JP2012177139A - Hot-dip galvanized steel sheet and method for production thereof - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a high-strength hot-dip galvanized steel sheet having excellent stretch-flangeability while having extremely high tensile strength, and to provide a method for production thereof.SOLUTION: The hot-dip galvanized steel sheet has a chemical composition containing, by mass, >0.070% to ≤0.15% C, 0.001-0.40% Si, >2.2% to ≤3.5% Mn, ≤0.05% P, ≤0.01% S, 0.001-0.40% sol. Al, 0.12-0.25% Ti, >0.0025% to ≤0.010% B and ≤0.01% N, and has a tensile strength (TS) of ≥980 MPa and a high specified hole expansion rate.

Description

  The present invention relates to a hot dip galvanized steel sheet and a method for producing the same. Specifically, the present invention relates to a high-strength hot-dip galvanized steel sheet having excellent stretch flangeability while having an extremely high tensile strength of 980 MPa or more and a method for producing the same.

  Here, in the present invention, “hot dip galvanizing” includes “hot galvanizing alloy plating” in which elements other than zinc (such as Al) are contained in the hot dip plating bath. The “hot dip galvanized steel sheet” includes “alloyed hot dip galvanized steel sheet” obtained by subjecting a steel sheet as a base material to a hot dip galvanizing process and further subjecting it to an alloying process.

  In recent years, improvement in fuel efficiency of automobiles has been demanded for the purpose of protecting the global environment, and thus there is an increasing need for high-strength steel sheets that can reduce the weight of the vehicle body while ensuring the safety of passengers. In particular, the weight reduction of automobile frame members greatly contributes to the weight reduction of the vehicle body. Therefore, high-strength steel sheets having a tensile strength of 980 MPa or more, particularly rust prevention are required for the steel sheets used for the vehicle frame members. There is a growing need for high-strength hot-dip galvanized steel sheets that can be applied to such members.

Steel sheets used for automotive framework members are required to satisfy not only high tensile strength but also various performances required during member molding, such as press formability, weldability, and plating adhesion. The Among them, since stretch flange molding is frequently used in the molding process of automobile frame parts such as pillars and members, it is necessary to have excellent stretch flangeability.
Therefore, there is a demand for a high-strength hot-dip galvanized steel sheet that has excellent tensile flangeability while having a tensile strength of 980 MPa or more.

  However, there is generally a trade-off relationship between tensile strength and stretch flangeability, and as shown in FIG. 1, stretch flangeability (hole expansion ratio: HER) is significantly reduced as the tensile strength is increased. For this reason, it is not easy to achieve both high tensile strength and excellent stretch flangeability.

  In addition, since the stretch flangeability significantly decreases with the increase in tensile strength, not only the stretch flangeability itself is improved, but also the tensile strength variation due to the fluctuation of manufacturing conditions is suppressed, in other words, the material stability. It is also important to increase

  By the way, the hot dip galvanized steel sheet is generally manufactured by continuous hot dip galvanizing equipment from the viewpoint of productivity. The manufacturing process in a continuous hot dip galvanizing facility is to heat a base steel plate such as a cold rolled steel plate and hold the base steel plate within a predetermined temperature range (this process is called “soaking”, holding in soaking) The temperature is referred to as “soaking temperature”), the base steel plate after cooling is cooled, and when cooled from this soaking temperature, it is immersed in a hot dip galvanizing bath maintained at a temperature of 400 ° C. or higher. , Has a characteristic temperature history of reheating and applying alloying treatment as necessary. That is, the cooling is temporarily interrupted in the temperature range of 400 ° C. or higher in the cooling process from the soaking temperature. In a high-strength steel sheet having a chemical composition adjusted to ensure high tensile strength, the above temperature range is essentially a temperature range where bainite transformation is likely to proceed. Therefore, the bainite transformation proceeds by being held in the temperature range once in the cooling process from the soaking temperature, but since the time to be held in the temperature range is short, most low carbon low alloy In steel, the bainite transformation is not completed, and C discharged from the transformed bainite is concentrated into untransformed austenite. When cooling from such a state to room temperature is performed, the austenite enriched with C is transformed into island martensite, and a structure including island martensite is easily formed as a final structure. Insular martensite is a very hard structure, which promotes non-uniform deformation and induces cracks from the flange portion in stretch flange molding. Therefore, when such a structure is formed, it becomes extremely difficult to ensure excellent stretch flangeability.

  Moreover, in the manufacturing process in the continuous hot dip galvanizing equipment, the cooling rate from the soaking temperature is usually about 0.5 to 50 ° C./second, which is smaller than that in the manufacturing process in the continuous annealing equipment. For this reason, it is not easy in itself to manufacture a high-strength hot-dip galvanized steel sheet having a tensile strength of 980 MPa or more.

  Thus, it is a very difficult technical problem to provide a high-strength hot-dip galvanized steel sheet that has a tensile strength of 980 MPa or more and is excellent in stretch flangeability, but several techniques have been proposed so far. Yes.

Many approaches to solve the above technical problems are to optimize the chemical composition of the steel sheet and the temperature history in the continuous hot dip galvanizing equipment.
In Patent Document 1, a cold-rolled steel sheet having a specific chemical composition is annealed at a maximum heating temperature of (Ac ₁ + Ac ₃ ) / 2 ° C. or higher and then held at 760 to 680 ° C. for 10 seconds or longer. After cooling between 680 ° C. and 550 ° C. at an average cooling rate of 1 ° C./second or more to (zinc plating bath temperature −40) ° C. to (zinc plating bath temperature +50) ° C. A method for producing a high-strength hot-dip galvanized steel sheet for cooling is disclosed.

