JP2017048412A - Hot-dip galvanized steel sheet, alloyed hot-dip galvanized steel sheet and production methods therefor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a hot-dip galvanized steel sheet and an alloyed hot-dip galvanized steel sheet each of which has good elongation property and bendability, and production methods therefor.SOLUTION: The hot-dip galvanized steel sheet is provided which comprises a steel sheet and a hot-dip galvanized layer, and in which the steel sheet includes a base material and a decarbonized ferrite layer, structure at a position of 1/4 depth of sheet thickness of the steel sheet contains 5.0 vol.% or more of tempered martensite and 0.5 vol.% or more and less than 7.0 vol.% of retained austenite and the balance mainly composed of 4 to 70 vol.% of ferrite and bainite, a part or all of the tempered martensite and retained austenite form M-A (Martensite-Austenite constituent), the decarbonized ferrite layer contains 120% or more of ferrite based on content of ferrite at depth of 1/4 of the sheet thickness, has ferrite average crystal particle diameter of 20 μm or less and a thickness of 5 μm to 200 μm, the structure of the decarbonized ferrite layer contains 1.0 vol.% or more of the tempered martensite and number density of the tempered martensite in the decarbonized ferrite layer is 0.01/μmor more.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a hot dip galvanized steel sheet, an alloyed hot dip galvanized steel sheet, and methods for producing them. The present invention is particularly suitable for applications such as automobile bodies, which are suitable for use in press molding, a high-strength hot-dip galvanized steel sheet and a high-strength alloyed hot-dip galvanized steel sheet excellent in both elongation characteristics and bendability. It relates to their manufacturing method. The hot-dip galvanized steel sheet and alloyed hot-dip galvanized steel sheet of the present invention are hot-dip galvanized steel sheet that has been hot-dip galvanized by hot-rolling steel sheet, and hot-dip steel sheet that has been hot-dip galvanized and alloyed. Galvannealed hot-rolled steel sheet, cold-rolled steel sheet obtained by cold rolling hot-rolled steel sheet, hot-rolled steel sheet And an alloyed hot-dip galvanized cold-rolled steel sheet that has been hot-dip galvanized and alloyed after annealing.

  In recent years, there has been a demand for improving the fuel efficiency of automobiles in order to protect the global environment, and there is an increasing need for high-strength steel sheets to reduce the weight of vehicle bodies and ensure the safety of passengers. A steel plate used for a member for automobiles is not sufficient if it has only high strength, and high corrosion resistance, good press formability, and good bendability are required.

  Steel strengthening methods include solid solution strengthening, precipitation strengthening and transformation strengthening. Of these, the use of transformation strengthening can effectively achieve high strength. For example, in Patent Document 1, by adding Mn, Cr, and Mo and further controlling the cooling rate, a mixed structure of ferrite, bainite, and martensite is obtained, and the tensile strength of the steel sheet becomes 780 MPa or more. However, when hard martensite is contained in the steel sheet for increasing the strength, there is a problem that the formability of the steel sheet deteriorates.

  Patent Document 2, Patent Document 3, Patent Document 4, and Patent Document 5 disclose a method of utilizing tempered martensite as a technique for improving the mechanical properties of a steel sheet.

  In the method described in Patent Document 2, martensite is generated on a steel sheet by rapidly cooling the steel sheet before hot dipping of the steel sheet, and then hot dipping is performed. Therefore, the steel sheet according to the method described in Patent Document 2 contains a lot of hard martensite which is newly generated by cooling after plating and which has an adverse effect on hole expansibility and is not tempered. Therefore, it has been difficult to obtain a steel sheet having high strength and excellent formability using the method described in Patent Document 2.

  Patent Document 3, Patent Document 4, and Patent Document 5 disclose methods of tempering a steel plate after plating. However, in the methods described in Patent Document 3, Patent Document 4, and Patent Document 5, since the end point temperature of cooling before tempering is not sufficiently low, hard martensite is newly generated in the steel sheet in the cooling process after tempering. This deteriorates the formability of the steel sheet.

  Moreover, as a technique for improving the bendability of the steel sheet, Patent Documents 6 to 10 disclose techniques for decarburizing the surface of the steel sheet by decarburization annealing.

Patent Document 6 discloses a manufacturing method in which decarburization annealing is performed on a steel plate, and then the steel plate is heated to _one or more points of Ac. The surface of the steel sheet obtained by this production method is a soft layer having a C content of 0.1% or less. However, in the decarburized layer obtained by the method of Patent Document 6, it is considered that the particle size of ferrite is coarsened by heating the steel plate to Ac ₁ point or more. The present inventors have found that when the ferrite in the decarburized layer is coarsened, the bendability of the steel sheet is not sufficiently improved.

  Patent Document 7 discloses a manufacturing method in which decarburization is performed in a high dew point atmosphere during continuous annealing. However, in patent document 7, it is not examined regarding the heating rate of the steel plate before starting a decarburization process. The present inventors have found that the heating rate before the decarburization treatment has a great influence on the structure of the decarburization layer obtained by the decarburization treatment, but Patent Document 7 discloses such matters at all. It has not been.

In Patent Document 8, a steel plate is heated under heating conditions in which a temperature range of 700 ° C. to (Ac ₃ -20 ° C.) is increased over 20 seconds before decarburization, and then (Ac ₃ -20 ° C.). ) To (Ac ₃ + 20 ° C.) in a high dew point atmosphere for 10 seconds or more is disclosed. However, in patent document 8, the heating rate before starting a decarburization process is not optimized. The present inventors have found that the heating rate before the decarburization treatment has a great influence on the structure of the decarburization layer obtained by the decarburization treatment, and Patent Document 8 discloses such matters at all. It has not been.

  Patent Document 9 discloses a manufacturing method in which the heating rate before the decarburization process is controlled to a value corresponding to the Si content of the steel sheet, and the decarburization process is performed in an atmosphere having a dew point of −30 ° C. However, it is considered that the martensite contained in the decarburized layer disclosed in Patent Document 9 is likely to be a starting point for the formation of microcracks. Therefore, it is considered that the method described in Patent Document 9 cannot sufficiently improve the bendability of the steel sheet.

In Patent Document 10, the steel sheet temperature is set within a temperature range of (Ac ₃ −20 ° C.) to (Ac ₃ + 100 ° C.) in a high dew point atmosphere in which the logarithm of water pressure / hydrogen partial pressure is −3 to 0. The manufacturing method which performs a decarburization process by hold | maintaining -600 second is disclosed. However, in patent document 10, it is not examined about the heating rate before starting a decarburization process. The present inventors have found that the heating rate before the decarburization treatment has a great influence on the structure of the decarburization layer obtained by the decarburization treatment, and Patent Document 10 discloses such matters at all. It has not been.

Welding Society Journal 50 (1981), No. 1, p37-46

JP-A-4-173946 Japanese Patent Laid-Open No. 6-93340 JP-A-6-108152 JP 2005-256089 A JP 2009-19258 A JP-A-5-195149 Japanese Patent Laid-Open No. 10-130782 JP 2006-70328 A JP 2011-231367 A JP 2013-163827 A

  It is an object of the present invention to provide a hot-dip galvanized steel sheet, an alloyed hot-dip galvanized steel sheet having good elongation characteristics and bendability, and a method for producing them.

  As a result of diligent experiments on hot-dip galvanized steel sheets and alloyed hot-dip galvanized steel sheets having sufficient elongation, the present inventors have determined that the form of martensite and retained austenite is MA (Martensite-Austenite constituent, also known as island martensite). It was found that a high elongation characteristic (hereinafter, the elongation characteristic is sometimes referred to as extensibility in this specification) can be obtained. Here, as described in Non-Patent Document 1, M-A is a concentration of C to untransformed austenite when steel is transformed into ferrite or bainite, and martensite is transformed during subsequent cooling. This is a region of a composite of martensite and retained austenite, and is scattered in islands in the matrix.

  However, excessively hard martensite deteriorates bendability. Therefore, the present inventors further advanced experiments for improving bendability, tempering MA at a relatively low temperature so that residual austenite remains, and additionally decarburizing the surface of the steel sheet with a properly formed structure. It has been found that by using a ferrite layer, improvement in bendability can be achieved while maintaining good elongation.

The present invention has been completed based on the above knowledge, and provides a hot-dip galvanized steel sheet and an alloyed hot-dip galvanized steel sheet that are excellent in workability, and a method for producing the same. In the present invention, “steel plate” means “steel strip”.
Here, the present invention is as follows.

