress the surface concentration of Si, so the upper limit of Si content is limited to 0.60% from the viewpoint of plating properties. It has the effect of extending to 1.00%. For this purpose, the total content of Cu and / or Ni is preferably 0.05% or more. Desirably, the total content is 0.10% or more. If each content exceeds 1.5%, the effect is saturated and the cost is increased. Therefore, each content is 1.5% or less.

  Moreover, Cu has the effect | action which improves the corrosion resistance in the edge part which is not hot-plated. For this purpose, the Cu content is preferably set to 0.03% or more, and if it exceeds 0.5%, the effect is saturated.

Cr: 1.0% or less, Mo: 1.0% or less, V: 1.0% or less, B: 0.01% or less In the steel sheet of the present invention, precipitation strengthening by Ti and Nb and transformation strengthening by Mn are performed. An increase in strength of 780 MPa or more can be achieved. However, since Mn makes the structure of a steel plate into a band shape, if bendability is required, one or more of Cr, Mo, V, B may be added to replace part of Mn. preferable. Further, even when the strength is further increased to 980 MPa or more, it is effective to add one or more of Cr, Mo, V, and B.

  However, if Cr, Mo and V are excessively contained, the wettability of hot dipping is deteriorated, so the respective contents are set to 1.0% or less. Further, if B is added excessively, the toughness deteriorates, so the B content is set to 0.01% or less. In order to obtain the effect of increasing the strength more surely, Cr, Mo and V are each preferably 0.03% or more, and B is preferably 0.0003% or more.

  In addition, B has the effect | action which suppresses a ferrite transformation and promotes the production | generation of a hard 2nd phase and strengthens a steel plate, but when it is made to contain especially with Mo, the effect which achieves high strengthening, improving bendability remarkably. Therefore, it is preferable to contain Mo and B in combination.

Ca: 0.050% or less, REM: 0.050% or less These elements have an action of changing the precipitation form of sulfides and improving stretch flangeability and toughness. Species can be included. Even if contained excessively, the effect is saturated and the cost is increased, so the respective contents are set to 0.050% or less. In order to obtain the effect of the above action more reliably, the total content is preferably 0.0005% or more, and more preferably 0.0010% or more.

(2) The structure of the steel sheet used as a base material The structure of the steel sheet according to the present invention is defined as follows.
In order to achieve good bendability in a region where the tensile strength is 780 MPa or more, the average crystal grain size of ferrite and the average grain size of the hard second phase are each set to 5.0 μm or less. Further, it is desirable that the thickness is 3.0 μm or less.

Here, the hard second phase is 1 to 5 μm of martensite, bainite, retained austenite, or a mixture thereof observed at the SEM level.
In the case of the present invention, austenite grains are refined during annealing (particle size 1 to 5 μm), and a part of the fine austenite grains are transformed into fine ferrite during subsequent cooling. Of the remaining fine austenite grains, some are transformed into bainite, some are transformed into martensite, and some are transformed into a martensite / austenite mixture. As shown in the SEM observation structure photograph of FIG. 1 described later, they are collectively referred to as “hard second phase”. Their particle size can be determined from the SEM observation photograph by a cutting method.

(3) Chemical composition of plating layer to be coated Fe: 8-15%
When the Fe content in the galvanized layer to be the coating is less than 8%, a soft part is likely to be formed on the surface layer portion of the plated layer after the alloying treatment, and the slidability is lowered, and the coating layer of the coating is reduced. Flaking-like peeling due to peeling from the interface with the base steel sheet increases. Therefore, the Fe content is 8% or more. Preferably it is 9.5% or more. On the other hand, if the Fe content exceeds 15%, when the steel sheet is subjected to bending, the amount of powdering peeling increases due to compression deformation of the galvannealed layer inside the bent portion. For this reason, Fe content shall be 15% or less. Preferably it is 14% or less.