Patent Document 2 discloses that a cold rolled steel sheet having a specific chemical composition is subjected to recrystallization annealing at A ₃ (° C.) or more and (A ₃ +30) (° C.) or less, and then 5 ° C. up to 600 ° C. After cooling at a rate of at least / s, and then pickling, heat treatment is performed at a temperature range of 500 ° C. or higher at a temperature A ₁ (° C.) or less defined by the contents of Si, Mn and Ni in the steel composition, followed by melting A method for manufacturing a hot-dip galvanized cold-rolled steel sheet that is subjected to galvanizing treatment is disclosed.

  Patent Document 3 discloses that austenite is prepared by properly adjusting alloy elements and strongly cooling to a temperature range of (Ms-100 ° C.) to (Ms-200 ° C.) during cooling from the soaking temperature in the annealing process. A method of manufacturing a hot-dip galvanized steel sheet is disclosed in which after partial quenching is performed to transform the part into martensite, reheating is performed and plating is performed.

On the other hand, there is also an approach for improving stretch flangeability by controlling inclusions.
Patent Document 4 discloses a technique for improving stretch flangeability by blowing Ar gas into molten steel at the time of casting, and has good stretch flangeability while having a tensile strength of 980 MPa or more. An example of a hot dip galvanized steel sheet is disclosed.

JP 2009-263686 A Japanese Patent Laid-Open No. 2004-211126 JP 2009-203548 A JP 2009-249732 A

  As described above, several techniques have been proposed for providing a high-strength hot-dip galvanized steel sheet having excellent tensile flangeability while having a tensile strength of 980 MPa or more, but none of them is sufficient. .

The technique disclosed in Patent Document 1 lacks material stability. That is, Patent Document 1 describes that annealing is performed at a maximum heating temperature of (Ac ₁ + Ac ₃ ) / 2 ° C. or higher, but as is clear from the description of the examples and the like, the maximum heating temperature is set at Ac ₃ ° C. or lower. Annealing, that is, annealing at a two-phase temperature. The technique disclosed in Patent Document 1 has a chemical composition in which a small amount of Ti is added. However, when a steel in which a small amount of Ti is added is annealed at a two-phase temperature, unrecrystallized ferrite remains. Thus, although such unrecrystallized ferrite remarkably increases the tensile strength, it is very difficult to control the fraction, so that it is difficult to secure a stable tensile strength and a stretch flange. Therefore, the technique disclosed in Patent Document 1 lacks material stability, and it is difficult to stably ensure a tensile strength of 980 MPa or more and good stretch flangeability.

The technique disclosed in Patent Document 2 lacks material stability and is difficult to apply to mass production techniques. That is, when a steel sheet heat treated in a continuous annealing furnace having a rapid cooling process is subjected to a high temperature heat treatment, the strength is lowered. In particular, the heat treatment is performed in a temperature range of 500 ° C. to A ₁ point where Mn diffusion becomes active. Such a tendency becomes remarkable in the technique disclosed in Japanese Patent Application Laid-Open No. H11-228707, and the fluctuation of the tensile strength accompanying the fluctuation of the heat treatment temperature becomes remarkable. Therefore, the technique disclosed in Patent Document 2 lacks material stability, and it is difficult to stably ensure a tensile strength of 980 MPa or more and good stretch flangeability. Moreover, since the manufacturing method which requires the heat processing hold | maintained in a high temperature range again after recrystallization annealing is inferior in productivity, the application to mass-production technology is not realistic.

  The technology disclosed in Patent Document 3 is difficult to apply to mass production technology. That is, in the technique disclosed in Patent Document 3, since the steel sheet is rapidly cooled to a temperature range below the Ms point after soaking, the flatness of the steel sheet is remarkably deteriorated, and the subsequent hot dip galvanizing process becomes difficult. Plating and uneven appearance appear. Therefore, application of the technique disclosed in Patent Document 3 to mass production technology is not realistic.

  The technique disclosed in Patent Document 4 lacks material stability. That is, in the technique disclosed in Patent Document 4, a large amount of Mo, which is a strengthening element, is added in order to ensure a tensile strength of 980 MPa or more for the hot-dip galvanized steel sheet (the steel of the example in Patent Document 4). No. I). This is because it is necessary to improve the hardenability of the steel itself in the manufacturing process of the continuous hot dip galvanizing equipment with a low cooling rate. However, when a large amount of Mo is added in this way, the tensile strength varies significantly with the soaking temperature, and the material stability deteriorates. Therefore, the technique disclosed in Patent Document 4 lacks material stability at least for hot-dip galvanized steel sheets, and it is difficult to stably secure a tensile strength of 980 MPa or more and good stretch flangeability. is there.

  As described above, several techniques have been proposed for providing a high-strength hot-dip galvanized steel sheet having excellent tensile flangeability while having a tensile strength of 980 MPa or more, but none has been said to be sufficient. .

The present invention provides a high-strength hot-dip galvanized steel sheet having excellent stretch flangeability while having an extremely high tensile strength of 980 MPa or more, and a method for producing the same, which have been difficult to manufacture as described above. With the goal. Here, “excellent stretch flangeability” refers to having mechanical properties such that the hole expansion ratio (HER) defined by the following formula (1) is 40% or more.
HER = (D _h −D ₀ ) / D ₀ × 100 (1)

Here, D ₀ is an initial hole diameter (mm), D _h is a hole diameter (mm) after fracture, and a circular punched hole having a diameter of 10 mm (= D ₀ ) punched with a clearance of 12.5% is used. The diameter of the punched hole when a crack that penetrates in the thickness direction at the edge of the punched hole is expanded by a cylindrical flat bottom punch (diameter: 33 mmφ, shoulder R: 3 mm) so that the burr is on the die side (= D _h ).

