JP2012219328A - Galvannealed steel sheet and method for manufacturing the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a high-strength hot-dip galvanized steel sheet that can be stably manufactured by a mass-producible method and has a high strength of 980 MPa or more and excellent stretch-flanging property.SOLUTION: The base steel sheet of a galvannealed steel sheet has a chemical composition containing >0.08 and ≤0.15 mass% C, >0.001 and ≤1.5 mass% Si, >2.2 and ≤3.5 mass% Mn, ≤0.02 mass% P, ≤0.01 mass% S, ≥0.001 and ≤0.40 mass% sol. Al, ≥0.015 and ≤0.060 mass% Ti, >0.0015 and ≤0.010 mass% B, and ≤0.01 mass% N, and has a steel microstructure including, by area, <5% ferrite and <0.5% non-recrystallization ferrite and <5% martensite with a particle size of ≤1.0 μm.

Description

  The present invention relates to an galvannealed 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.

  In recent years, there has been a demand for improving the fuel efficiency of automobiles for the purpose of protecting the global environment. Therefore, there is an increasing need for high-strength steel sheets that can reduce the weight of vehicle bodies while ensuring the safety of passengers. In particular, the weight reduction of the automobile frame member contributes greatly to the weight reduction of the vehicle body. Therefore, the steel sheet used for the vehicle frame member is a high-strength steel sheet having a tensile strength of 980 MPa or more, especially rust prevention. There is a growing need for high-strength galvannealed steel sheets that can be applied to members that require high strength.

  Steel sheets used for automobile framework members are required to satisfy not only high tensile strength but also various performances required at the time of member molding, such as press formability, weldability, and plating adhesion. Among them, since stretch flange molding is frequently used in the molding process of automobile frame parts such as rockers and pillars, it is necessary to have excellent stretch flangeability.

Accordingly, there is a need for a high-strength galvannealed steel sheet that has excellent tensile flangeability while having a tensile strength of 980 MPa or more.
However, the tensile strength and the stretch flangeability are generally in a trade-off relationship, and the stretch flangeability is significantly lowered as the tensile strength increases. 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 galvannealed steel sheet is generally manufactured by continuous 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. And having a characteristic temperature history of reheating and alloying. 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 plate whose chemical composition is adjusted so as to ensure high tensile strength, a temperature range of 400 ° C. or higher and 500 ° C. or lower is a temperature range in which bainite transformation essentially proceeds. Therefore, the bainite transformation proceeds by being once held in the temperature range in the cooling process from the soaking temperature. However, since the time for which the temperature range is maintained is short, in the high-strength steel containing a large amount of Mn and B, the bainite transformation is not completed and C discharged from the transformed bainite is not transformed. Thicken to austenite. When cooling from such a state to room temperature is performed, the austenite enriched with C becomes hard martensite. Hard martensite promotes non-uniform deformation and induces cracks in stretch flange forming. Therefore, when such a structure is formed, it becomes extremely difficult to ensure excellent stretch flangeability. In particular, the conventional alloyed hot-dip galvanized steel sheet having a tensile strength of 980 MPa or more has a metal structure having ferrite as a main phase as shown in FIG. 1, so that the strain is at the interface between ferrite and hard martensite. Concentrated and stretch flangeability was poor.

  Moreover, in the manufacturing process in the continuous hot dip galvanizing equipment, the cooling rate from the soaking temperature is usually about 1 to 30 ° C./second, which is smaller than the cooling rate employed in the manufacturing process in the continuous annealing equipment. For this reason, producing a high-strength galvannealed steel sheet having a tensile strength of 980 MPa or more is not easy in itself. Therefore, it is necessary to add a large amount of reinforcing elements such as Nb, V, Cr, Mo, Cu and Ni. However, as will be described later, the addition of these elements makes rolling, particularly cold rolling after hot rolling, very difficult.

  Thus, it is a very difficult technical problem to provide a high-strength galvannealed steel sheet that has excellent tensile flangeability while having a tensile strength of 980 MPa or more, but several techniques have been proposed so far. Has been.

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, for a cold-rolled steel sheet having a specific chemical composition, a temperature range of 580 to 750 ° C. is heated at 0.8 ° C./second or more, and after annealing at 750 ° C. or more and 900 ° C. or less, A method for producing a high-strength hot-dip galvanized steel sheet that is cooled to (zinc plating bath temperature −40) ° C. to (zinc plating bath temperature +50) ° C. and then immersed in a zinc plating bath and cooled to room temperature is disclosed.

In Patent Document 2, 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.