Al: 0.15 to 0.50%
When the Al content in the galvanized layer to be a coating is less than 0.20%, the effect of suppressing the development of the alloy layer in the plating bath becomes insufficient, and it becomes difficult to control the amount of coating. Therefore, the Al content is 0.15% or more. Preferably it is 0.20% or more, more preferably 0.25% or more. On the other hand, when the Al content exceeds 0.50%, the alloying speed decreases, and therefore, the normal alloying temperature must be higher than 530 ° C. in order to realize the Fe content at a normal line speed. As will be described later, it becomes difficult to set the interfacial adhesion strength between the steel sheet and the galvannealed layer to 20 MPa or more. Therefore, the Al content is 0.50% or less. Preferably it is 0.45% or less, more preferably 0.40% or less.

Other:
In the alloying process, Si, Mn, P, S, Ti, Nb, Cr, Mo, V, B, Ca, REM, etc. are usually taken into the galvanized layer as the coating, If it is within the range that is incorporated in the plating layer when the hot dipping and alloying treatment is performed under the above conditions, there is no problem because the plating quality is not adversely affected. The normal plating conditions here are, as will be described later, a plating bath temperature of 400 ° C. to 500 ° C., a steel plate penetration temperature of 400 ° C. to 500 ° C., and an alloying temperature of 460 to 600 ° C.

Thus, the balance of the chemical composition of the plating film is substantially Zn.
(4) Manufacturing conditions of steel plate used as base material The steel plate according to the present invention is manufactured through hot rolling, cold rolling, and hot dip galvanizing. Suitable manufacturing conditions are shown below.

For example, the molten steel adjusted to the above chemical composition is made into a slab by continuous casting or casting and ingot rolling, and then hot rolled. Hot rolling usually consists of a rough hot rolling process in which a slab is a rough bar and a finishing hot rolling process in which the rough bar is a hot rolled steel sheet. It is preferable to equalize the temperature of the entire length of the rough bar by induction heating or the like with respect to the rough bar before rolling, because fluctuations in characteristics in the coil can be suppressed. Moreover, it is desirable to perform finish hot rolling at ₃ or more Ar points.

  Cooling is started within 4 seconds after completion of hot rolling, cooling is performed to a temperature range of 700 ° C. or lower within 10 seconds after completion of hot rolling, and winding is performed at 400 to 700 ° C. When the cooling start time exceeds 4 seconds after completion of hot rolling, or when cooling is not performed to a temperature range of 700 ° C. or lower within 10 seconds after completion of hot rolling, the structure becomes coarse and bendability deteriorates.

  As for the coiling temperature, if it is less than 400 ° C., it is extremely hardened and cold rolling becomes difficult. Preferably it is 500 degreeC or more. On the other hand, when it exceeds 700 ° C., the yield deteriorates due to scale loss. Therefore, the winding temperature is set to 700 ° C. or less.

  Conventional methods may be used for pickling and cold rolling performed after hot rolling. Before or after pickling, it is advantageous in terms of ensuring flatness if mild rolling of about 0 to 5% is performed and the shape is corrected. In addition, the mild rolling improves pickling properties, promotes removal of surface-enriched elements, and has the effect of expanding the preferred range of Si and P that are restricted from the viewpoint of hot-plated adhesion.

There is no particular problem with the rolling reduction of cold rolling in the range of 35 to 80%. If the rolling reduction is increased, the transformation to austenite during annealing is promoted, so that the effect of expanding the preferred range of annealing is obtained.
The cold rolled steel sheet thus obtained is allowed to stay in a temperature range of Ac ₃ points to 950 ° C. for 5 to 200 seconds, and then hot dip galvanized. It is preferable from the viewpoint of productivity and manufacturing cost that the soaking process and the hot dip galvanizing treatment for staying in a temperature range of Ac _{3 to} 950 ° C. for 5 to 200 seconds are performed in a continuous hot dip galvanizing line.

If the soaking temperature is less than ₃ Ac, the austenite transformation is insufficient and it is difficult to secure the desired bendability, and if it exceeds 950 ° C., the austenite grain growth is excessively promoted and the structure becomes coarse. It becomes difficult to ensure the intended strength and bendability.