(1) A hot-dip galvanized steel sheet according to an aspect of the present invention is a hot-dip galvanized steel sheet including a steel sheet and a hot-dip galvanized layer formed on the surface of the steel sheet, and the steel sheet is removed from the base material. The base metal has a chemical composition in unit mass%, C: 0.03 to 0.40%, Si: 0.001 to 1.80%, Mn: 1.0 to 5. 0%, P: 0.10% or less, S: 0.0100% or less, sol. Al: 0.001 to 1.500%, N: 0.02% or less, Ti: 0 to 0.300%, Nb: 0 to 0.300%, V: 0 to 0.300%, Cr: 0 to 0 2.000%, Mo: 0 to 2.000%, Cu: 0 to 2.000%, Ni: 0 to 2.000%, B: 0 to 0.0200%, Ca: 0 to 0.0100%, REM: 0 to 0.1000% and Bi: 0 to 0.0500%, the balance is made of iron and impurities, and the base material has a depth of 1/4 of the thickness of the steel plate from the surface of the steel plate. The structure at the position contains 5.0% by volume or more of tempered martensite and 0.5% by volume or more and less than 7.0% by volume of retained austenite, with the balance being mainly 4 to 70% by volume of ferrite, And bainite, in the base material, the tempered martensite and the residual austenite. The decarburized ferrite layer has a structure whose depth is ¼ of the thickness of the steel plate from the surface of the steel plate of the base metal. 120% or more of ferrite with respect to the content of the ferrite in the structure of the position of, the average grain size of the ferrite of the decarburized ferrite layer is 20 μm or less, the thickness of the decarburized ferrite layer 5 μm or more and 200 μm or less, the structure of the decarburized ferrite layer contains 1.0% by volume or more of the tempered martensite, and the number density of the tempered martensite of the decarburized ferrite layer is 0.01 / and the [mu] m ² or more, the galvannealed steel sheet, yield strength in tensile test perpendicular to the rolling direction is not less than 420 MPa, the mechanical properties tensile strength is not less than 780MPa Yes That.
(2) In the hot dip galvanized steel sheet according to the above (1), the steel sheet is further unit mass%, Ti: 0.001% or more and 0.300% or less, Nb: 0.001% or more and 0.300%. One or more selected from the group consisting of the following and V: 0.001% or more and 0.300% or less may be contained.
(3) In the hot dip galvanized steel sheet according to the above (1) or (2), the steel sheet is further in unit mass%, Cr: 0.001% to 2.000% and Mo: 0.001% You may contain 1 type or 2 types selected from the group which consists of more than 2.000%.
(4) In the hot-dip galvanized steel sheet according to any one of (1) to (3), the steel sheet is further unit mass%, Cu: 0.001% or more and 2.000% or less, and Ni: You may contain 1 type or 2 types selected from the group which consists of 0.001% or more and 2.000% or less.
(5) In the hot-dip galvanized steel sheet according to any one of (1) to (4), the steel sheet further contains B: 0.0001% to 0.0200% in unit mass%. May be.
(6) In the hot-dip galvanized steel sheet according to any one of (1) to (5), the steel sheet is further unit mass%, Ca: 0.0001% to 0.0100%, and REM. : You may contain 1 type or 2 types selected from the group which consists of 0.0001% or more and 0.100% or less.
(7) In the hot-dip galvanized steel sheet according to any one of (1) to (6), the steel sheet further contains Bi: 0.0001% to 0.0500% in unit mass%. May be.
(8) In the galvannealed steel sheet according to another aspect of the present invention, the galvanized layer of the galvanized steel sheet according to any one of (1) to (7) is alloyed. Yes.
(9) The manufacturing method of the hot dip galvanized steel sheet which concerns on another aspect of this invention is a unit mass%, C: 0.03-0.40%, Si: 0.001-1.80%, Mn: 1 0.0 to 5.0%, P: 0.10% or less, S: 0.0100% or less, sol. Al: 0.001 to 1.500%, N: 0.02% or less, Ti: 0 to 0.300%, Nb: 0 to 0.300%, V: 0 to 0.300%, Cr: 0 to 0 2.000%, Mo: 0 to 2.000%, Cu: 0 to 2.000%, Ni: 0 to 2.000%, B: 0 to 0.0200%, Ca: 0 to 0.0100%, REM: 0 to 0.1000%, and Bi: 0 to 0.0500%, with the balance being a steel plate made of iron and impurities at an average heating rate of 1 to 50 ° C./second in a temperature range of 100 to 720 ° C. In the atmosphere in which the component comprises 2 to 20% by volume of hydrogen, the balance containing nitrogen and impurities, and the dew point is higher than −30 ° C. and lower than or equal to 20 ° C. after the heating step and the heating step. A step of annealing the steel sheet in a temperature range of 720 to 1000 ° C. for 10 to 600 seconds; After the step of annealing, in the temperature range of 720 to 650 ° C., the step of performing the first cooling on the material steel plate at an average cooling rate of 0.5 to 10.0 ° C./second, and the first cooling After the step to be performed, in the temperature range of 650 to 500 ° C., the step of performing the second cooling on the material steel plate at the average cooling rate of 2.0 to 100.0 ° C./second, and the step of performing the second cooling After the step of applying hot dip galvanizing to the material steel plate and the step of applying hot dip galvanizing, the material steel plate is thirdly cooled at an average cooling rate of 2 ° C./second or higher from the hot dip galvanizing temperature to 200 ° C. or lower. And a step of performing a tempering treatment for 1 second to 48 hours in a temperature range of 100 to 600 ° C. after the third cooling step and the third cooling step.
(10) In the manufacturing method of the hot dip galvanized steel sheet according to (9), the material steel sheet is further in unit mass%, Ti: 0.001% or more and 0.300% or less, Nb: 0.001% or more. You may contain 1 type (s) or 2 or more types selected from the group which consists of 0.300% or less and V: 0.001% or more and 0.300% or less.
(11) In the method for producing a hot dip galvanized steel sheet according to (9) or (10), the material steel sheet is further in unit mass%, Cr: 0.001% to 2.000%, and Mo: You may contain 1 type or 2 types selected from the group which consists of 0.001% or more and 2.000% or less.
(12) In the manufacturing method of the hot-dip galvanized steel sheet according to any one of (9) to (11), the material steel sheet is further in unit mass%, Cu: 0.001% or more and 2.000%. 1 type or 2 types selected from the group which consists of below and Ni: 0.001% or more and 2.000% or less may be contained.
(13) In the manufacturing method of the hot dip galvanized steel sheet according to any one of (9) to (12), the material steel sheet is further in unit mass%, and B: 0.0001% or more and 0.0200%. You may contain the following.
(14) In the manufacturing method of the hot dip galvanized steel sheet according to any one of (9) to (13), the material steel sheet is further in unit mass%, and Ca: 0.0001% or more and 0.0100%. 1 type or 2 types selected from the group which consists of below and REM: 0.0001% or more and 0.100% or less may be contained.
(15) In the method for producing a hot dip galvanized steel sheet according to any one of (9) to (14), the material steel sheet is further in unit mass%, and Bi: 0.0001% or more and 0.0500%. You may contain the following.
(16) The manufacturing method of the galvannealed steel sheet according to another aspect of the present invention is unit mass%, C: 0.03 to 0.40%, Si: 0.001 to 1.80%, Mn : 1.0-5.0%, P: 0.10% or less, S: 0.0100% or less, sol. Al: 0.001 to 1.500%, N: 0.02% or less, Ti: 0 to 0.300%, Nb: 0 to 0.300%, V: 0 to 0.300%, Cr: 0 to 0 2.000%, Mo: 0 to 2.000%, Cu: 0 to 2.000%, Ni: 0 to 2.000%, B: 0 to 0.0200%, Ca: 0 to 0.0100%, REM: 0 to 0.1000%, and Bi: 0 to 0.0500%, with the balance being a steel plate made of iron and impurities at an average heating rate of 1 to 50 ° C./second in a temperature range of 100 to 720 ° C. In the atmosphere in which the component comprises 2 to 20% by volume of hydrogen, the balance containing nitrogen and impurities, and the dew point is higher than −30 ° C. and lower than or equal to 20 ° C. after the heating step and the heating step. A step of annealing the steel sheet in a temperature range of 720 to 1000 ° C. for 10 to 600 seconds; After the step of annealing, in the temperature range of 720 to 650 ° C., the step of performing the first cooling on the material steel plate at an average cooling rate of 0.5 to 10.0 ° C./second, and the first cooling After the step to be performed, in the temperature range of 650 to 500 ° C., the step of performing the second cooling on the material steel plate at the average cooling rate of 2.0 to 100.0 ° C./second, and the step of performing the second cooling , After the step of hot dip galvanizing the raw steel plate, the step of hot galvanizing, the step of alloying the raw steel plate, and the step of performing the alloying treatment, the alloying treatment From the temperature to 200 ° C. or less, at a mean cooling rate of 2 ° C./second or more, after the step of performing the third cooling on the material steel plate and the step of performing the third cooling, the material steel plate is subjected to 100 to 600 ° C. Tempering for 1 second to 48 hours in the temperature range And a step of performing.
(17) In the method for producing an alloyed hot-dip galvanized steel sheet according to (16) above, the material steel sheet is further in unit mass%, Ti: 0.001% to 0.300%, Nb: 0.001. % Or more and 0.300% or less, and V: One or more selected from the group consisting of 0.001% or more and 0.300% or less may be contained.
(18) In the method for producing an alloyed hot-dip galvanized steel sheet according to (16) or (17), the material steel sheet is further in unit mass%, Cr: 0.001% or more and 2.000% or less, and Mo: You may contain 1 type or 2 types selected from the group which consists of 0.001% or more and 2.000% or less.
(19) In the method for producing an alloyed hot-dip galvanized steel sheet according to any one of (16) to (18) above, the material steel sheet is further unit mass%, and Cu: 0.001% or more. One or two selected from the group consisting of 000% or less and Ni: 0.001% or more and 2.000% or less may be contained.
(20) In the method for producing an alloyed hot-dip galvanized steel sheet according to any one of (16) to (19) above, the material steel sheet is further in unit mass%, and B: 0.0001% or more and 0.00. You may contain 0200% or less.
(21) In the method for producing an alloyed hot-dip galvanized steel sheet according to any one of (16) to (20) above, the material steel sheet is further unit mass%, and Ca: 0.0001% or more and 0.00. You may contain 1 type or 2 types selected from the group which consists of 0100% or less and REM: 0.0001% or more and 0.100% or less.
(22) In the method for producing an alloyed hot-dip galvanized steel sheet according to any one of (16) to (21), the material steel sheet is further in unit mass%, Bi: 0.0001% or more and 0.00. You may contain 0500% or less.