In Patent Document 3, 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./s up to 600 ° C. After cooling at the above speed, and then pickling, heat treatment is performed at a temperature A ₁ (° C.) or less, 500 ° C. or more defined by the contents of Si, Mn and Ni in the steel composition, and then hot dip galvanizing The manufacturing method of the hot dip galvanized cold-rolled steel plate which processes is disclosed.

Patent Document 4 discloses that austenite is prepared by appropriately adjusting the alloy elements and by strongly cooling to a temperature range of (M _s −100 ° C.) to (M _s −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.

JP 2007-231369 A JP 2009-263686 A Japanese Patent Laid-Open No. 2004-211126 JP 2009-203548 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. Not a thing.

  The technology disclosed in Patent Document 1 is difficult to apply to mass production technology. That is, in the technique disclosed in Patent Document 1, in order to ensure a tensile strength of 980 MPa or more for the hot dip galvanized steel sheet, not only Ti and B but also Mo which is a strengthening element is added in a large amount. 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 0.1% or more of Mo is added to the steel containing Ti and B in this way, the hot-rolled sheet, which is the original sheet of the hot-dip galvanized steel sheet, is remarkably hardened and cold rolling becomes difficult. For this reason, not only the productivity is lowered, but also the thickness accuracy of the hot dip galvanized steel sheet is remarkably deteriorated. Therefore, application of the technology disclosed in Patent Document 1 to mass production technology is not realistic.

The technique disclosed in Patent Document 2 lacks material stability. That is, Patent Document 2 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, the maximum heating temperature is actually set to Ac ₃ ° C. What is annealed below, that is, annealed at a two-phase region temperature. The technique disclosed in Patent Document 2 has a chemical composition in which a small amount of Ti is added. However, when steel with a small amount of Ti is annealed at a two-phase temperature, unrecrystallized ferrite remains. Resulting in. Such unrecrystallized ferrite significantly increases the tensile strength, but it is extremely difficult to control the fraction. Therefore, it becomes difficult to ensure a stable tensile strength and stretch flange. 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.

The technique disclosed in Patent Document 3 lacks material stability and is difficult to apply to mass production techniques. That is, when a high-temperature heat treatment is performed on a steel plate that has been heat-treated in a continuous annealing furnace that generally has a rapid cooling process, the strength is reduced. In particular, in the technique disclosed in Patent Document 3 where heat treatment is performed in a temperature range of 500 ° C. or more and A ₁ point or less where diffusion of Mn becomes active, such a tendency becomes remarkable, and tensile strength associated with fluctuations in heat treatment temperature. The fluctuation of becomes remarkable. Therefore, the technique disclosed in Patent Document 3 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 again in a high temperature range after recrystallization annealing is inferior in productivity, application to mass-production technology is not realistic.

The technology disclosed in Patent Document 4 is difficult to apply to mass production technology. That is, in the technique disclosed in Patent Document 4, since the steel sheet is rapidly cooled to a temperature range below the M _s point after soaking, the flatness of the steel sheet is significantly deteriorated. Therefore, the subsequent hot dip galvanizing process becomes difficult, and non-plating and appearance irregularities are scattered. Therefore, application of the technology disclosed in Patent Document 4 to mass production technology is not realistic.

  As described above, several techniques have been proposed for providing a high-strength hot-dip galvanized steel sheet having excellent stretch flangeability while having a tensile strength of 980 MPa or more, all of which are productivity and / or material stability. It was not enough.

  As described above, the present invention is 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 stably mass-produce in the past. The purpose is to provide.

  Here, “excellent stretch flangeability” means having a mechanical property that the hole expansion ratio (HER) defined by the Japan Iron and Steel Federation standard JFS T 1001-1996 is 40% or more.

  The present inventor has intensively studied to solve the above-mentioned problems, and the contents of C, Si, Mn, Ti and B are extremely limited with respect to the chemical composition of the steel sheet which is the plating base material of the galvannealed steel sheet. In addition, the optimum production conditions for steel of the chemical composition are applied and the metallographic structure is made extremely uniform, so that it is difficult to produce with a conventional technique. We obtained new knowledge that high strength alloyed hot dip galvanized steel sheet with high tensile strength and excellent stretch flangeability can be manufactured stably.