  In the present invention, since a large amount of Ti + Nb is contained, recrystallization of ferrite processed by cold rolling is remarkably suppressed. Therefore, processing strain remains up to the austenite region during heating, and the phase transformation to austenite is significantly promoted. As a result, the transformation from the processed ferrite to austenite is completed by soaking for only 5 seconds or more, and the processing strain is removed. If the soaking time is less than 5 seconds, the transformation to austenite is not sufficient, so that processing strain remains and the ductility of the final product deteriorates.

  On the other hand, the soaking time is 200 seconds or less. In the present invention, since a large amount of Ti + Nb is contained, austenite grain growth during soaking can be effectively suppressed. Therefore, there is no problem if the upper limit is up to 200 seconds. However, from the viewpoint of productivity, it is desirable to be within 120 seconds. When the soaking time exceeds 200 seconds, austenite during soaking grows excessively and fine grains, and thus good bendability may not be obtained, so the soaking time is set to 200 seconds or less.

  There is no particular restriction on the cooling after soaking, but it is desirable that the temperature is 40 ° C./second or less up to 700 ° C. Due to the combined effect of a cooling rate of 40 ° C./second or less and a large amount of Ti + Nb, the ferrite transformation is remarkably promoted, and the ferrite particle size can be reduced to 5.0 μm or less. There is no particular limitation on the cooling from 700 ° C. to the cooling stop temperature in the temperature range of 400 to 600 ° C. However, for example, there is no problem if it is 70 ° C./s or less. Further, by setting the cooling stop temperature in the temperature range of 500 to 600 ° C., it is possible to effectively increase the strength. On the other hand, by setting the temperature range to 400 to 500 ° C., the bendability can be further improved instead of suppressing the increase in strength.

(5) Plating conditions Al concentration in the plating bath: 0.08 to 0.20%
When the Al concentration in the plating bath is less than 0.08%, an excessive Fe—Zn interface alloy layer is already formed in the plating bath before the alloying treatment, making it difficult to control the amount of adhesion. Therefore, the Al concentration in the plating bath is preferably 0.08% or more. More preferably, it is 0.09% or more.

  On the other hand, if the Al concentration in the plating bath exceeds 0.20%, the concentration of Al in the plating film proceeds excessively, resulting in a decrease in the alloying speed, and the above-mentioned Fe content is realized at a normal line speed. Therefore, the alloying treatment temperature may be forced to exceed 540 ° C., and it becomes difficult to make the interfacial adhesion strength between the steel sheet and the galvannealed layer 20 MPa or more as will be described later. Therefore, the Al concentration in the plating bath is preferably 0.20% or less. More preferably, it is 0.15% or less.

About immersion time, if it is less than 5 second, a performance and operativity will not be inhibited especially. About other plating conditions, the range generally employ | adopted may be sufficient, and if a plating bath temperature is 400-500 degreeC and an intrusion board temperature is the range of 400-500 degreeC, there will be no problem in particular. Even if 0.1% or less of Fe and Pb, Cd, Cr, Ni, W, Ti, Mg, and Si, which are inevitable elements, are contained as components other than Al in the plating bath, this performance is affected. Absent. The adhesion amount may be in the range of 25 to 70 g / m ² per side generally used as a product.

(6) Alloying treatment Alloying treatment temperature: 460-540 ° C
When the alloying treatment temperature is less than 460 ° C., coarse crystals of ζ phase are easily formed on the surface layer portion of the alloyed hot-dip galvanized layer, and the Fe content in the galvanized layer is less than 8%. There is. Therefore, the alloying treatment temperature is preferably set to 460 ° C. or higher. More preferably, it is 470 degreeC or more, Most preferably, it is 480 degreeC or more.

  On the other hand, when the alloying treatment temperature exceeds 540 ° C., the effect of promoting the diffusion of Zn in the plating film due to the inclusion of Si into the steel plate described above to the grain boundary of the steel plate that is the plating base material is weakened. Since the diffusion into the grains becomes dominant, the interfacial adhesion strength between the steel sheet and the galvannealed layer is lowered. Therefore, the alloying temperature is set to 540 ° C. or lower. Preferably it is 520 degrees C or less. As a heating means in the alloying treatment, any means such as radiant heating, high frequency induction heating, energization heating and the like may be used.