The clearance is the ratio of the gap between the die and the punch when punched into the test piece by punching as a ratio to the thickness of the test piece, and is defined by the following formula (2).
c = (d _d −d _p ) / 2t × 100 (2)

Here, c is the clearance (%), d _d is the inner diameter (mm) of the punching die, d _p is the diameter of the punching punch (d _p = 10 mm), and t is the thickness (mm) of the test piece.

  The present inventor has intensively studied to solve the above problems, and the C, Si, Mn, Ti, and B contents are extremely limited with respect to the chemical composition of the steel sheet that is the plating base of the hot dip galvanized steel sheet. High strength with excellent stretch flangeability while having an extremely high tensile strength of 980 MPa or more, which was difficult to manufacture with conventional technology, by applying optimal manufacturing conditions for the control New knowledge that hot dip galvanized steel sheet can be obtained was obtained.

The present invention has been made on the basis of the above new findings, and the gist thereof is as follows.
(1) A hot dip galvanized steel sheet provided with a hot dip galvanized layer on the surface of the steel sheet, wherein the steel sheet is in mass%, C: more than 0.070%, 0.15% or less, Si: 0.001% or more, 0 .40% or less, Mn: more than 2.2% and 3.5% or less, P: 0.05% or less, S: 0.01% or less, sol.Al: 0.001% or more and 0.40% or less, Ti : A chemical composition containing 0.12% or more and 0.25% or less, B: more than 0.0025% and 0.010% or less and N: 0.01% or less, and the hot dip galvanized steel sheet has a tensile strength. A hot-dip galvanized steel sheet having mechanical properties (TS) of 980 MPa or more and a hole expansion rate (HER) defined by the following formula (i) of 40% or more.
HER = (D _h −D ₀ ) / D ₀ × 100 (i)
Here, D ₀ is an initial hole diameter (mm), D _h is a hole diameter (mm) after fracture, and a circular punched hole having a diameter of 10 mm (= D ₀ ) punched with a clearance of 12.5% is used. The diameter of the punched hole when a crack that penetrates in the thickness direction at the edge of the punched hole is expanded by a cylindrical flat bottom punch (diameter: 33 mmφ, shoulder R: 3 mm) so that the burr is on the die side (= D _h ).

  (2) The chemical composition is mass%, Nb: 0.5% or less, V: 0.5% or less, Cr: 0.5% or less, Mo: 0.1% or less, Cu: 0.5% The hot-dip galvanized steel sheet according to the above (1), further comprising one or more selected from the group consisting of: Ni and 0.5% or less.

  (3) The chemical composition was selected from the group consisting of Ca: 0.01% or less, Mg: 0.01% or less, REM: 0.01% or less, and Zr: 0.01% or less in mass%. The hot-dip galvanized steel sheet according to (1) or (2) above, further containing one or more kinds.

  (4) The hot-dip galvanized steel sheet according to any one of (1) to (3) above, wherein the chemical composition further contains, by mass%, Bi: 0.05% or less.

(5) A method for producing a hot dip galvanized steel sheet comprising the following steps (A) to (C):
(A) The steel material having the chemical composition described in any one of (1) to (4) above is hot-rolled at 1100 ° C. or higher and 1300 ° C. or lower, and in a temperature range of 800 ° C. or higher and 1000 ° C. or lower. A hot rolling step in which the hot rolling is completed and wound into a hot rolled steel sheet in a temperature range of 400 ° C. or higher and 750 ° C. or lower;
(B) Pickling and cold rolling step of pickling and cold rolling the hot-rolled steel sheet to obtain a cold-rolled steel sheet; and (C) Heating the cold-rolled steel sheet to Ac ₃ point or more and 950 ° C or less. The average cooling rate from 750 ° C. to 580 ° C. is set to 1.0 ° C./second or more and 50 ° C./second or less, and is cooled to a temperature range of 400 ° C. or more and 560 ° C. or less. A continuous hot-dip galvanizing step in which a hot-dip galvanized steel sheet is maintained for 10 seconds or more and 500 seconds or less, including when immersed in a plating bath, in a temperature range of 600 ° C. or lower.

  (6) In the step (C), the hot dip galvanized steel sheet according to the above (5), wherein the hot dip galvanized steel sheet immersed in the plating bath is subjected to an alloying treatment in a temperature range of 430 ° C. to 600 ° C. A method of manufacturing a steel sheet.

  According to the present invention, a high-strength hot-dip galvanized steel sheet having excellent stretch flangeability while having an extremely high tensile strength of 980 MPa or more can be obtained. The hot-dip galvanized steel sheet according to the present invention can be used widely in industry, particularly in the automobile field. In particular, it is suitable for applications in which press molding, such as the body of an automobile, and in particular, stretch flange molding, which has been difficult to apply in the past, is indispensable.

It is a graph which shows the relationship between the tensile strength and stretch flangeability in the conventional high-strength hot-dip galvanized steel sheet.

Hereinafter, modes for carrying out the present invention will be described.
1. Chemical composition The reason which prescribed | regulated the chemical composition of the steel plate which is a plating base material of the hot dip galvanized steel plate which concerns on this invention as mentioned above is demonstrated. In the following description, “%” representing the content of each element means mass% unless otherwise specified.

(C: more than 0.070% and less than 0.15%)
C is an element having an effect of increasing the strength of the steel sheet. When the C content is 0.070% or less, it becomes difficult to ensure a tensile strength of 980 MPa or more. Therefore, the C content is more than 0.070%. On the other hand, when the C content exceeds 0.15%, the deterioration of stretch flangeability becomes significant. Therefore, the C content is 0.15% or less. Preferably it is 0.13% or less.