The present invention has been made on the basis of the above new findings, and the gist thereof is as follows.
(1) An alloyed hot-dip galvanized steel sheet provided with an alloyed hot-dip galvanized layer on the surface of the steel sheet, wherein the steel sheet is in mass%, C: more than 0.08%, 0.15% or less; Si: 0 More than 0.001%, 1.5% or less; Mn: more than 2.2%, 3.5% or less; P: 0.02% or less; S: 0.01% or less; Al: not less than 0.001% and not more than 0.40%; Ti: not less than 0.015% and not more than 0.060%; B: more than 0.0015% and not more than 0.010%; and N: not more than 0.01% And a steel structure having an area% of ferrite: less than 5%, non-recrystallized ferrite: less than 0.5%, and martensite having a grain size of 1.0 μm or less: less than 5% The alloyed hot-dip galvanized steel sheet has mechanical properties having a tensile strength (TS) of 980 MPa or more.

  (2) The chemical composition is mass%, Nb: 0.03% or less, V: 0.03% or less, Cr: 0.8% or less, Mo: 0.1% or less, Cu: 0.4% The alloyed hot-dip galvanized steel sheet according to the above (1), further comprising one or more selected from the group consisting of the following and Ni: 0.4% 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 alloyed hot-dip galvanized steel sheet according to (1) or (2) above, further comprising one or more kinds.

(4) The alloyed 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 an galvannealed steel sheet comprising the following steps (A) to (C):
(A) A steel material having the chemical composition according to any one of (1) to (4) above is hot-rolled at a temperature of 1100 ° C. or higher and 1300 ° C. or lower, and a temperature of 800 ° C. or higher and 1000 ° C. or lower. A hot rolling step in which hot rolling is completed in a region and wound in a temperature region of 400 ° C. or higher and 680 ° C. or lower to form a hot rolled steel sheet;
(B) A pickling / cold rolling step in which the hot-rolled steel sheet is pickled and cold-rolled to form a cold-rolled steel sheet; and (C) the cold-rolled steel sheet is defined by the following formula (i): The average cooling rate in the temperature range of 580 ° C. or more and 800 ° C. or less is 3 ° C./second or more after being held in the temperature range of T (° C.) or more and 810 ° C. or more and 950 ° C. or less for 5 seconds or more and 150 seconds or less, Cooling to a temperature range of 400 ° C. or more and 560 ° C. or less as 100 ° C./second or less, and subsequently 25 seconds or more including the time of plating bath immersion and alloying treatment in the temperature range of 400 ° C. or more and 580 ° C. or less, Hold for 500 seconds or less, perform alloying treatment in a temperature range of 500 ° C. or higher and 580 ° C. or lower, hold in a temperature range of 300 ° C. or higher and 500 ° C. or lower for 20 seconds or longer and 200 seconds or shorter, and cool to room temperature. Alloying hot dip zinc Continuous hot dip galvanizing process for making steel sheets.

T = 910−203 × (C ^0.5 ) −15.2 × Ni + 44.7 × Si + 104 × V +
31.5 * Mo-30 * Mn-11 * Cr-20 * Cu + 700 * P +
400 × Al + 50 × Ti (i)
Here, the element symbol in a formula shows content (unit: mass%) of each element in the chemical composition of the said steel plate material.

  According to the present invention, it is possible to stably supply a high-strength galvannealed steel sheet having excellent stretch flangeability while having an extremely high tensile strength of 980 MPa or more by a method capable of mass production. The alloyed hot-dip galvanized steel sheet according to the present invention can be widely used industrially, 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.

The electron micrograph which shows the metal structure of the conventional high intensity | strength galvannealed steel plate with a tensile strength of 980 MPa.

Hereinafter, modes for carrying out the present invention will be described.
1. Chemical Composition First, the reason why the chemical composition of the steel sheet that is the plating base of the galvannealed steel sheet according to the present invention is defined as described above will be described. In the following description, “%” representing the content of each element means mass% unless otherwise specified.

(C: more than 0.08%, less than 0.15%)
C is an element having an effect of increasing the strength of the steel sheet. If the C content is 0.08% or less, it becomes difficult to ensure a tensile strength of 980 MPa or more. Therefore, the C content is more than 0.08%. On the other hand, if the C content exceeds 0.15%, a structure containing hard martensite is formed, and the stretch flangeability is significantly deteriorated. Therefore, the C content is 0.15% or less. From the viewpoint of improving productivity by reducing the load during cold rolling, the C content is preferably set to 0.13% or less. More preferably, it is 0.11% or less.

(Si: more than 0.001%, 1.5% or less)
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 0.001% or less, it is difficult to obtain the above effect. Therefore, the Si content is more than 0.001%. When the Si content is 0.05% or more, the plating adhesion is further improved. Therefore, the Si content is preferably 0.05% or more. From the viewpoint of improving ductility, the Si content is preferably 0.5% or more. On the other hand, when the Si content exceeds 1.5%, the plating wettability is remarkably lowered, and non-plating occurs frequently. Therefore, the Si content is 1.5% or less.