  The hot dip galvanized steel sheet and the galvannealed steel sheet according to the present invention have both good extensibility and bendability and excellent formability, and are therefore optimal for structural parts of automobiles such as pillars. .

It is a schematic diagram of the hot dip galvanized steel sheet which concerns on 1 aspect of this invention. It is a schematic diagram of the galvannealed steel plate which concerns on 1 aspect of this invention. It is a figure which shows typically the relationship between the depth from a steel plate surface, and the amount of ferrite. It is a flowchart which shows the manufacturing method of the hot dip galvanized steel plate which concerns on 1 aspect of this invention. It is a flowchart which shows the manufacturing method of the galvannealed steel plate which concerns on 1 aspect of this invention.

  In the following, regarding the hot dip galvanized steel sheet and alloyed hot dip galvanized steel sheet according to the present embodiment and their production methods, (A) the chemical composition of the base material, (B) the structure of the decarburized ferrite layer, (C) Composition of base material, (D) Composition of hot-dip galvanized layer and alloyed hot-dip galvanized layer, (E) Mechanical properties of hot-dip galvanized steel sheet and alloyed hot-dip galvanized steel sheet, and (F) Hot-dip galvanized steel sheet and alloy The manufacturing method of the hot dip galvanized steel sheet will be specifically described in order. In the following description, all percentages relating to the chemical composition of the base material are mass%. As shown in FIG. 1, the hot dip galvanized steel sheet 1 according to the present embodiment includes a steel sheet 10 including a decarburized ferrite layer 12 and a base material 13, and a hot dip galvanized layer 11. As shown in FIG. 2, the galvannealed steel plate 2 according to the present embodiment includes a steel plate 10 including a decarburized ferrite layer 12 and a base material 13, and an galvannealed layer 21. The characteristic of a steel plate means the characteristic of the steel plate of the state provided with both a base material and a decarburized ferrite layer unless there is particular notice. The characteristics of the steel sheet and the characteristics of the hot dip galvanized steel sheet and the alloyed hot dip galvanized steel sheet are substantially the same except for some characteristics such as corrosion resistance. Various characteristics of the hot dip galvanized steel sheet may be simply referred to as various characteristics of the steel sheet.

(A) Chemical composition of base material [C: 0.03% to 0.40%]
C is an effective component for obtaining high tensile strength. If the C content is less than 0.03%, the required tensile strength cannot be obtained. On the other hand, when C is contained exceeding 0.40%, the weldability of a steel plate will fall. Therefore, the C content is 0.03 to 0.40%. A preferable lower limit of the C content is 0.05%, and a preferable upper limit of the C content is 0.30%.

[Si: 0.001% to 1.80%]
Si has a deoxidizing action, suppresses generation of surface flaws on the steel sheet, and improves the production yield of the steel sheet. In order to obtain a desired effect, it is necessary to contain 0.001% or more of Si. On the other hand, when Si is contained exceeding 1.80%, there is a possibility that cracking may occur in the edge portion of the steel sheet during cold rolling. Therefore, the Si content is determined to be 0.001 to 1.80%. Si is an effective element that suppresses the precipitation of cementite, promotes austenite retention, increases elongation, strengthens ferrite, and also has a function of homogenizing the structure. Therefore, the lower limit of the preferable Si content The value is 0.10%. Since the growth of the decarburized ferrite layer and the generation of MA are facilitated, the more preferable lower limit of the Si content is 0.40%. On the other hand, the preferable upper limit of the Si content is 0.95%.

[Mn: 1.0% to 5.0%]
Mn is an essential element for dispersing tempered martensite in the ferrite of the decarburized ferrite layer. Further, since Mn generates MA while suppressing precipitation of cementite, it also has an effect of obtaining both strength and elongation. In order to obtain a desired effect, it is necessary to contain 1.0% or more of Mn. On the other hand, when Mn is contained exceeding 5.0%, the weldability of the steel sheet is lowered. Therefore, the Mn content is determined to be 1.0 to 5.0%. The preferable lower limit of Mn content is 1.9%, the preferable upper limit of Mn content is 4.2%, and the more preferable upper limit is 3.5%.

[P: 0.10% or less]
P is an undesirable element that is contained as an impurity and deteriorates weldability. Therefore, the P content is set to 0.10% or less. A preferable P content is 0.02% or less.

[S: 0.0100% or less]
S is contained as an impurity, and deteriorates hole expansibility by forming MnS in the steel. Therefore, the S content is determined to be 0.0100% or less. The S content is preferably 0.0050% or less, and more preferably 0.0012% or less.

[Sol. Al: 0.001% to 1.500%]
Al has a deoxidizing action, suppresses generation of surface flaws on the steel sheet, and improves the production yield of the steel sheet. In order to obtain a desired effect, 0.001% or more of sol. It is necessary to contain Al. On the other hand, when Al is contained exceeding 1.500%, inclusions increase and the hole expandability deteriorates. Therefore, sol. Al content is defined as 0.001-1.500%. Further, Al is also effective in suppressing the precipitation of cementite and increasing the amount of retained austenite, as is the case with Si, so the preferable lower limit of the Si content is 0.020%. On the other hand, the preferable upper limit of the Al content is 1.000%.

[N: 0.02% or less]
N is contained as an impurity and forms a nitride during continuous casting, causing cracks in the slab. Therefore, it is preferable that the N content is low. Therefore, the N content is determined to be 0.02% or less. Preferably, the N content is 0.01% or less.
The following elements are optional elements that may optionally be included. However, since the following elements are not necessarily contained, the lower limit value of the following elements is 0%.

[Ti: 0 to 0.300%]
[Nb: 0 to 0.300%]
[V: 0 to 0.300%]
Since there is no need to contain Ti, Nb, and V, the lower limit value of each content of Ti, Nb, and V is 0%. On the other hand, since Ti, Nb, and V become precipitates that act as nuclei of crystal grains, they have the effect of refining the crystal grains. Therefore, Ti, Nb, and V may be contained in the steel sheet for the purpose of improving strength and toughness. However, when Ti, Nb, and V are each less than 0.001%, sufficient effects cannot be obtained. Further, when Ti, Nb, and V are contained in excess of 0.300%, the effect is saturated, resulting in excessive cost. Therefore, when Ti, Nb, and V are contained, any element of Ti, Nb, and V may have a content of 0.001% to 0.300%. Ti and Nb promote C concentration to austenite due to the formation of ferrite in steel in which the structure is partially or completely austenitized by heat treatment, and facilitate formation of MA. Therefore, it is preferable to contain one or both of Ti and Nb in a total of 0.010% or more, and it is more preferable to contain a total of 0.030% or more.

[Cr: 0 to 2.000%]
[Mo: 0 to 2.000%]
Since it is not necessary to contain Cr and Mo, the lower limit of the content of Cr and Mo is 0%. On the other hand, Cr and Mo, like Mn, stabilize the austenite, thereby promoting the transformation strengthening due to martensite generation. That is, since Cr and Mo are effective for increasing the strength of the steel sheet, they may be contained. However, when the contents of Cr and Mo are less than 0.001%, the effect cannot be sufficiently obtained. On the other hand, when Cr and Mo are contained in excess of 2.000%, the effect is saturated and excessive costs are generated. Therefore, when Cr and Mo are contained, both the Cr content and the Mo content may be 0.001% or more and 2.000% or less. A preferable Cr content is 0.100% or more and 1.000% or less, and a preferable Mo content is 0.050% or more and 0.500% or less.

[Cu: 0 to 2.000%]
[Ni: 0 to 2.000%]
Since there is no need to contain Cu and Ni, the lower limit of the contents of Cu and Ni is 0%. On the other hand, Cu and Ni have a corrosion inhibiting effect. Further, Cu and Ni have a function of suppressing the penetration of hydrogen into the steel sheet by concentrating on the surface of the steel sheet and suppressing delayed fracture of the steel sheet. Therefore, Cu and Ni may be contained. However, when the contents of Cu and Ni are less than 0.001%, the effect cannot be sufficiently obtained. On the other hand, when Cu and Ni are each contained exceeding 2.000%, the effect is saturated and excessive cost is generated. Therefore, when Cu and Ni are contained, both the Cu content and the Ni content may be 0.001% or more and 2.000% or less. Preferably, both Cu content and Ni content are 0.010% or more and 0.800% or less.