(Si: 0.001% to 0.40%)
Si is an element having an effect of increasing the strength of the steel sheet without deteriorating the ductility so much or improving the ductility. Moreover, it is also an element which has the effect | action which improves plating adhesiveness. If the Si content is less than 0.001%, it is difficult to obtain the above effect. Therefore, the Si content is 0.001% or more. When the Si content is 0.05% or more, the TRIP effect is promoted and the ductility is further improved. Therefore, the Si content is preferably 0.05% or more. On the other hand, if the Si content exceeds 0.40%, austenite is excessively generated, and the stretch flangeability is significantly deteriorated. Therefore, the Si content is 0.40% or less.

(Mn: more than 2.2% and 3.5% or less)
Mn is an element that has the effect of increasing the strength of the steel sheet and improving the material stability. When the Mn content is 2.2% or less, it becomes difficult to stably secure a tensile strength of 980 MPa or more. Therefore, the Mn content is over 2.2%. When the Mn content is 2.4% or more, the soaking temperature can be set to 880 ° C. or less in the production process in the continuous hot dip galvanizing equipment, thereby suppressing the soaking furnace from being damaged and increasing the productivity. It becomes possible to improve. For this reason, it is preferable that Mn content shall be 2.4% or more. On the other hand, if the Mn content exceeds 3.5%, a band structure develops and a large amount of MnS is generated, and the stretch flangeability deteriorates remarkably. Therefore, the Mn content is 3.5% or less. From the viewpoint of improving productivity by reducing the load during cold rolling, it is preferably 3.0% or less, and more preferably 2.7% or less.

(P: 0.05% or less)
P is an impurity that is inevitably contained in steel in general, but P has an effect of increasing the strength of the steel sheet by solid solution strengthening, and therefore may be positively contained. However, when the P content exceeds 0.05%, the weldability is significantly deteriorated. Therefore, the P content is 0.05% or less. Preferably, it is 0.012% or less. In order to obtain the above action more reliably, the P content is preferably set to 0.005% or more.

(S: 0.01 or less)
S is an impurity inevitably contained in steel, and is preferably as low as possible from the viewpoint of weldability. If the S content exceeds 0.01%, the weldability is significantly reduced. Therefore, the S content is 0.01% or less. Preferably it is 0.003% or less, More preferably, it is 0.0015% or less.

(Sol.Al: 0.001% or more and 0.40% or less)
Al is an element having an action of deoxidizing steel to make the steel material sound, and is also an element having an action of improving the yield of carbonitride forming elements such as Ti. sol. If the Al content is less than 0.001%, it is difficult to obtain the above effect. Therefore, sol. Al content shall be 0.001% or more. Preferably it is 0.015% or more. On the other hand, sol. When the Al content exceeds 0.40%, the weldability is significantly lowered, and the oxide inclusions are increased, so that the surface properties are significantly deteriorated. Therefore, sol. The Al content is 0.40% or less. Preferably it is 0.080% or less.

(Ti: 0.12% or more and 0.25% or less)
Ti is an important element in the present invention, and is an element having an effect of remarkably increasing the strength of a steel sheet by forming fine precipitates which are carbide, nitride, or carbonitride in steel. And, it has a very high tensile strength of 980 MPa or more by stipulating strictly with C content, Si content, Mn content and B content, and further by combining continuous hot dip galvanizing treatment conditions as described later However, a high-strength hot-dip galvanized steel sheet having excellent stretch flangeability can be obtained. When the Ti content is less than 0.12%, it is difficult to obtain the effect by the above action. Therefore, the Ti content is 0.12% or more. Preferably it is 0.14% or more. On the other hand, if the Ti content exceeds 0.25%, the precipitates become coarse, making it difficult to obtain an effect of significantly increasing the strength of the steel sheet, and it is difficult to ensure a tensile strength of 980 MPa or more. Therefore, the Ti content is set to 0.25% or less. Preferably it is 0.22% or less.

(B: more than 0.0025% and 0.010% or less)
B is an important element in the present invention, has an effect of increasing the strength of the steel sheet, and includes an appropriate amount of B, thereby ensuring a tensile strength of 980 MPa or more and accompanying a change in the B content. It becomes possible to remarkably suppress the fluctuation of the tensile strength. That is, the material stability is improved. When the B content is 0.0025% or less, it is difficult to secure a tensile strength of 980 MPa or more, and the fluctuation of the tensile strength due to the fluctuation of the B content is large, so that sufficient material stability can be ensured. It becomes difficult. Therefore, the B content is more than 0.0025%. On the other hand, if the B content exceeds 0.010%, an oxide containing B is generated on the surface of the steel sheet, and the surface properties deteriorate. Therefore, the B content is set to 0.010% or less.

(N: 0.01% or less)
N is an impurity inevitably contained in the steel, and is preferably as low as possible from the viewpoint of stretch flangeability. If the N content exceeds 0.01%, the stretch flangeability will be significantly reduced. Therefore, the N content is 0.01% or less. Preferably it is 0.006% or less.

(Nb: not more than 0.5%, V: not more than 0.5%, Cr: not more than 0.5%, Mo: not more than 0.1%, Cu: not more than 0.5% and Ni: not more than 0.5% One or more selected from the group)
These elements are all elements that have an effect of increasing the strength of the steel sheet. Therefore, you may contain 1 type, or 2 or more types of these elements. However, when Nb and V are contained in amounts exceeding 0.5%, deterioration of the surface properties due to inclusions containing Nb and V may become apparent. Further, even if Cr, Cu and Ni are contained in amounts exceeding 0.5%, the effects of the above action are saturated and disadvantageous economically, and hot rolling and cold rolling become difficult. Moreover, when Mo is contained exceeding 0.1%, the material stability deteriorates. Therefore, the content of each element is as described above. In order to obtain the effect of the above operation more surely, Nb: 0.003% or more, V: 0.003% or more, Cr: 0.005% or more, Mo: 0.005% or more, Cu: 0.00. It is preferable to satisfy any of 005% or more and Ni: 0.005% or more.