(Mn: over 2.2%, up to 3.5%)
Mn is an element having an effect of increasing the strength of the steel plate. If the Mn content is 2.2% or less, it becomes difficult to ensure a tensile strength of 980 MPa or more. Therefore, the Mn content is over 2.2%. When the Mn content is 2.3% 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. Therefore, the Mn content is preferably 2.3% or more. On the other hand, if the Mn content exceeds 3.5%, the hot-rolled sheet is markedly cured, and the thickness accuracy is deteriorated. Therefore, the Mn content is 3.5% or less. From the viewpoint of improving productivity by reducing the load during cold rolling, the Mn content is preferably 3.1% or less. More preferably, it is 2.8% or less.

(P: 0.02% or less)
In general, P is an impurity inevitably contained in steel, but may be positively contained because it has an action of increasing the strength of the steel sheet by solid solution strengthening. However, when the P content exceeds 0.02%, the weldability is significantly deteriorated. Therefore, the P content is 0.02% or less. The P content is preferably 0.012% or less. In order to obtain the above action more reliably, the P content is preferably 0.003% 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. When the S content exceeds 0.01%, the weldability is significantly reduced. Therefore, the S content is 0.01% or less. The S content is preferably 0.003% or less, more preferably 0.0015% or less.

(Sol.Al: 0.001% or more, 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: 0.015% or more, 0.060% or less)
Ti is an important element in the present invention. Fine precipitates that are carbides, nitrides, or carbonitrides are formed in the steel, and by including an appropriate amount of Ti, the ferrite transformation is significantly reduced. It becomes possible to suppress, and the tensile strength of a steel plate is raised remarkably. And it is excellent elongation while having extremely high tensile strength of 980 MPa or more by strictly defining C content, Mn content and B content, and further combining continuous hot dip galvanizing treatment conditions as described later It becomes possible to obtain a high-strength galvannealed steel sheet having flangeability. If the Ti content is less than 0.015%, ferrite transformation cannot be suppressed, and it is difficult to ensure a tensile strength of 980 MPa or more. Therefore, the Ti content is set to 0.015% or more. Preferably it is 0.020% or more, More preferably, it is 0.030% or more. On the other hand, even if the Ti content exceeds 0.060%, ferrite transformation is promoted and hard martensite is formed, and the deterioration of stretch flangeability becomes remarkable. Therefore, the Ti content is set to 0.060% or less. Preferably it is 0.055% or less, More preferably, it is 0.050% or less.

(B: more than 0.0015%, 0.010% or less)
B is also an important element in the present invention, and has an effect of increasing the strength of the steel sheet. By containing an appropriate amount of B, ferrite transformation is suppressed and a tensile strength of 980 MPa or more is secured. It becomes possible to remarkably suppress the fluctuation of the tensile strength accompanying the fluctuation of the content. That is, the material stability is improved. When the B content is 0.0015% 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 secured. It becomes difficult. Therefore, the B content is more than 0.0015%. Preferably it is 0.000020% or more. 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. Preferably it is 0.0050% 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. When the N content is more than 0.01%, the stretch flangeability is remarkably lowered. Therefore, the N content is 0.01% or less. Preferably it is 0.006% or less.

(Nb: not more than 0.03%, V: not more than 0.03%, Cr: not more than 0.8%, Mo: not more than 0.1%, Cu: not more than 0.4% and Ni: not more than 0.4% 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 0.03%, hot rolling and cold rolling become difficult. Even if Cr exceeds 0.8%, Mo exceeds 0.1%, and Cu and Ni each exceed 0.4%, the effect of the above action is not It becomes saturated and economically disadvantageous, and hot rolling and cold rolling become difficult. 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 at least one of 005% or more and Ni: 0.005% or more.

(One 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 stretch flangeability. 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. Accordingly, the content of each element is as described above. In order to obtain the effect of the above action more reliably, the content of at least one of these elements is preferably set to 0.0003% or more.

  Here, REM refers to 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, Bi may be included. However, when Bi is contained in an amount exceeding 0.05%, 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 | operation more reliably, it is preferable to make Bi content 0.0003% or more.

2. Steel structure Next, the steel structure of the steel sheet which is the plating base material of the galvannealed steel sheet according to the present invention will be described.