[B: 0 to 0.0200%]
Since it is not necessary to contain B, the lower limit of the B content is 0%. On the other hand, B is an element that contributes to increasing the strength of the steel sheet by suppressing ferrite nucleation from the grain boundaries and improving the hardenability of the steel sheet. Moreover, since B produces | generates MA effectively, it contributes to the improvement of the elongation of a steel plate. Therefore, B may be contained. However, when the B content is less than 0.0001%, the effect cannot be sufficiently obtained. Moreover, when B is contained exceeding 0.0200%, an effect will be saturated and excess cost will generate | occur | produce. Therefore, when B is contained, the B content may be set to 0.0001 to 0.0200%.

[Ca: 0 to 0.0100%]
[REM: 0 to 0.1000%]
Since there is no need to contain Ca and REM, the lower limit of the content of each of Ca and REM is 0%. On the other hand, Ca and REM may be contained because they have the effect of improving the hole expanding property of the steel sheet by making the sulfide spheroidized. However, when the Ca content is less than 0.0001%, the effect cannot be sufficiently obtained. On the other hand, when the Ca content exceeds 0.0100%, the effect is saturated and excessive costs are generated. Therefore, when Ca is contained, the Ca content may be 0 to 0.0100%. Further, when the REM content is less than 0.0001%, the effect cannot be sufficiently obtained. On the other hand, when the REM content exceeds 0.1000%, the effect is saturated and excessive costs are generated. Therefore, when REM is contained, the REM content may be 0 to 0.1000%.
Here, REM refers to a total of 17 elements of Sc, Y, and lanthanoid. In the case of lanthanoid, it is added industrially in the form of misch metal. In the present invention, the content of REM refers to the total content of these elements.

[Bi: 0 to 0.0500%]
Since there is no need to contain Bi, the lower limit of the Bi content is 0%. On the other hand, Bi has the function of concentrating on the solidification interface to narrow the dendrite interval and reduce the solidification segregation, so it may be contained. When Mn and the like are microsegregated, a band structure with non-uniform hardness develops and the workability is lowered, but Bi can alleviate defects due to this microsegregation. However, when the Bi content is less than 0.0001%, the effect is insufficient. Moreover, when Bi is contained exceeding 0.0500%, surface quality will be deteriorated. Therefore, when Bi is contained, the Bi content may be 0 to 0.0500%. A preferable range of the Bi content is 0.0003 to 0.0100%, and a more preferable range is 0.0003 to 0.0050%.

[Balance: Iron and impurities]
The balance of the chemical components of the base material of the hot-dip galvanized steel sheet and the galvannealed steel sheet according to this embodiment is made of iron and impurities. Impurities are components that are mixed due to various factors in the manufacturing process, such as ore or scrap, when industrially manufacturing steel materials, and have an adverse effect on the characteristics of the steel sheet according to this embodiment. It means what is allowed in the range.

(B) Configuration of decarburized ferrite layer [steel plate surface: decarburized ferrite layer]
[Definition of decarburized ferrite layer: Area containing 120% or more of ferrite relative to the ferrite content of the base metal structure at a depth of 1/4 of the thickness of the steel sheet from the surface of the steel sheet]
The surface of the steel sheet which the hot dip galvanized steel sheet and the alloyed hot dip galvanized steel sheet according to this embodiment have is a decarburized ferrite layer having a larger ferrite content than the inside of the steel sheet. Specifically, as shown in FIG. 3, the decarburized ferrite layer in the present embodiment corresponds to the amount of ferrite in the structure of the base material at a depth of ¼ of the thickness of the steel plate from the surface of the steel plate. This is a region containing 120% or more of ferrite. More specifically, the decarburized ferrite layer has a ferrite volume fraction of 120 relative to the ferrite volume fraction of the base metal structure at a depth of ¼ of the steel plate thickness from the steel sheet surface. It is a region where the volume fraction is at least%. Hereinafter, a region having a depth of about 1/4 of the plate thickness of the steel plate from the surface of the steel plate may be abbreviated as a plate thickness 1/4 position.

  The decarburized ferrite layer is formed by decarburizing the surface of the steel sheet. The decarburized ferrite layer exists between the base material of the steel plate and the hot dip galvanized layer or the alloyed hot dip galvanized layer. Since the decarburized ferrite layer is softer than the base material of the steel plate, it prevents the surface of the steel plate from cracking when the steel plate is bent. In addition, since the decarburized ferrite layer is uniformly deformed, the decarburized ferrite layer prevents constriction from occurring on the surface of the steel sheet. Therefore, the decarburized ferrite layer has a function of improving the bendability of the steel sheet. The inventors of the present invention have made extensive studies focusing on the fact that the decarburized ferrite layer according to the prior art cannot sufficiently improve the bendability. As a result, the average grain size of ferrite in the decarburized ferrite layer according to the prior art is 20 μm or more, and when the steel sheet is bent, deformation is concentrated at the ferrite grain boundaries, so that fine cracks are generated in the decarburized ferrite layer. The present inventors have found that this occurs. In order to solve this problem, the present inventors reduced the average particle diameter of the ferrite in the decarburized ferrite layer, dispersed martensite in the ferrite in the decarburized ferrite layer, and dispersed the ferrite. It was found that tempering martensite is effective. The configuration of the decarburized ferrite layer of the hot-dip galvanized steel sheet and the alloyed hot-dip galvanized steel sheet according to the present embodiment obtained based on this knowledge is as follows.

[Average grain size of ferrite in decarburized ferrite layer: 20 μm or less]
The decarburized ferrite layer of the hot-dip galvanized steel sheet and the alloyed hot-dip galvanized steel sheet according to this embodiment is mainly composed of ferrite, and the average crystal grain size of this ferrite is 20 μm or less. When the average grain size of the ferrite in the decarburized ferrite layer is more than 20 μm, the total area of the ferrite grain boundary is reduced, and deformation is concentrated in a narrow region, so that the bendability of the steel sheet is lowered. Although it is better that the average particle diameter of ferrite is small, in view of the current technical level, it is difficult to make the average particle diameter of ferrite 0.5 μm or less.

[Decarburized ferrite layer structure: 1.0% by volume or more of tempered martensite, with the balance mainly consisting of ferrite]
The decarburized ferrite layer of the hot-dip galvanized steel sheet and the alloyed hot-dip galvanized steel sheet according to this embodiment contains 1.0% by volume or more of tempered martensite. When the tempered martensite is less than 1.0% by volume, uneven deformation occurs in the steel sheet, so that the bendability of the steel sheet is lowered. The upper limit of the amount of tempered martensite in the decarburized ferrite layer of the steel sheet is equal to the amount of tempered martensite contained in the base material of the steel sheet (that is, the portion of the steel sheet excluding the decarburized ferrite layer). Since the decarburized ferrite layer is formed by decarburizing the steel sheet, the amount of tempered martensite in the decarburized ferrite layer does not exceed the amount of tempered martensite in the base material. If the amount of tempered martensite in the decarburized ferrite layer exceeds the amount of tempered martensite in the base material, decarburization has not occurred in the decarburized ferrite layer. In addition, by making the martensite contained in the decarburized ferrite layer tempered martensite instead of fresh martensite (ferrite that has not been tempered), the occurrence of cracks at the interface between ferrite and martensite can be suppressed. .

  The balance of the structure of the decarburized ferrite layer is ferrite. As described above, the amount of ferrite in the decarburized ferrite layer is 120% or more of the amount of ferrite at the ¼ position of the steel plate. The balance of the structure of the decarburized ferrite layer is within a range that does not affect the characteristics of the hot-dip galvanized steel sheet and the alloyed hot-dip galvanized steel sheet according to the present embodiment (for example, 5% by volume). The following may also be included.

[Thickness of decarburized ferrite layer: 5 μm or more and 200 μm or less]
In the hot-dip galvanized steel sheet and the alloyed hot-dip galvanized steel sheet according to the present embodiment, a decarburized ferrite layer is formed in a region from the surface of the steel sheet to a depth of 5 μm to 200 μm. That is, the thickness of the decarburized ferrite layer is 5 μm or more and 200 μm or less. When the thickness of the decarburized ferrite layer is less than 5 μm, the bending improvement effect of the decarburized ferrite layer is not sufficiently exhibited. Therefore, when bending occurs in the steel plate, the base material of the steel plate having high strength is deformed, Cracks occur. When the thickness of the decarburized ferrite layer is more than 200 μm, the tensile strength of the steel sheet is lowered. The thickness of the decarburized ferrite layer is obtained, for example, by observing a cross section of the steel plate. Specifically, the area ratio of ferrite is measured every 1 μm from the surface of the steel sheet, and the position where the amount of ferrite is 120% of the ¼ position of the steel sheet thickness is defined as the interface between the decarburized ferrite layer and the base material. The thickness of the decarburized ferrite layer is determined by measuring the distance from the surface of the steel plate to the interface.