(7) Temper rolling There are no particular restrictions on temper rolling. From the viewpoint of rolling load, the elongation is preferably 0.5% or less.

(8) Post-treatment The product surface after plating may be untreated, but it may be subjected to post-treatment such as known chromic acid treatment, phosphate treatment, and resin coating. In addition, rust preventive oil may be applied, and the rust preventive oil used for the application may be a commercially available general one, but it is highly lubricious and contains an extreme pressure additive such as S or Ca. Rust oil may be applied.

  Steels having chemical components shown in Table 1 were melted in a converter and slabs having a thickness of 245 mm were obtained by continuous casting. The obtained slab was hot-rolled under the conditions shown in Table 2. The obtained hot-rolled steel sheet was pickled and cold-rolled at the cold pressure rate shown in Table 2. The obtained cold-rolled steel sheet was annealed and hot-dip galvanized under the conditions shown in Table 2, and the obtained hot-dip galvanized steel sheet was examined for tensile tests, limit bending, structure, and plating characteristics. The results are shown in Table 3.

For mechanical properties, YS, TS, and El were measured by the method specified in JIS Z 2241 using No. 5 test piece specified in JIS Z 2201 collected in the direction perpendicular to the rolling.
In the bending test, No. 3 test piece defined in JIS Z 2204 was used, and 180 ° bending was performed by the push bending method defined in JIS Z2248, and evaluation was performed with a limit bend r that does not cause cracking.

  1 is a structure photograph of the specimen obtained in Experiment No. 1 by SEM observation. It can be seen that a mixture of martensite and austenite as the hard second phase is precipitated at the grain boundaries of the ferrite phase.

The plating characteristics were investigated as follows.
1) Collection of sample piece A sample piece of 25 mmφ was collected from the alloyed sample, and the plating layer was dissolved with a 10% HCl aqueous solution containing 0.5 vol% inhibitor (trade name: “Ibit 710N” manufactured by Asahi Chemical). This was subjected to composition analysis of the plating layer by the ICP method.

2) Measurement of interfacial adhesion strength between steel plate and alloyed hot-dip galvanized layer The sample subjected to the alloying treatment was cut into 20 mm × 100 mm so that the longitudinal direction was the rolling direction, and one liquid manufactured by Sunstar Co., Ltd. Type epoxy-based structural adhesive (trade name: E-6773) as an adhesive, stacking margin: 12.5 mm, adhesive film thickness: 200 μm, baking conditions: 180 × 20 minutes, tensile speed: 5 mm / minute, A tensile test was carried out in the longitudinal direction under conditions at room temperature. The interfacial adhesion strength in this test is affected by the substrate strength due to the deformation of the base material, but is almost negligible for the base material having a YP of 350 MPa or more as in this case. As a result of the test, those having a strength of 20 MPa or more were considered to have good adhesion strength, and those having a strength of less than 20 MPa were judged to be defective.

  As shown in Table 3, when satisfying the scope of the present invention, it has a good balance between strength and ductility, good bendability and plating characteristics. On the other hand, the experiment Nos. 20 and 23 in which the components depart from the present invention have lower strength, Nos. 21 and 22 have poor plating adhesion strength, No. 24 has poor strength ductility balance and bendability, Nos. 25 to 27 had poor bendability due to their poor structure.

  For No. 1, 3, 14, and 16 in Table 1, hot rolling, pickling, cold rolling, and hot dip galvanizing in a continuous hot dip galvanizing line under the conditions shown in Table 4, followed by a tensile test, The limit bending, structure and plating characteristics were investigated. The results are shown in Table 5.

  Nos. 1, 32, 3, 38, 14, 44, 16, and 50 whose production conditions are within the scope of the present invention have a good balance between strength and ductility and good bendability and plating characteristics. On the other hand, No. 29, 30, 35, 36, 41, 42, 47, and 48, whose manufacturing conditions deviate from the present invention, have a defective structure and have poor bendability. Nos. 31, 37, 43, and 49 include unrecrystallized, and the strength ductility balance and bendability are poor. No. 33, 34, 39, 40, 45, 46, 51 and 52 have poor plating adhesion strength.