(Area ratio of ferrite: less than 5%)
When the area ratio of ferrite is 5% or more, it is difficult to obtain the desired stretch flangeability in a region where the tensile strength is 980 MPa or more. Therefore, the area ratio of ferrite is less than 5% (including the case of 0%). This ferrite means recrystallized ferrite and does not include unrecrystallized ferrite described below.

(Area ratio of non-recrystallized ferrite: less than 0.5%)
“Non-recrystallized ferrite” refers to a ferrite phase elongated in the rolling direction as confirmed by microscopic observation.

  When the area ratio of non-recrystallized ferrite is 0.5% or more, it is difficult to obtain the desired stretch flangeability in a region where the tensile strength is 980 MPa or more. Therefore, the area ratio of non-recrystallized ferrite is set to less than 0.5% (including the case of 0%).

(Area ratio of martensite having a particle size of 1.0 μm or less: less than 5%)
If the area ratio of martensite having a particle size of 1.0 μm or less is 5% or more, it is difficult to obtain the desired stretch flangeability in a region where the tensile strength is 980 MPa or more. Therefore, the area ratio of the martensite is less than 5% (including 0%). Here, the particle size of martensite is a circle-equivalent diameter determined from the area of each martensite particle. The area ratio of the martensite is preferably less than 4.5%.

  The method for measuring the area ratio of each phase in the steel structure described above is well known to those skilled in the art, and can also be measured by a conventional method in the present invention. As will be shown later in the examples, these area ratios are measured in cross sections in both the rolling direction and the direction perpendicular to the rolling direction, and are obtained as an average value.

3. Alloyed hot-dip galvanized layer The chemical composition of the alloyed hot-dip galvanized layer of the alloyed hot-dip galvanized steel sheet according to the present invention is not particularly limited, but preferably satisfies the following conditions.

(Fe: 8% by mass or more and 15% by mass or less)
By setting the Fe content in the alloyed hot-dip galvanized layer to 8% by mass or more, formation of a soft portion in the surface layer portion of the plated layer after the alloying treatment is suppressed, slidability is increased, and the plated layer becomes Flakes-like peeling due to peeling from the interface with the steel plate as the substrate 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, 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 make the tensile strength 980 MPa or more. Accordingly, 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, in the alloying process, Si, Mn, P, S, Ti, Nb, V, Cr, Mo, Cu, Ni, B, Ca, Mg, Zr, REM, Bi are used from the base material. However, there is no problem because the plating quality is not adversely affected 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. 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 plate penetration temperature of 400 ° C. or higher and 500 ° C. or lower, an alloying temperature of 500 ° C. or higher, 600 It is below ℃.

4). Manufacturing Method Next, a preferable manufacturing method of the galvannealed steel sheet according to the present invention having the above characteristics will be described.

(A) Hot rolling process It is preferable to melt the molten steel having the above-described chemical 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 consists of a rough rolling process and a finish rolling process, if the temperature of the full length is uniformed by induction heating or the like for the rough bar after the rough rolling and before the finish rolling, the characteristic fluctuation will occur. Can be suppressed, which is preferable.

(Temperature of steel used for hot rolling: 1100 ° C or higher and 1300 ° C or lower)
The temperature of the steel material used for hot rolling is 1100 ° C. or higher and 1300 ° C. or lower.
The alloyed 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 holding 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 in order to make Ti or the like more solid solution, more preferably 30 minutes More preferably, the above is used. 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, 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 properties of the alloyed hot-dip galvanized steel sheet deteriorate significantly. Accordingly, the rolling completion temperature is set to 1000 ° C. or less.

(Winding temperature: 400 ° C or higher, 680 ° C or lower)
The coiling temperature is 400 ° C or higher and 680 ° 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 exceeds 680 ° C., the grain boundary oxidation becomes remarkable, and the surface property of the hot dip galvanized steel sheet is significantly deteriorated. Accordingly, the coiling temperature is 680 ° C. or lower. Preferably it is 600 degrees C or less.

(B) Pickling / cold rolling process The hot-rolled steel sheet obtained in the hot-rolling process is subjected to pickling by a conventional method, and then subjected to cold rolling to form a cold-rolled steel sheet.