[Dispersion of tempered martensite in decarburized ferrite layer: number density of tempered martensite 0.01 / μm ² or more]
The decarburized ferrite layer of the hot-dip galvanized steel sheet and the alloyed hot-dip galvanized steel sheet according to the present embodiment contains 0.01 pieces / μm ² or more of tempered martensite. When the number density of tempered martensite in the decarburized ferrite layer is less than 0.01 / μm ² , uneven deformation occurs in the steel sheet, so that the bendability of the steel sheet decreases. Although it is better that the number density of tempered martensite in the decarburized ferrite layer is larger, in view of the current technical level, it is difficult to make the tempered martensite 1 piece / μm ² or more.

(C) Composition of base material [tempered martensite at 1/4 position of base material thickness: 5.0% by volume or more]
[Retained austenite at 1/4 position of base metal thickness: 0.5% by volume or more and less than 7.0% by volume]
[Remainder of structure at ¼ position of base metal thickness: 4-70 volume% ferrite and bainite]
[Part or all of tempered martensite and retained austenite forms MA in base material]
In order to obtain hot-dip galvanized steel sheet and alloyed hot-dip galvanized steel sheet with good workability and tensile strength of 780 MPa or more, the retained austenite remains in the structure of the base material of the steel sheet and the structure containing MA. It is effective to make the structure tempered at such a relatively low temperature. Thereby, the bendability is improved while maintaining the good total elongation provided by MA.

  Therefore, the structure of the base thickness of the base material of the hot dip galvanized steel sheet and the galvannealed steel sheet according to the present embodiment needs to contain tempered martensite at 5.0% by volume or more. In addition, the structure of the base thickness of the galvanized steel sheet and the alloyed hot dip galvanized steel sheet needs to contain 0.5% by volume or more and less than 7.0% by volume of retained austenite. In order to obtain higher strength, the tempered martensite at the 1/4 thickness position of the base material is preferably 8.0% by volume or more. Moreover, in the base material of the hot dip galvanized steel sheet and the alloyed hot dip galvanized steel sheet according to this embodiment, part or all of the tempered martensite and the retained austenite need to form MA.

  The part which prescribes | regulates the structure | tissue of a base material shall be a board thickness 1/4 position. This is because, in general, the plate thickness ¼ position is considered to be a portion having an average characteristic and configuration of the steel plate. The structure of the base material at a position other than the 1/4 position of the thickness is generally substantially the same as the structure at the 1/4 position of the thickness.

  The remainder of the base metal structure is preferably mainly ferrite, or mainly ferrite and bainite. Moreover, it is preferable not to contain 5 μm or more of cementite in the base material ferrite grains and martensite grains in order to promote the formation of MA. As described above, the elongation is improved by tempering MA at a relatively low temperature so that residual austenite remains. In order to improve bendability, it is preferable that all martensite contained is tempered. The ferrite content of the base material is preferably 4 to 70% by volume in order to bring mechanical properties such as tensile strength within an appropriate range.

(D) Hot-dip galvanized layer and alloyed hot-dip galvanized layer Hot-dip galvanized layer and alloyed hot-dip galvanized layer are the usual in the technical field to which the hot-dip galvanized steel sheet and alloyed hot-dip galvanized steel sheet according to this embodiment belong. It should be. However, if the Fe concentration of the alloyed hot-dip galvanized layer is less than 7% by mass, weldability and slidability may be insufficient. Therefore, the Fe concentration of the alloyed hot-dip galvanized layer is preferably 7% by mass or more. The upper limit of the Fe concentration of the alloyed hot-dip galvanized layer is preferably 20% or less, more preferably 15% or less from the viewpoint of powdering resistance. The Fe content of the plating layer is adjusted by the conditions of heat treatment (alloying treatment) after hot dipping. When the alloying treatment is not performed, the Fe concentration of the hot dip galvanizing may be less than 7% by mass. Hot dip galvanizing has lower weldability than galvannealed alloying. However, hot dip galvanizing is preferred because of its good corrosion resistance. The adhesion amount per one side of hot dip galvanizing is preferably in the range of ^{20 to} 120 g / m ² .

(E) Mechanical properties of hot-dip galvanized steel sheet and alloyed hot-dip galvanized steel sheet [yield strength is 420 MPa or more in a tensile test in the direction perpendicular to rolling]
[Tensile strength is 780 MPa or more in the tensile test in the direction perpendicular to rolling]
The hot-dip galvanized steel sheet and the alloyed hot-dip galvanized steel sheet according to the present embodiment have mechanical properties having a tensile strength (TS) of 780 MPa or more in a tensile test in the direction perpendicular to the rolling direction. In this tensile test, if the tensile strength is less than 780 MPa, it may be difficult to ensure sufficient shock absorption in the case of an automobile part. The tensile strength is preferably 800 MPa or more, more preferably 900 MPa or more. In consideration of application to automobile parts that require high plastic deformation starting strength at the time of collision, the yield strength (YS) is preferably 420 MPa or more. More preferably, it is 600 MPa or more. In consideration of application to automobile parts that require formability, the total elongation is preferably 10% or more and the hole expansion ratio is preferably 35% or more. In addition, the bendability of the hot dip galvanized steel sheet and the galvannealed steel sheet according to the present embodiment is characterized by no cracks and no constriction of 10 μm or more in the 90-degree V bending test. It is preferable.

(F) Manufacturing method of hot-dip galvanized steel sheet and alloyed hot-dip galvanized steel sheet In order to produce MA in the base material, in the manufacturing process of hot-dip galvanized steel sheet and alloyed hot-dip galvanized steel sheet, austenite is used. It is necessary to concentrate C. As austenite undergoes martensitic transformation during cooling, C is unevenly distributed in untransformed austenite, and a part of the austenite remains after cooling to obtain MA. Since M-A contains retained austenite and martensite is hard, strain concentrates on a relatively soft matrix, and high strength and good elongation can be obtained. However, excessively hard martensite impairs the bendability. Therefore, by appropriately tempering so that residual austenite remains in the base material, hot dip galvanized steel sheets and galvannealed steel sheets with excellent elongation and bendability can be obtained. Manufacturable. In addition, by performing tempering after plating, the problem of surface oxidation and the problem of remaining untempered martensite can be suppressed. Moreover, in order to produce | generate a decarburized ferrite layer, it is necessary to heat a raw material steel plate under a suitable average heating rate, and to anneal in a suitable atmosphere.

  Specifically, the manufacturing method of the hot dip galvanized steel sheet according to the present embodiment shown in FIG. 4 is a method in which a raw steel sheet having the above-described components is subjected to an average heating rate of 1 to 50 ° C./second in a temperature range of 100 to 720 ° C. And in the atmosphere in which the component is composed of 2 to 20% by volume of hydrogen, the balance containing nitrogen and impurities, and the dew point is higher than −30 ° C. and lower than or equal to 20 ° C. after the heating step, An average cooling rate of 0.5 to 10.0 ° C./second in a temperature range of 720 to 650 ° C. after the step of annealing the steel sheet in a temperature range of 720 to 1000 ° C. for 10 to 600 seconds and the step of annealing. Then, after the step of performing the first cooling on the material steel plate and the step of performing the first cooling, in the temperature range of 650 to 500 ° C., the average cooling rate is 2.0 to 100.0 ° C./sec. Second cooling is applied to the material steel plate After the step, the step of performing the second cooling, the step of hot dip galvanizing the raw steel plate, and the step of applying the hot dip galvanization, the average cooling rate from the hot dip galvanizing temperature to 200 ° C. or less is 2 ° C. Tempering for 1 second or more and 48 hours or less in the temperature range of 100 to 600 ° C. after the step of performing the third cooling on the raw steel plate and the step of performing the third cooling at / second or more. And a process of performing processing.

  The manufacturing method of the alloyed hot-dip galvanized steel sheet according to this embodiment shown in FIG. 5 heats the material steel sheet having the above-described components at an average heating rate of 1 to 50 ° C./second in a temperature range of 100 to 720 ° C. After the step and the heating step, the material steel plate is contained in an atmosphere consisting of 2 to 20% by volume of hydrogen, nitrogen and the balance containing impurities, and having a dew point of -30 ° C to 20 ° C. In the temperature range of 720 to 650 ° C. after the step of annealing for 10 to 600 seconds in the temperature range of 720 to 1000 ° C. and the step of annealing, the average cooling rate is 0.5 to 10.0 ° C./second, In the temperature range of 650 to 500 ° C., the raw steel plate is cooled at an average cooling rate of 2.0 to 100.0 ° C./second after the first cooling of the raw steel plate and the first cooling step. The second cooling step After the second cooling step, the step of hot dip galvanizing the raw steel plate, the step of subjecting the raw steel plate to alloying after the step of hot dip galvanizing, and the alloying treatment After the step of performing the third cooling to the raw steel plate at an average cooling rate of 2 ° C./second or more from the alloying treatment temperature to 200 ° C. or less, and the step of performing the third cooling, And a step of tempering the raw steel plate for 1 second to 48 hours in a temperature range of 100 to 600 ° C.

  Henceforth, the steel plate in the intermediate process of the manufacturing method which concerns on this embodiment is called raw material steel plate, and it distinguishes from the hot dip galvanized steel plate or alloyed hot dip galvanized steel plate which concerns on this embodiment.