It is a SEM observation organization photograph of a sample by experiment No. 1 of an example.

Claims (6)

In the steel plate provided with the alloyed hot-dip galvanized layer on the surface of the steel plate, the steel plate is mass%, C: 0.03 to 0.25%, Si: 0.12 to 0.60%, Mn: 2.0. -4.0%, P: 0.05% or less, S: 0.01% or less, sol. Al: 0.8% or less, N: 0.0020 to 0.015%, Ti: 0.500% or less and Nb: 0.500% or less A metal having a chemical composition consisting of Fe and impurities with the balance being Fe and impurities, and having an average crystal grain size of ferrite of 5.0 μm or less and an average grain size of hard second phase of 5.0 μm or less have a tissue, 540 ° C. or less of the high tensile galvannealed steel sheet tensile strength, characterized by comprising subjected the alloying treatment is not less than 780MPa in a temperature range. In the steel plate provided with the alloyed hot-dip galvanized layer on the surface of the steel plate, the steel plate is mass%, C: 0.03 to 0.25%, Si: 0.12 to 0.60 %, Mn: 2.0. -4.0%, P: 0.05% or less, S: 0.01% or less, sol. Al: 0.8% or less, N: 0.0020 to 0.015%, Ti: 0.500% or less and Nb: 0.500% or less in a 0.050% or more on the free, and has a chemical composition the balance being Fe and impurities, the average crystal grain size of the ferrite is not more than 5.0μm average particle diameter of the hard second phase is less than or equal to 5.0μm have a certain metal structure, the steel sheet and high tensile galvannealed tensile strength of more than 780MPa, wherein the interfacial adhesion strength of the galvannealed layers is not less than 20MPa is measured by the following tensile test Plated steel sheet.
Tensile test: An alloyed hot-dip galvanized steel sheet as a sample is cut into 20 mm × 100 mm so that the longitudinal direction is the rolling direction, and is a one-component epoxy structural adhesive (trade name: E, manufactured by Sunstar Co., Ltd.). -6973) is used as an adhesive, and the stacking margin is 12.5 mm, the adhesive film thickness is 200 μm, the baking condition is 180 ° C. × 20 minutes, the tensile speed is 5 mm / minute, and the longitudinal direction is performed at room temperature. The chemical composition is a group in which, instead of a part of Fe, mass%, Cr: 1.0% or less, Mo: 1.0% or less, V: 1.0% or less, and B: 0.01% or less The high-tensile galvannealed steel sheet having a tensile strength of 780 MPa or more according to claim 1 or 2, which contains one or more selected from The said chemical composition contains 1 type or 2 types chosen from the group of Ca: 0.050% or less and REM: 0.050% or less by mass% instead of a part of Fe. A high-strength galvannealed steel sheet having a tensile strength of 780 MPa or more . The alloyed hot-dip galvanized layer contains, by mass%, Fe: 8 to 15% and Al: 0.15 to 0.50%, and the balance having a chemical composition composed of Zn and impurities. A high-strength galvannealed steel sheet having a tensile strength of 780 MPa or more . The slab having the chemical composition according to any one of claims 1 to 4 is hot-rolled, starts cooling within 4 seconds after completion of the hot rolling, and has a temperature of 700 ° C or less within 10 seconds after the hot rolling. The steel sheet is cooled to an area of 400 ° C. to 700 ° C. and rolled into a hot-rolled steel sheet, and the obtained hot-rolled steel sheet is pickled and cold-rolled at a reduction rate of 35 to 80%. It was made into a cold rolled steel plate, and the obtained cold rolled steel plate was allowed to stay in a temperature range of Ac _{3 to} 950 ° C. for 5 to 200 seconds, then cooled to a temperature range of 400 to 600 ° C., and a temperature of 400 to 600 ° C. The tensile strength according to any one of claims 1 to 5, wherein the tensile strength is 780 MPa or more, wherein the region is allowed to stay for 5 to 200 seconds, and then subjected to hot dip galvanizing treatment and alloying treatment in a temperature range of 540 ° C or lower . A method for producing a high-strength galvannealed steel sheet.

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