  When the shape is corrected by performing mild rolling of about 0 to 5% before or after pickling, it is advantageous in terms of ensuring flatness, which is preferable. Moreover, pickling is improved by performing mild rolling before pickling, and the removal of the surface concentrating element is promoted, and the 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. Furthermore, Ti and B significantly affect the nucleation and rate of ferrite transformation. On the other hand, Mn and B also affect the rate of bainite transformation, that is, the formation process of hard martensite-like structures. Therefore, the conditions in the continuous hot dip galvanizing process are extremely important to ensure high tensile strength and excellent stretch flangeability with the steel structure as a suitable range. Therefore, it is preferable to perform the treatment under the following conditions of the continuous hot dip galvanizing treatment from the viewpoint of obtaining a steel plate having the intended performance.

(T (° C.) or higher, 810 ° C. or higher and 950 ° C. or lower, maintained for 5 seconds or more and 150 seconds or less)
The soaking process is performed by holding in a temperature range of T (° C.) or higher and 810 ° C. or higher and 950 ° C. or lower defined by the following formula (i) for 5 seconds or more and 150 seconds or less.

T = 910−203 × (C ^0.5 ) −15.2 × Ni + 44.7 × Si + 104 × V +
31.5 * Mo-30 * Mn-11 * Cr-20 * Cu + 700 * P +
400 × Al + 50 × Ti (i)
The element symbol in the above formula indicates the content (unit: mass%) of each element in the chemical composition of the steel sheet material.

  If the soaking temperature is less than T (° C.) or less than 810 ° C., a large amount of unrecrystallized material remains, the tensile strength of the steel sheet is not stable, and stretch flangeability may deteriorate. Accordingly, the soaking temperature is set to T (° C.) or more and 810 ° C. or more. On the other hand, when the soaking temperature exceeds 950 ° C., damage to the annealing furnace becomes obvious and productivity decreases. Therefore, the soaking temperature is 950 ° C. or less. Preferably it is 880 degrees C or less.

  If the soaking time is less than 5 seconds, it becomes difficult to control the temperature in the continuous hot dip galvanizing process, and it becomes difficult to ensure a stable tensile strength. Therefore, the soaking time is 5 seconds or more. On the other hand, when the soaking time exceeds 150 seconds, not only the productivity is lowered, but also the grain boundary oxidation becomes remarkable, and the surface properties of the galvannealed steel sheet may be deteriorated.

  In the heating to the soaking temperature, the average heating rate in the temperature range of 100 ° C. or more and 810 ° C. or less is preferably 1 ° C./second or more and 50 ° C./second or less. High productivity can be maintained by setting the average heating rate to 1 ° C./second or more. Further, the soaking temperature can be easily controlled by setting the average heating rate to 50 ° C./second or less.

(The average cooling rate in the temperature range of 580 ° C. or higher and 800 ° C. or lower is set to 3 ° C./second or higher and 100 ° C./second or lower to cool to a temperature range of 400 ° C. or higher and 560 ° C. or lower)
After the soaking, the average cooling rate in the temperature range of 580 ° C. or higher and 800 ° C. or lower is set to 3 ° C./second or higher and 100 ° C./second or lower to cool to a temperature range of 400 ° C. or higher and 560 ° C. or lower. Cooling in a temperature range of 580 ° C. or higher and 800 ° C. or lower is important in order to suppress the ferrite transformation and secure a target steel structure. If the average cooling rate in the above temperature range is less than 3 ° C./second, the ferrite transformation proceeds, making it difficult to achieve both high tensile strength and excellent stretch flangeability. Therefore, the average cooling rate in the temperature range is set to 3 ° C./second or more. Preferably, it is 10 ° C./second or more. On the other hand, when the average cooling rate in the temperature range exceeds 100 ° C./second, uneven cooling occurs in the width direction of the coil, and the flatness of the steel sheet is significantly deteriorated. Therefore, the average cooling rate in the temperature range is set to 100 ° C./second or less. Preferably it is 50 degrees C / sec or less.

  In the present invention, since Mn is contained in a large amount and B is further contained, the bainite transformation is remarkably suppressed. For this reason, the condition after cooling becomes important in order to make the area ratio of the extremely hard martensite having a particle diameter of 1.0 μm or less appropriate.

  When the cooling stop temperature is lower than 400 ° C., heat removal during immersion in 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., the bainite transformation does not proceed sufficiently, and as a result, the martensite is excessively formed, and the stretch flangeability may be deteriorated. Therefore, the cooling stop temperature is set to 560 ° C. or lower. In addition, in hot dip galvanization, it is performed by immersing in a hot dip galvanizing bath at 400 ° C. or higher and 490 ° C. or lower according to a conventional method.