In the manufacturing method of the hot dip galvanized steel sheet or the alloyed hot dip galvanized steel sheet according to this embodiment, a raw steel sheet having the above-described chemical components is used as a material. As described above, the raw steel plate may be either a hot-rolled steel plate or a cold-rolled steel plate, and the conditions for hot rolling and cold rolling are not particularly limited. However, a hot-rolled steel sheet that has been hot-rolled under conditions where the rolling finishing temperature is in the range of 800 ° C. to 1100 ° C. and the winding temperature is in the range of 350 ° C. to 750 ° C. is preferable. Also, when the content of Si is less than 1.0%, the tempered martensite decarburization ferrite layer of galvanized steel sheet and galvannealed steel sheet in order to contain 0.01 pieces / [mu] m ² or more Is preferably a hot-rolled steel sheet containing 0.01 / μm ² or more of pearlite and cementite at a depth of 20 μm from the surface of the steel sheet.

[Average heating rate in the heating step: 1 to 50 ° C./second in a temperature range of 100 to 720 ° C.]
In the step of heating the material steel plate, the heating rate in the temperature range of 100 to 720 ° C. is set to 1 to 50 ° C./second. When the heating rate is less than 1 ° C./second, the cementite of the raw steel sheet does not dissolve in the step of heating the raw steel sheet, so the tensile strength of the finally obtained hot-dip galvanized steel sheet or alloyed hot-dip galvanized steel sheet decreases. . Furthermore, when the heating rate is less than 1 ° C./second, it becomes difficult to disperse tempered martensite in the ferrite of the decarburized ferrite layer. On the other hand, when the heating rate is higher than 50 ° C./second, coarse ferrite is generated in the raw steel plate in the step of heating the raw steel plate. Furthermore, when the heating rate is higher than 50 ° C./second, it becomes difficult to disperse tempered martensite in the ferrite of the decarburized ferrite layer, so that the finally obtained hot-dip galvanized steel sheet or alloyed hot-dip galvanized steel sheet The bendability decreases.

[Annealing temperature in the annealing step: 720 to 1000 ° C.]
[Holding time in annealing process: 10 to 600 seconds]
After the heating step, the material steel plate is annealed. The annealing temperature (holding temperature) in the annealing step is set to 720 ° C. or higher in order to generate austenite during heating. In other words, when the annealing temperature is less than 720 ° C., martensite is not generated. In order to obtain a uniform structure advantageous for improving the bendability, it is preferable to maintain the raw steel plate temperature in the austenite single-phase region (Ac ₃ points or more). Moreover, when maintaining a raw material steel plate temperature in an austenite single phase area, it is preferable to make it heat from 720 degreeC to Ac ₃ point over 30 second in order to produce | generate a desired decarburized ferrite layer on the raw material steel plate surface. On the other hand, when the annealing temperature exceeds 1000 ° C., the decarburized ferrite layer disappears. When the holding time in the annealing step is less than 10 seconds, the thickness of the decarburized ferrite layer cannot be grown to 5 μm or more. On the other hand, when the holding time in the annealing step is more than 600 seconds, the effect of annealing is saturated, and thus productivity is lowered. Furthermore, when the holding time in the annealing process is longer than 600 seconds, the thickness of the decarburized ferrite layer grows excessively, so that the tensile strength decreases.

[Atmospheric components in the temperature range of 720 to 1000 ° C. in the annealing step: the balance containing 2 to 20% by volume of hydrogen, nitrogen and impurities]
[Dew point of atmosphere in a temperature range of 720 to 1000 ° C. in the annealing step: more than −30 ° C. and not more than 20 ° C.]
In the annealing step, when the hydrogen concentration of the atmosphere when the temperature of the raw steel plate is in the temperature range of 720 to 1000 ° C. is less than 2%, the oxide film on the surface of the raw steel plate cannot be reduced. The wettability of plating deteriorates. On the other hand, if the hydrogen concentration in this atmosphere is more than 50%, the dew point cannot be kept below 20 ° C. Moreover, in the annealing step, when the dew point of the atmosphere when the temperature of the raw steel plate is in the temperature range of 720 to 1000 ° C. is −30 ° C. or less, the thickness of the decarburized ferrite layer can be grown to 5 μm or more. Can not. On the other hand, when the dew point of this atmosphere is higher than 20 ° C., dew condensation occurs in the facility, which hinders the operation of the facility.

[Average cooling rate in the first cooling step: 0.5 to 10.0 ° C./second in a temperature range of 720 to 650 ° C.]
After the annealing step, the material steel plate is subjected to the first cooling. In the first cooling, it is necessary to set the average cooling rate in the temperature range of 720 to 650 ° C. to 0.5 to 10.0 ° C./second. The average cooling rate is a value obtained by dividing the difference between the cooling start temperature and the cooling end temperature by the cooling time. When the cooling rate in the step of performing the first cooling is less than 0.5 ° C./sec, precipitation of cementite occurs, so that martensite is not generated in the decarburized ferrite layer. On the other hand, when the cooling rate in the step of performing the first cooling exceeds 10.0 ° C./second, the C concentration in the austenite is not sufficiently generated, so that the retained austenite is hardly generated. The cooling rate in the step of performing the first cooling is preferably 1.0 to 8.0 ° C./second, more preferably 1.5 to 6.0 ° C./second.

[Average cooling rate in the second cooling step: 2.0 to 100.0 ° C./second in a temperature range of 650 to 500 ° C.]
After the step of performing the first cooling, the material steel plate is subjected to the second cooling. In the second cooling, the average cooling rate in the temperature range of 650 to 500 ° C. needs to be 2.0 to 100.0 ° C./second. When the cooling rate in the second cooling step is less than 2 ° C./second, precipitation of pearlite occurs, so that martensite is hardly generated. Moreover, when the cooling rate in the process of performing the second cooling exceeds 100 ° C./second, the flatness of the steel sheet deteriorates, so that the plating process becomes non-uniform. The cooling rate in the second cooling step is preferably 5 to 60 ° C./second, more preferably 8 to 40 ° C./second.

[Process of hot dip galvanizing]
[Process for alloying]
After the second cooling step, the raw steel plate is subjected to hot dip galvanization after isothermal holding or cooling is performed on the raw steel plate as necessary. Further, if necessary, the raw steel plate is heated to a temperature necessary for alloying the plating to be alloyed. The raw steel plate having the hot dip galvanizing or alloying hot dip galvanizing thus obtained is cooled.

The bath temperature and bath composition of hot dip plating may be general, and there is no particular limitation. The plating adhesion amount is not particularly limited, and may be within a normal range. For example, the amount of adhesion per side is in the range of ²⁰ to 120 g / m ² . The alloying treatment is preferably performed under conditions such that the Fe concentration in the plating layer is 7% by mass or more. Necessary conditions vary depending on the plating adhesion amount, but are performed, for example, by heating at a temperature of 490 to 560 ° C. for 5 to 60 seconds. When the alloying treatment is not performed, the Fe concentration in the hot dip galvanizing may be less than 7% by mass. The weldability of the hot dip galvanized steel sheet is lower than that of the galvannealed steel sheet. However, the corrosion resistance of the hot dip galvanized steel sheet is good.

[Average cooling rate in the third cooling step: 2 ° C./second or more in the temperature range from the alloying temperature or the hot dip galvanizing temperature to 200 ° C. or less]
In the cooling after the hot dip galvanizing process or after the alloying process, the steel sheet is cooled from the alloying temperature or the hot dip galvanizing temperature to 200 ° C. or less at an average cooling rate of 2 ° C./second or more. It is important to. The plating temperature is the bath temperature of hot dip galvanizing. By controlling the cooling rate, stable austenite is generated. Most of the stable austenite remains as austenite after tempering. In the second cooling, hard martensite is generated simultaneously with stable austenite, but the hard martensite becomes ductile martensite by subsequent tempering. A preferable cooling end temperature is 100 ° C. or lower. A preferable cooling rate is 5 ° C./second or more. The upper limit of the cooling rate is not particularly defined, but is preferably 500 ° C./second or less from the viewpoint of economy.

[Tempering temperature in the tempering process: 100 to 600 ° C.]
[Tempering time in the tempering process: 1 second to 48 hours]
After the second cooling step, the tempering treatment is performed on the raw steel plate. Tempering is performed in order to properly temper the martensite constituting the M-A. Thereby, since the martensite is softened, the bendability of the steel sheet is improved. Furthermore, tempering has the function of concentrating C in untransformed retained austenite. Hardening of retained austenite due to C enrichment improves the elongation of the steel sheet.

  Tempering is performed within a range of 1 second to 48 hours in a state where the temperature of the raw steel plate is within a temperature range of 100 to 600 ° C. If the steel plate temperature is low or the temperature holding time is short, the base material of the steel plate and the martensite of the decarburized ferrite layer become hard, and unstable residual austenite is generated excessively, so sufficient bendability is achieved. I can't get it. On the other hand, when the material steel plate temperature is high, the base material of the steel plate and the retained austenite of the decarburized ferrite layer are decomposed or the martensite becomes too soft, so that the elongation deteriorates. In tempering, it is necessary to set the maximum temperature of the steel sheet within the range of 100 to 600 ° C. and hold it within this temperature range for 1 second to 48 hours. However, it is preferable to keep the material steel plate isothermally at the maximum attained temperature in order to suppress variations in the properties of the steel plate. A preferred maximum temperature is in the range of 250-500 ° C. In addition, it is preferable that all the MA martensite of the base material of the raw steel plate is tempered at this point.