(One or two or more selected from the group consisting of Ca: 0.01% or less, Mg: 0.01% or less, REM: 0.01% or less, and Zr: 0.01% or less)
Any of these elements contributes to inclusion control, in particular, fine dispersion of inclusions, and has an effect of enhancing bendability. Therefore, you may contain 1 type, or 2 or more types of these elements. However, when the content exceeds 0.01%, deterioration of surface properties may become obvious. Therefore, the content of each element is as described above. In order to obtain the effect of the above action more reliably, the content of any element is preferably set to 0.0005% or more.
Here, REM is a generic name for a total of 17 elements of Sc, Y, and lanthanoid, and the content of REM means the total content of these elements. In the case of a lanthanoid, it is industrially added in the form of misch metal.

(Bi: 0.05% or less)
Bi is an element having an action of improving bendability. Therefore, you may make it contain. However, if the content exceeds 0.05%, the hot workability deteriorates and hot rolling becomes difficult. Therefore, the Bi content is set to 0.05% or less. In addition, in order to acquire the effect by the said action more reliably, it is preferable to make Bi content into 0.0005% or more.

2. Steel structure The steel structure of the steel sheet which is a plating base material of the hot dip galvanized steel sheet according to the present invention is not particularly limited. However, in order to obtain the target tensile strength and stretch flangeability, a steel structure that satisfies the following conditions is preferable.

(Area ratio of non-recrystallized ferrite: less than 0.5%)
In order to achieve the target stretch flangeability in a region where the tensile strength is 980 MPa or more, the area ratio of non-recrystallized ferrite is preferably less than 0.5% (including the case of 0%). The non-recrystallized ferrite described here is a phase elongated in the rolling direction confirmed by microscopic observation.

(Area ratio of retained austenite: 3% or less)
In order to achieve the target stretch flangeability in a region where the tensile strength is 980 MPa or more, the area ratio of retained austenite is preferably 3% or less (including the case of 0%).

3. Hot-dip galvanized layer The chemical composition of the hot-dip galvanized layer of the hot-dip galvanized steel sheet according to the present invention is not particularly limited. When the hot dip galvanized layer is alloyed hot dip galvanized, it is preferable to satisfy the following conditions.

(Fe: 8% to 15% by mass)
By setting the Fe content in the hot dip galvanized layer to 8% by mass or more, the formation of a soft portion in the surface layer portion of the plated layer after the alloying treatment is suppressed, the slidability is increased, and the plated layer is the base material. Flakes-like peeling due to peeling from the interface with a certain steel sheet is suppressed. Therefore, the Fe content is preferably 8% by mass or more. More preferably, it is 9.5 mass% or more. On the other hand, when the Fe content is 15% by mass or less, powdering peeling caused by compressive deformation of the alloyed hot-dip galvanized layer inside the bent portion when the steel sheet is bent is suppressed. Therefore, the Fe content is preferably 15% by mass or less. More preferably, it is 14 mass% or less.

(Al: 0.15 mass% or more and 0.50 mass%)
By setting the Al content in the hot dip galvanized layer to 0.15% by mass or more, the development of the alloy layer in the hot dip galvanizing bath can be more appropriately suppressed, and the control of the coating amount becomes easy. . Therefore, the Al content is preferably 0.15% by mass or more. More preferably, it is 0.20 mass% or more, Most preferably, it is 0.25 mass% or more. On the other hand, by setting the Al content to 0.50% by mass or less, an appropriate alloying rate can be secured, and the above Fe content is secured at an alloying treatment temperature of 540 ° C. or less even at a normal line speed. It becomes easy to ensure a tensile strength of 980 MPa or more. Therefore, the Al content is preferably 0.50% by mass or less. More preferably, it is 0.45 mass% or less, Most preferably, it is 0.40 mass% or less.

(Other)
In the galvanized layer, Si, Mn, P, S, Ti, Nb, V, Cr, Mo, Cu, Ni, B, Ca, REM, etc. are taken in from the base material in the alloying process. As long as it is within the range that can be incorporated into the plating layer when hot-dip plating and alloying are performed under normal conditions, there is no problem because the plating quality is not adversely affected. The normal plating conditions here are, as described later, a plating bath temperature of 400 ° C. or higher and 490 ° C. or lower, a steel sheet penetration temperature of 400 ° C. or higher and 500 ° C. or lower, and an alloying temperature of 430 ° C. or higher and 600 ° C. or lower. is there.

4). Manufacturing method Next, the reason for limitation of the manufacturing method of the hot dip galvanized steel sheet of this invention is demonstrated.
(A) Hot rolling process It is preferable to melt the molten steel having the above-described steel composition by a conventional melting method such as a converter or an electric furnace and to obtain a steel material such as a slab by a continuous casting method. In place of the continuous casting method, an ingot casting method, a thin slab casting method, or the like may be employed. This steel material is hot-rolled to obtain a hot-rolled steel sheet. Hot rolling is a direct feed rolling in which a cast steel material is not cooled to room temperature but is charged in a heating furnace while being heated and heated and then rolled, or a direct rolling in which rolling is performed immediately after performing a slight heat retention, Or any of the reheating rolling which reheats and rolls after once cooling a steel material may be sufficient. At this time, when the hot rolling process is composed of a rough rolling process and a finish rolling process, the temperature fluctuation of the entire length of the rough bar after the rough rolling and before the finish rolling is equalized by induction heating or the like. Can be suppressed, which is preferable.