(Hold for 25 seconds or more and 500 seconds or less in the temperature range of 400 ° C or more and 580 ° C or less, including immersion in the plating bath and alloying treatment)
After the cooling, a hot dip galvanizing process is performed, and an alloying process is further performed. Under the present circumstances, it hold | maintains at the temperature range of 400 degreeC or more and 580 degrees C or less including the time of plating bath immersion and the time of alloying treatment for 25 seconds or more and 500 seconds or less. When the holding time in the temperature range of 400 ° C. or higher and 580 ° C. or lower is less than 25 seconds, the bainite transformation does not proceed sufficiently, and as a result, hard martensite is excessively formed and stretch flangeability deteriorates. There is. Therefore, the holding time is set to 25 seconds or longer. On the other hand, if the holding time exceeds 500 seconds, the bainite transformation may proceed excessively and the tensile strength may be significantly reduced. Accordingly, the holding time is 500 seconds or less. From the viewpoint of productivity, the holding time is preferably 300 seconds or less.

(Alloying in a temperature range of 500 ° C or higher and 580 ° C or lower)
The alloying treatment temperature when the alloying treatment is performed after immersion in the plating bath is 500 ° C. or more and 580 ° C. or less. When the alloying treatment temperature is less than 500 ° C., martensite is excessively formed, and stretch flangeability may be deteriorated. Therefore, the alloying treatment temperature is set to 500 ° C. or higher. Preferably it is 510 degreeC or more. On the other hand, when the alloying treatment temperature exceeds 580 ° C., the tensile strength may be significantly reduced. Therefore, the alloying temperature is 580 ° C. or lower. Preferably it is 530 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.

(Hold for 20 seconds or more and 200 seconds or less in a temperature range of 300 ° C or more and 500 ° C or less)
After the alloying treatment, the temperature is maintained at 300 ° C. or more and 500 ° C. or less for 20 seconds or more and 200 seconds or less. The reason why the above temperature range is maintained after the alloying treatment is that the bainite transformation is gradually advanced again to suppress the formation of martensite. When the holding time in the above temperature range is less than 20 seconds, the bainite transformation cannot proceed appropriately, and martensite is excessively generated, and the stretch flangeability may deteriorate. Accordingly, the holding time in the temperature range is set to 20 seconds or longer. On the other hand, if the holding time in the above temperature range exceeds 200 seconds, alloying may proceed excessively and the adhesion of plating may be significantly reduced. Accordingly, the holding time in the above temperature range is 200 seconds 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. By temper rolling, the yield point elongation is suppressed and the yield strength is adjusted.

The present invention will be described more specifically with reference to examples.
1. Production of Steel Plate for Evaluation Steel having the chemical composition shown in Table 1 was melted in a converter and made into a 245 mm thick slab 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 (rolling ratio 54%).

  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, after heating and soaking under the conditions shown in Table 3, cooling and holding at the cooling stop temperature for a predetermined time (holding time before immersion) for 4 seconds to 460 ° C., which is an assumed plating bath temperature. Cooled and held at 460 ° C. for 2 seconds. Subsequently, the alloying treatment temperature shown in Table 3 was heated for 4 seconds, and held at each alloying treatment temperature for 5 seconds so as to simulate the alloying treatment, and then a temperature of 300 ° C. or higher and 500 ° C. or lower. The region was held for the time shown in Table 3, and further 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 evaluation steel sheets.

  The annealed cold-rolled steel sheet produced in this example is not hot-dip galvanized, but receives the same thermal history as the alloyed hot-dip galvanized steel sheet, so the mechanical properties and steel structure of the steel sheet have the same thermal history. It is substantially the same as the alloyed hot-dip galvanized steel sheet.

2. Evaluation method The steel structure was analyzed with respect to the annealed cold-rolled steel sheet obtained under various production conditions, and a tensile test, a bending test, and a stretch flange test were performed, and each mechanical characteristic was evaluated. The method of each evaluation is as follows.

(Ferrite area ratio)
Test specimens were taken from the rolling direction of each annealed cold-rolled steel sheet and the direction perpendicular to the rolling direction, and the structure of the cross section in the rolling direction and the cross section perpendicular to the rolling direction (etched by a conventional method) was observed with a scanning electron microscope. The area of 8 mm ² was photographed and the area ratio of ferrite was investigated by image analysis. The numerical value is shown as an average of the measured value in the rolling direction and the measured value in the direction perpendicular to the rolling direction.

(Area ratio of non-recrystallized ferrite)
Test pieces were taken from the direction perpendicular to the rolling direction and the rolling direction of the annealed cold-rolled steel sheet was observed by the same manner as described above electron microscopy tissue perpendicular cross section to the rolling direction cross-section and the rolling direction, 8 mm ² This area was photographed, and the area ratio of unrecrystallized ferrite was investigated by image analysis. The numerical value is similarly shown as an average of the measured value in the rolling direction and the measured value in the direction perpendicular to the rolling direction.