  After this tempering, there is no problem even if it is treated with a leveler to correct the flatness, and a film having oiling or lubricating action may be applied.

  Steels A to N having the chemical compositions shown in Table 1 were melted in an experimental furnace to produce a slab having a thickness of 40 mm. These slabs were hot-rolled at the rolling completion temperatures shown in Table 2 to the thicknesses shown in Table 2, then water spray cooled at a cooling rate of about 30 ° C./second, and further displayed in Table 2. Hot-rolled steel plates (material steel plates) 1 to 28 were manufactured by performing a heat treatment equivalent to winding at a winding temperature. In the above heat treatment, after water spray cooling to the coiling temperature, the steel plates 1 to 28 are charged into the furnace, the steel plates 1 to 28 are held at the coiling temperature for 60 minutes, and then a cooling rate of 20 ° C./hour. Thus, the steel sheet 1 is cooled to 100 ° C. or lower in the furnace, so that substantially the same heat history as that of the hot-rolled steel sheet can be applied to the steel sheets 1 to 28. The obtained hot-rolled steel sheets 1 to 28 were subjected to a scale removal treatment by pickling, and the steel sheets 14 and 17 were removed, and cold rolling was performed so that the thickness became 1.4 mm. The steel plates 14 and 17 were not cold rolled.

  From the hot-rolled steel sheet and the cold-rolled steel sheets 1 to 28 obtained in this manner, heat treatment test materials 1 to 28 were sampled and heated, annealed, first cooled, and secondly according to various conditions shown in Tables 2 and 3. Cooling, hot dip galvanizing and optionally alloying treatment, third cooling, and tempering. That is, first, the test materials 1 to 28 were heated to the annealing temperature at the average heating rate shown in the table, and were annealed while being held at that temperature for the annealing time shown in the table. Next, the test materials 1 to 28 were cooled to 500 ° C. at the primary cooling rate (first cooling rate) and the secondary cooling rate (second cooling rate) shown in the table. Thereafter, the test materials 1 to 28 are held in the temperature range of 500 to 460 ° C. for the residence time indicated in the table, and then heat treatment simulating hot dip galvanizing at 460 ° C. is performed on the test materials 1 to 28. Some test materials were subjected to heat treatment simulating alloying heat treatment at 510 ° C. Then, the test materials 1 to 28 were cooled at the tertiary cooling rate (third cooling rate) shown in the table. The test materials 1 to 28 thus cooled were tempered with the temperature maintained under the tempering conditions shown in the table immediately after cooling, except for some test materials. The “heat treatment temperature” shown in the table is the highest temperature reached during the tempering treatment. The rate of temperature increase when raising the temperature of the test material to the tempering temperature was 20 ° C./second. By this tempering, all martensite generated before tempering is tempered. Whether or not martensite has been tempered was determined by corroding the cross sections of the test materials 1 to 28 and then performing SEM observation to confirm the presence of carbides in the martensite.

  The following measurements were performed on the obtained test materials 1 to 28. These measurement results are summarized in Table 4.

Thickness of decarburized ferrite layer: electron microscope observation image of a cross section of test materials 1 to 46 orthogonal to the rolling direction and a cross section of test materials 1 to 46 orthogonal to the plate width direction (direction orthogonal to the rolling direction) It is an average value of values calculated by performing analysis. The area ratio of ferrite is measured every 1 μm from the surface of the steel sheet, and the position where the ferrite content is 120% of the thickness 1/4 position of the steel sheet is regarded as the interface between the decarburized ferrite layer and the base material. Was taken as the thickness of the decarburized ferrite layer.

  Decarburized ferrite layer structure: ferrite particle size of decarburized ferrite layer, tempered martensite volume ratio of decarburized ferrite layer, number density of tempered martensite of decarburized ferrite layer, the electron microscope observation image of decarburized ferrite layer Calculation was performed by image analysis.

  Structure of plated steel base material: Obtained by observing the base material at a 1/4 depth position. Specifically, image analysis of the electron microscope observation image is performed to measure the MA volume fraction, the residual austenite volume fraction is measured by X-ray diffraction, and the residual austenite volume fraction is subtracted from the MA volume fraction. Was regarded as the tempered martensite volume fraction.

  Tensile test: JIS No. 5 tensile test specimens were taken from various heat-treated materials so that the direction perpendicular to the rolling direction was the tensile direction, yield strength (YS), tensile strength (TS), and total elongation (El). Was measured. The total elongation (El) was 10% or more.

  Bending test: In a 90-degree V bending test in which the bending radius is twice the plate thickness, a test material having no cracks and no constriction of 10 μm or more was judged as “good”.

  Test No. which is an invention example satisfying all the provisions of the present invention. 1, 2, 7, 10, 14, 16, 17, 20, 23-26, 28 hot-dip galvanized steel sheets and alloyed hot-dip galvanized steel sheets have a tensile strength of 780 MPa or more, good elongation, and even better bending. Showed sex.

In contrast, test no. Reference numerals 3 to 6, 8, 9, 11 to 13, 15, 18, 19, 21, 22, and 27 are comparative examples that do not satisfy the provisions of the present invention.
No. No. 3 had a low annealing temperature, tempered martensite in the decarburized ferrite layer was not generated, and retained austenite was not generated, so the elongation and tensile strength were low and the bendability was poor.
No. In No. 4, the annealing temperature was excessive, the decarburized ferrite layer disappeared, and the bendability was poor.
No. In No. 5, the heat treatment temperature in the tempering process was excessive and austenite was decomposed, so that the elongation and the tensile strength were low.

No. In No. 6, the average heating rate in the heating step was large, the ferrite in the decarburized ferrite layer became coarse, and the tempered martensite was not dispersed and the number density was insufficient, so the bendability was poor.
No. In No. 8, the dew point of the atmosphere in the annealing step was below the specified range, and the decarburized ferrite layer was not generated, so the bendability was poor.
No. No. 9 had a low C content, so its tensile strength was low.

No. No. 11 had poor bendability because the heat treatment temperature in the tempering process was insufficient and the martensite of the decarburized ferrite layer was not tempered.
No. No. 12 had a high average cooling rate in the temperature range of 720 to 650 ° C. in the step of performing the first cooling, and no retained austenite was generated, so the elongation was low.
No. No. 13 had a low average heating rate in the heating step, tempered martensite was not dispersed in the decarburized ferrite layer, and the number density was insufficient, so the tensile strength was low and the bendability was poor.

No. No. 15 had a low tensile strength because the average cooling rate in the temperature range of 650 to 500 ° C. in the second cooling step was insufficient, and pearlite was generated, thereby suppressing the generation of martensite.
No. No. 18 had a low elongation because the average cooling rate in the temperature range of 650 to 500 ° C. in the step of performing the third cooling after the alloying treatment was insufficient and austenite was decomposed.
No. No. 19 had poor bendability because the average cooling rate in the temperature range of 720 to 650 ° C. in the first cooling step was insufficient and the tempered martensite of the decarburized ferrite layer was insufficient.

No. No. 21 was poor in bendability because annealing time was insufficient in the annealing step and no decarburized ferrite layer was formed.
No. No. 22 had poor Mn content, low tensile strength, and lack of tempered martensite in the decarburized ferrite layer, so the bendability was poor.
No. No. 27 had a long annealing time in the annealing step, and the decarburized ferrite layer grew excessively, so the tensile strength was low.

DESCRIPTION OF SYMBOLS 1 Hot-dip galvanized steel plate 2 Alloyed hot-dip galvanized steel plate 10 Steel plate 11 Hot-dip galvanized layer 12 Decarburized ferrite layer 13 Base material 21 Alloyed hot-dip galvanized layer

Claims (22)