(Temperature of steel used for hot rolling: 1100 ° C to 1300 ° C)
The temperature of the steel material used for hot rolling is 1100 ° C. or higher and 1300 ° C. or lower.
The hot-dip galvanized steel sheet according to the present invention ensures the intended tensile strength by dispersing fine precipitates such as Ti. Therefore, it is necessary to make Ti etc. into a solid solution state at the stage of hot rolling. When the temperature of the steel material used for hot rolling is less than 1100 ° C., it may be difficult to make Ti or the like into a solid solution state. Therefore, the temperature of the steel material used for hot rolling is set to 1100 ° C. or higher. On the other hand, even if the temperature of the steel material used for hot rolling exceeds 1300 ° C., not only the effect of making Ti or the like into a solid solution state is saturated, but also the yield decreases due to an increase in scale loss. Therefore, the temperature of the steel material used for the hot-rolled steel sheet is 1300 ° C. or less. The time for maintaining in the temperature range of 1100 ° C. or higher and 1300 ° C. when being subjected to hot rolling is not particularly specified, but it is preferably 10 minutes or longer, more preferably 30 minutes or longer in order to make Ti or the like more solid solution. More preferably. Moreover, in order to suppress an excessive scale loss, it is preferable to set it as 10 hours or less, and it is more preferable to set it as 5 hours or less. In addition, when direct feed rolling or direct rolling is performed and Ti or the like is in a solid solution state, it may be directly subjected to hot rolling without being subjected to heat treatment.

(Rolling completion temperature: 800 ° C or higher and 1000 ° C or lower)
Rolling completion temperature shall be 800 degreeC or more and 1000 degrees C or less.
If rolling completion temperature is less than 800 degreeC, the deformation resistance at the time of rolling will be large and operation will become difficult. Therefore, the rolling completion temperature is 800 ° C. or higher. On the other hand, when the rolling completion temperature exceeds 1000 ° C., the grain boundary oxidation becomes remarkable, and the surface property of the hot dip galvanized steel sheet is significantly deteriorated. Therefore, the rolling completion temperature is 1000 ° C. or less.

(Winding temperature: 400 ° C or higher and 750 ° C or lower)
The coiling temperature is 400 ° C. or higher and 750 ° C. or lower.
When the coiling temperature is less than 400 ° C., hard bainite and martensite are generated, and subsequent cold rolling becomes difficult. Therefore, the coiling temperature is 400 ° C. or higher. Preferably it is 500 degreeC or more. On the other hand, when the coiling temperature is higher than 750 ° C., the grain boundary oxidation becomes remarkable, and the surface properties of the hot dip galvanized steel sheet deteriorate significantly. Therefore, the coiling temperature is 750 ° C. or lower. Preferably it is 700 degrees C or less.

(B) Pickling / cold rolling process The hot-rolled steel sheet is pickled by a conventional method and then cold-rolled to obtain a cold-rolled steel sheet.
Before or after pickling, it is preferable to perform a mild rolling of about 0 to 5% and correct the shape because it is advantageous in ensuring flatness. In addition, when mild rolling is performed before pickling, pickling performance is improved, removal of surface concentrating elements is promoted, and plating adhesion is improved.
From the viewpoint of refining the structure of the steel sheet after continuous hot dip galvanizing, the rolling reduction in cold rolling is preferably 30% or more. Moreover, from the viewpoint of suppressing breakage during cold rolling, the rolling reduction of cold rolling is preferably 70% or less.

(C) Continuous hot-dip galvanizing step In the present invention, since Mn is contained in a large amount and Ti and B are further contained, recrystallization of the processed ferrite is remarkably suppressed. Therefore, processing strain remains at the time of temperature rise during soaking, the remaining of non-recrystallized grains is remarkably promoted, and the tensile strength and stretch flangeability are affected by continuous hot dip galvanizing conditions. Therefore, the target performance is achieved by performing the continuous hot dip galvanizing treatment under the following conditions.

(Soaking temperature: Ac ₃ points or more and 950 ° C. or less)
The soaking temperature is Ac ₃ points or more and 950 ° C. or less.
If the soaking temperature is less than Ac ₃ points, unrecrystallized remains and a uniform structure cannot be obtained, and the material stability and stretch flangeability deteriorate. Therefore, the soaking temperature is Ac ₃ points or more. On the other hand, if the soaking temperature exceeds 950 ° C., damage to the annealing furnace becomes obvious and productivity decreases. Therefore, the soaking temperature is 950 ° C. or lower. Preferably, it is 880 degrees C or less.

  The soaking time maintained in the above temperature range is preferably 10 seconds or longer in order to completely remove unrecrystallized materials and ensure good material stability and stretch flangeability. Moreover, it is preferable to set it as 300 seconds or less from a viewpoint of productivity.

  The heating to the soaking temperature is preferably performed at an average heating rate of 1 ° C./second or more. By setting the average heating rate to 1 ° C./second or more, uneven grain growth in the heating process is suppressed, a more uniform structure can be obtained, and good bendability can be ensured.

(Average cooling rate from 750 ° C. to 580 ° C .: 1.0 ° C./second or more and 50 ° C./second or less)
In cooling after soaking, the average cooling rate from 750 ° C. to 580 ° C. is 1.0 to 50 ° C./second. The reason for defining the average cooling rate in the temperature range from 750 ° C. to 580 ° C. is to secure the tensile strength of 980 MPa or more while ensuring the material stability by controlling the cooling rate in the temperature range. .

  When the average cooling rate is less than 1.0 ° C./second, it is difficult to ensure a tensile strength of 980 MPa or more. Therefore, the average cooling rate is set to 1.0 ° C./second or more. Preferably, it is 4 ° C./second or more. On the other hand, if the average cooling rate exceeds 50 ° C./second, it is difficult to control the cooling rate and cooling stop temperature over the entire coil, and it is difficult to operate in a continuous hot dip galvanizing facility. Therefore, the average cooling rate is set to 50 ° C./second or less. Preferably it is 30 degrees C / sec or less.

(Cooling stop temperature: 400 ° C or more and 560 ° C or less)
The cooling stop temperature for cooling after soaking is 400 ° C. or more and 560 ° C. or less.
When the cooling stop temperature is less than 400 ° C., the amount of heat removal at the time of entering the plating bath is large and operation becomes difficult. Therefore, the cooling stop temperature is set to 400 ° C. or higher. On the other hand, when the cooling stop temperature exceeds 560 ° C., operation becomes difficult and stretch flangeability deteriorates. Therefore, the cooling stop temperature is set to 560 ° C. or lower. In hot dip galvanizing, a cold-rolled steel sheet soaked in a hot dip galvanizing bath at 400 ° C. or higher and 490 ° C. or lower is immersed in a conventional method.

(Dwelling time in a temperature range of 400 ° C. or more and 600 ° C. or less: 10 seconds or more and 500 seconds or less, but also during plating immersion)
After the cooling, a hot dip galvanizing treatment, and further an alloying treatment as necessary. Here, since the bath temperature of the hot dip galvanizing bath is usually 400 ° C. or higher and 490 ° C. or lower, in order to avoid excessive heat removal from the hot dip galvanizing bath and making the operation difficult, and stable In order to ensure plating quality, the temperature before immersion in the hot dip galvanizing bath is usually 400 ° C. or higher and 500 ° C. or lower. The alloying treatment temperature is preferably 430 ° C. or more and 600 ° C. or less as will be described later. For this reason, in order to perform a hot dip galvanization process and also an alloying process as needed, it will inevitably stay in the temperature range of 400 degreeC or more and 600 degrees C or less. However, the temperature range is the temperature range where the bainite transformation is most advanced, in other words, the temperature range that affects the tensile strength of the hot dip galvanized steel sheet as the final product. Therefore, the control of the residence time in the temperature range is extremely important.

  If the residence time in the temperature range of 400 ° C. or more and 600 ° C. or less is less than 10 seconds, it is difficult to perform plating immersion and control the amount of plating adhesion. Therefore, the stay time is 10 seconds or more. On the other hand, if the residence time exceeds 500 seconds, it is difficult to ensure the 980 MPa tensile strength. Therefore, the stay time is 500 seconds or less. In addition, it is preferable that the said residence time shall be 30 second or more and 150 second or less from a viewpoint of improving material stability especially.

(Alloying temperature: 430 ° C or higher and 600 ° C or lower)
When the alloying treatment is performed after immersion in the plating bath, the alloying treatment temperature is set to 430 ° C. or more and 600 ° C. or less.

  When the alloying treatment temperature is less than 430 ° C., unalloyed treatment occurs, and the surface properties of the steel sheet deteriorate. Therefore, the alloying treatment temperature is set to 430 ° C. or higher. Preferably it is 500 degreeC or more. On the other hand, when the alloying treatment temperature exceeds 600 ° C., the plating adhesion deteriorates. Therefore, the alloying temperature is 600 ° C. or lower. Preferably it is 550 degrees C or less. Although the alloying treatment time is not particularly defined, it is preferably 5 seconds or more and 60 seconds or less from the viewpoint of securing a suitable degree of alloying (Fe content of the alloyed hot-dip galvanized layer). By doing in this way, it is preferable to make an alloying degree into 8 mass% or more and 15 mass% or less.

  After the continuous hot dip galvanizing treatment, it is preferable to further perform temper rolling in the range of 0.05% to 1% elongation. Yield point elongation is suppressed by temper rolling, and yield strength is adjusted.

Furthermore, the present invention will be described more specifically with reference to examples.
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 to produce a 2.6 mm thick hot rolled steel sheet.

The obtained hot-rolled steel sheet was pickled and cold-rolled to produce a 1.2 mm-thick cold-rolled steel sheet.
The obtained cold-rolled steel sheet was subjected to heat treatment under the conditions shown in Table 3 so as to simulate the thermal history in the continuous hot-dip galvanizing process, thereby producing an annealed cold-rolled steel sheet. That is, cooling is performed after soaking under the soaking conditions shown in Table 3 (soaking temperature, soaking time), and after cooling at the cooling stop temperature, a predetermined time (holding time before soaking) is maintained from the start of soaking. It cooled over 4 second to 460 degreeC which is plating bath temperature, and also hold | maintained at 460 degreeC for 2 second. Specimen No. Except for 18, it was heated for 4 seconds until the alloying treatment temperature shown in Table 3 was maintained for 10 seconds at each alloying treatment temperature so as to simulate the alloying treatment, and an average cooling rate of 20 ° C. / Cooled to room temperature in seconds. Specimen No. 18 was held at 460 ° C. for 2 seconds and then cooled to room temperature at an average cooling rate of 20 ° C./second. The annealed cold-rolled steel sheet thus obtained was temper-rolled at an elongation of 0.1% to prepare various test pieces for evaluation.

  The annealed cold rolled steel sheet produced in this example is not hot dip galvanized, but has the same thermal history as a hot dip galvanized steel sheet (including “alloyed hot dip galvanized steel sheet” as described above). Therefore, the mechanical properties of the steel sheet are substantially the same as those of the hot dip galvanized steel sheet having the same thermal history.

Steel structures were analyzed for annealed cold-rolled steel sheets obtained under various production conditions, tensile tests and stretch flange tests were performed, and the respective mechanical properties were evaluated.
[Test method]
(Area ratio of non-recrystallized ferrite)
Test specimens were taken from the rolling direction of each annealed cold-rolled steel sheet and the direction perpendicular to the rolling direction, the cross section in the rolling direction and the structure of the cross section perpendicular to the rolling direction were observed with an electron microscope, and a region of 8 mm ² was photographed. The area ratio of unrecrystallized ferrite was investigated by photographing and image analysis.