A hot-dip galvanized steel sheet comprising a steel sheet and a hot-dip galvanized layer formed on the surface of the steel sheet,
The steel sheet includes a base material and a decarburized ferrite layer,
The chemical composition of the base material is unit mass%,
C: 0.03 to 0.40%,
Si: 0.001-1.80%,
Mn: 1.0 to 5.0%
P: 0.10% or less,
S: 0.0100% or less,
sol. Al: 0.001-1.500%,
N: 0.02% or less,
Ti: 0 to 0.300%,
Nb: 0 to 0.300%,
V: 0 to 0.300%,
Cr: 0 to 2.000%,
Mo: 0 to 2.000%,
Cu: 0 to 2.000%,
Ni: 0 to 2.000%,
B: 0 to 0.0200%,
Ca: 0 to 0.0100%,
REM: 0 to 0.1000%, and Bi: 0 to 0.0500%
The balance consists of iron and impurities,
The structure of the base material at a depth of 1/4 of the thickness of the steel sheet from the surface of the steel sheet is tempered martensite of 5.0% by volume or more and 0.5% by volume or more of 7.0% by volume. Less residual austenite, the balance mainly consisting of 4 to 70% by volume of ferrite and bainite,
In the base material, part or all of the tempered martensite and the retained austenite forms MA,
The structure of the decarburized ferrite layer is 120% or more with respect to the ferrite content of the structure at the position of the base material at a depth of 1/4 of the plate thickness of the steel plate from the surface of the steel plate. Containing ferrite,
The average grain size of the ferrite of the decarburized ferrite layer is 20 μm or less,
The thickness of the decarburized ferrite layer is 5 μm or more and 200 μm or less,
The structure of the decarburized ferrite layer contains 1.0% by volume or more of the tempered martensite, and the number density of the tempered martensite of the decarburized ferrite layer is 0.01 pieces / μm ² or more,
The alloyed hot-dip galvanized steel sheet has a mechanical property that yield strength is 420 MPa or more and tensile strength is 780 MPa or more in a tensile test in a direction perpendicular to rolling. The steel sheet is further in unit mass%,
Ti: 0.001% to 0.300%,
2. One or more selected from the group consisting of Nb: 0.001% or more and 0.300% or less, and V: 0.001% or more and 0.300% or less. The hot-dip galvanized steel sheet according to 1. The steel sheet is further in unit mass%,
1 or 2 types selected from the group consisting of Cr: 0.001% or more and 2.000% or less and Mo: 0.001% or more and 2.000% or less 2. The hot dip galvanized steel sheet according to 2. The steel sheet is further in unit mass%,
It contains one or two selected from the group consisting of Cu: 0.001% to 2.000% and Ni: 0.001% to 2.000%. The hot-dip galvanized steel sheet according to any one of the above. The steel sheet is further in unit mass%,
B: The hot-dip galvanized steel sheet according to any one of claims 1 to 4, which contains 0.0001% or more and 0.0200% or less. The steel sheet is further in unit mass%,
1 or 2 types selected from the group consisting of Ca: 0.0001% or more and 0.0100% or less, and REM: 0.0001% or more and 0.100% or less. The hot-dip galvanized steel sheet according to any one of 5. The steel sheet is further in unit mass%,
Bi: 0.0001% or more and 0.0500% or less are contained, The hot dip galvanized steel plate as described in any one of Claims 1-6 characterized by the above-mentioned.   An alloyed hot-dip galvanized steel sheet, wherein the hot-dip galvanized layer of the hot-dip galvanized steel sheet according to any one of claims 1 to 7 is alloyed. In unit mass%
C: 0.03 to 0.40%,
Si: 0.001-1.80%,
Mn: 1.0 to 5.0%
P: 0.10% or less,
S: 0.0100% or less,
sol. Al: 0.001-1.500%,
N: 0.02% or less,
Ti: 0 to 0.300%,
Nb: 0 to 0.300%,
V: 0 to 0.300%,
Cr: 0 to 2.000%,
Mo: 0 to 2.000%,
Cu: 0 to 2.000%,
Ni: 0 to 2.000%,
B: 0 to 0.0200%,
Ca: 0 to 0.0100%,
REM: 0 to 0.1000%, and Bi: 0 to 0.0500%
And heating the material steel plate made of iron and impurities at an average heating rate of 1 to 50 ° C./second in a temperature range of 100 to 720 ° C.,
After the step of heating, in the atmosphere where the component is composed of 2 to 20% by volume of hydrogen, the balance containing nitrogen and impurities, and the dew point is more than −30 ° C. and not more than 20 ° C., Annealing for 10 to 600 seconds in a temperature range of 1000 ° C .;
After the step of annealing, in the temperature range of 720 to 650 ° C., the step of performing the first cooling on the material steel plate at an average cooling rate of 0.5 to 10.0 ° C./second;
After the step of performing the first cooling, in the temperature range of 650 to 500 ° C., the step of performing the second cooling on the material steel plate at an average cooling rate of 2.0 to 100.0 ° C./second;
After the step of performing the second cooling, a step of hot dip galvanizing the raw steel plate,
After the step of applying the hot dip galvanizing, a step of performing a third cooling on the material steel plate at an average cooling rate of 2 ° C./second or more from the hot dip galvanizing temperature to 200 ° C. or less;
A method for producing a hot-dip galvanized steel sheet, comprising a step of performing a tempering treatment for 1 second to 48 hours in a temperature range of 100 to 600 ° C. after the third cooling step. The material steel plate is further in unit mass%,
Ti: 0.001% to 0.300%,
10. Nb: 0.001% or more and 0.300% or less, and V: One or more selected from the group consisting of 0.001% or more and 0.300% or less. The manufacturing method of the hot-dip galvanized steel sheet as described in 2. The material steel plate is further in unit mass%,
10. One or two selected from the group consisting of Cr: 0.001% or more and 2.000% or less, and Mo: 0.001% or more and 2.000% or less. 10. A method for producing a hot dip galvanized steel sheet according to 10. The material steel plate is further in unit mass%,
It contains one or two selected from the group consisting of Cu: 0.001% or more and 2.000% or less and Ni: 0.001% or more and 2.000% or less. The manufacturing method of the hot dip galvanized steel plate as described in any one of these. The material steel plate is further in unit mass%,
B: 0.0001% or more and 0.0200% or less is contained, The manufacturing method of the hot dip galvanized steel sheet as described in any one of Claims 9-12 characterized by the above-mentioned. The material steel plate is further in unit mass%,
It contains one or two selected from the group consisting of Ca: 0.0001% or more and 0.0100% or less, and REM: 0.0001% or more and 0.100% or less. The method for producing a hot-dip galvanized steel sheet according to any one of 13. The material steel plate is further in unit mass%,
Bi: 0.0001% or more and 0.0500% or less are contained, The manufacturing method of the hot dip galvanized steel sheet as described in any one of Claims 9-14 characterized by the above-mentioned. In unit mass%
C: 0.03 to 0.40%,
Si: 0.001-1.80%,
Mn: 1.0 to 5.0%
P: 0.10% or less,
S: 0.0100% or less,
sol. Al: 0.001-1.500%,
N: 0.02% or less,
Ti: 0 to 0.300%,
Nb: 0 to 0.300%,
V: 0 to 0.300%,
Cr: 0 to 2.000%,
Mo: 0 to 2.000%,
Cu: 0 to 2.000%,
Ni: 0 to 2.000%,
B: 0 to 0.0200%,
Ca: 0 to 0.0100%,
REM: 0 to 0.1000%, and Bi: 0 to 0.0500%
And heating the material steel plate made of iron and impurities at an average heating rate of 1 to 50 ° C./second in a temperature range of 100 to 720 ° C.,
After the step of heating, in the atmosphere where the component is composed of 2 to 20% by volume of hydrogen, the balance containing nitrogen and impurities, and the dew point is more than −30 ° C. and not more than 20 ° C., Annealing for 10 to 600 seconds in a temperature range of 1000 ° C .;
After the step of annealing, in the temperature range of 720 to 650 ° C., the step of performing the first cooling on the material steel plate at an average cooling rate of 0.5 to 10.0 ° C./second;
After the step of performing the first cooling, in the temperature range of 650 to 500 ° C., the step of performing the second cooling on the material steel plate at an average cooling rate of 2.0 to 100.0 ° C./second;
After the step of performing the second cooling, a step of hot dip galvanizing the raw steel plate,
After the step of performing the hot dip galvanization, a step of alloying the raw steel plate,
After the step of performing the alloying treatment, a step of performing a third cooling on the material steel plate at an average cooling rate of 2 ° C./second or more from the alloying treatment temperature to 200 ° C. or less;
A method for producing an galvannealed steel sheet, comprising, after the third cooling step, a step of tempering the raw steel plate in a temperature range of 100 to 600 ° C. for 1 second to 48 hours. The material steel plate is further in unit mass%,
Ti: 0.001% to 0.300%,
17. Nb: 0.001% or more and 0.300% or less, and V: One or more selected from the group consisting of 0.001% or more and 0.300% or less. The manufacturing method of the galvannealed steel plate as described in 2. The material steel plate is further in unit mass%,
17 or more kinds selected from the group consisting of Cr: 0.001% or more and 2.000% or less, and Mo: 0.001% or more and 2.000% or less. The method for producing the galvannealed steel sheet according to 17. The material steel plate is further in unit mass%,
It contains 1 type or 2 types selected from the group which consists of Cu: 0.001% or more and 2.000% or less and Ni: 0.001% or more and 2.000% or less. The manufacturing method of the galvannealed steel plate as described in any one of these. The material steel plate is further in unit mass%,
B: 0.0001% or more and 0.0200% or less is contained, The manufacturing method of the galvannealed steel plate as described in any one of Claims 16-19 characterized by the above-mentioned. The material steel plate is further in unit mass%,
It contains one or two selected from the group consisting of Ca: 0.0001% or more and 0.0100% or less, and REM: 0.0001% or more and 0.100% or less. The method for producing an alloyed hot-dip galvanized steel sheet according to any one of 20. The material steel plate is further in unit mass%,
Bi: 0.0001% or more and 0.0500% or less are contained, The manufacturing method of the galvannealed steel plate as described in any one of Claims 16-21 characterized by the above-mentioned.

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