(Area ratio of retained austenite)
A specimen (width 25 mm x length 25 mm x thickness 1.2 mm) taken from each annealed cold rolled steel sheet is subjected to chemical polishing to reduce the thickness by 0.3 mm, and X-ray diffraction is performed on the surface of the specimen after chemical polishing. Conducted three times. The obtained X-ray diffraction profile was analyzed to determine the area ratio of retained austenite, and the average value of the three numerical values obtained was taken as the area ratio of retained austenite of the corresponding annealed cold rolled sheet.

(Tensile test)
From each annealed cold-rolled steel sheet, a JIS No. 5 tensile specimen was taken from the direction perpendicular to the rolling direction, and TS (tensile strength) and El (total elongation) were measured.

(Stretch flange test)
A round hole with a diameter of 10 mm (= D ₀ ) is formed in the center of a specimen (width 100 mm × length 100 mm × plate thickness 1.2 mm) collected from each annealed cold rolled steel sheet under the condition that the clearance is 12.5%. Punched and stretched flange specimens were prepared. With the burrs in the punched portion on the die side, it was expanded with a cylindrical flat bottom punch with a diameter of 33 mm and a shoulder R3 mm, and the diameter D _h of the hole immediately after the crack penetrated the plate thickness at the edge of the round hole was measured. HER (hole expansion rate) obtained by the following formula was calculated.
HER = (D _h −D ₀ ) / D ₀ × 100 (1)

(Explanation of test results)
These results are shown in Table 4.

  In addition, the numerical value underlined in Tables 1-4 has shown that content, conditions, or a mechanical characteristic shown by the numerical value is outside the range of this invention.

Specimen Nos. 3, 4, 7, 9, 12, 14, 17-19, and 23 in Table 4 are steel plates of the present invention examples that satisfy all the conditions of the present invention.
On the other hand, since the test materials No. 1, 11 and 15 deviated from the range defined by the invention in the chemical composition, the target stretch flangeability was not obtained. Since the test materials No. 2, 6, 21, and 22 deviated from the range defined by the invention in the chemical composition, the intended tensile strength was not obtained. Since the test materials No. 5, 10 and 20 were out of the range defined in the present invention by the manufacturing conditions, the intended tensile strength could not be obtained. Since sample conditions No. 8 and 13 deviated from the range specified in the present invention, the intended stretch flangeability could not be obtained. Since test material No. 16 was outside the range which Ti content prescribes | regulates by this invention, the target tensile strength and stretch flangeability were not obtained.

Claims (6)

A hot-dip galvanized steel sheet provided with a hot-dip galvanized layer on the surface of the steel sheet,
The steel sheet is, by mass%, C: more than 0.070% and less than 0.15%, Si: 0.001% to 0.40%, Mn: more than 2.2% and 3.5% or less, P: 0 0.05% or less, S: 0.01% or less, sol.Al: 0.001% or more and 0.40% or less, Ti: 0.12% or more and 0.25% or less, B: more than 0.0025% Having a chemical composition containing 010% or less and N: 0.01% or less,
The hot dip galvanized steel sheet has mechanical properties such that the tensile strength (TS) is 980 MPa or more and the hole expansion rate (HER) defined by the following formula (1) is 40% or more. Plated steel sheet.
HER = (D _h −D ₀ ) / D ₀ × 100 (1)
Here, D ₀ is an initial hole diameter (mm), D _h is a hole diameter (mm) after fracture, and a circular punched hole having a diameter of 10 mm (= D ₀ ) punched with a clearance of 12.5% is used. The diameter of the punched hole when a crack that penetrates in the thickness direction at the edge of the punched hole is expanded by a cylindrical flat bottom punch (diameter: 33 mmφ, shoulder R: 3 mm) so that the burr is on the die side (= D _h ).   When the chemical composition is mass%, Nb: 0.5% or less, V: 0.5% or less, Cr: 0.5% or less, Mo: 0.1% or less, Cu: 0.5% or less, and Ni The galvanized steel sheet according to claim 1, further comprising one or more selected from the group consisting of 0.5% or less.   The chemical composition may be one selected from the group consisting of Ca: 0.01% or less, Mg: 0.01% or less, REM: 0.01% or less, and Zr: 0.01% or less in terms of mass%. The hot-dip galvanized steel sheet according to claim 1 or 2, further comprising two or more kinds.   The hot dip galvanized steel sheet according to any one of claims 1 to 3, wherein the chemical composition further contains, by mass%, Bi: 0.05% or less. A method for producing a hot-dip galvanized steel sheet comprising the following steps (A) to (C):
(A) The steel material having the chemical composition according to any one of claims 1 to 4 is hot-rolled at a temperature of 1100 ° C to 1300 ° C and hot in a temperature range of 800 ° C to 1000 ° C. A hot rolling step in which rolling is completed and wound into a hot rolled steel sheet in a temperature range of 400 ° C. or higher and 750 ° C. or lower;
(B) Pickling and cold rolling step of pickling and cold rolling the hot-rolled steel sheet to obtain a cold-rolled steel sheet; and (C) Heating the cold-rolled steel sheet to Ac ₃ point or more and 950 ° C or less. The average cooling rate from 750 ° C. to 580 ° C. is set to 1.0 ° C./second or more and 50 ° C./second or less, and is cooled to a temperature range of 400 ° C. or more and 560 ° C. or less. A continuous hot-dip galvanizing step in which a hot-dip galvanized steel sheet is maintained for 10 seconds or more and 500 seconds or less, including when immersed in a plating bath, in a temperature range of 600 ° C. or lower.   The method for producing a hot dip galvanized steel sheet according to claim 5, wherein in the step (C), the hot dip galvanized steel sheet immersed in the plating bath is subjected to an alloying treatment in a temperature range of 430 ° C or higher and 600 ° C or lower. .

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