(Area ratio of martensite with a particle size of 1.0 μm or less)
Test pieces were taken from the direction perpendicular to the rolling direction and the rolling direction of the annealed cold-rolled steel sheet was observed by the same manner as described above electron microscopy tissue perpendicular cross section to the rolling direction cross-section and the rolling direction, 8 mm ² This area was photographed, and the area ratio of martensite having a particle size of 1.0 μm or less was examined by image analysis. The numerical value is similarly shown as an average of the measured value in the rolling direction and the measured value in the direction perpendicular to the rolling direction.

(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 the tensile strength (TS) and total elongation (El) were measured.

(Stretch flange test)
About each annealing cold-rolled steel plate, the hole expansion rate (HER) was measured by the hole expansion test method prescribed | regulated by Japan Iron and Steel Federation specification JFS T1001-1996.

3. Evaluation results Table 4 shows the results of the above evaluations.

  The numerical values underlined in Tables 1 to 4 indicate that the content, conditions, or mechanical properties indicated by the numerical values are outside the scope of the present invention. Sample No. in Table 3 Since the soaking temperature of 12 is lower than the value of T of the steel material F shown in Table 1, it is outside the scope of the present invention.

Sample No. in Table 4 2, 5, 9, 11, 13, 15, 19, and 21 are steel sheets of inventive examples that satisfy all the conditions of the present invention.
On the other hand, the test material No. In Nos. 1 and 18, since the chemical composition deviated from the range specified in the invention, the area ratio of ferrite was 5% or more, and the intended tensile strength was not obtained.

  Specimen No. Since 3, 4, 6 and 14 were out of the range specified in the present invention, the area ratio of martensite having a particle size of 1.0 μm or less was 5% or more, and the stretch flangeability was poor.

Specimen No. Nos. 7, 10 and 22 did not achieve the intended tensile strength because the manufacturing conditions were outside the range defined in the present invention.
Specimen No. In Nos. 8 and 23, the chemical composition was out of the range specified in the invention, so the intended tensile strength could not be obtained.

Specimen No. In Nos. 12 and 16, the manufacturing conditions were outside the range specified in the present invention, so the area ratio of non-recrystallized ferrite was 0.5% or more, and the stretch flangeability was poor.
Specimen No. In No. 17, since the chemical composition was outside the range specified in the invention, the area ratio of martensite having a particle size of 1.0 μm or less was 5% or more, and the stretch flangeability was poor.

Specimen No. In No. 20, since the manufacturing conditions were outside the range defined in the present invention, the area ratio of ferrite was 5% or more, and the stretch flangeability was poor.
Specimen No. 24, because the chemical composition is outside the range specified by the invention, the area ratio of ferrite is 5% or more, and the area ratio of martensite having a particle size of 1.0 μm or less is 5% or more, and stretch flangeability is improved. It was bad.

Claims (5)

  An alloyed hot-dip galvanized steel sheet provided with an alloyed hot-dip galvanized layer on the surface of the steel sheet, the steel sheet being in mass%, C: more than 0.08%, 0.15% or less, Si: 0.001% More than 1.5%, Mn: more than 2.2%, 3.5% or less, P: 0.02% or less, S: 0.01% or less, sol. Al: not less than 0.001% and not more than 0.40%, Ti: not less than 0.015% and not more than 0.060%, B: more than 0.0015%, not more than 0.010% and N: not more than 0.01% And having a steel structure having a chemical composition containing, in area%, ferrite: less than 5%, non-recrystallized ferrite: less than 0.5%, and martensite having a grain size of 1.0 μm or less: less than 5%, The alloyed hot-dip galvanized steel sheet has mechanical properties having a tensile strength (TS) of 980 MPa or more.   When the chemical composition is mass%, Nb: 0.03% or less, V: 0.03% or less, Cr: 0.8% or less, Mo: 0.1% or less, Cu: 0.4% or less, and Ni The alloyed hot-dip galvanized steel sheet according to claim 1, further comprising one or more selected from the group consisting of 0.4% 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 galvannealed steel sheet according to claim 1 or 2, further comprising two or more kinds.   The alloyed 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 an alloyed 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 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 in a temperature range of 400 ° C. or higher and 680 ° C. or lower to obtain a hot rolled steel sheet;
(B) A pickling / cold rolling step in which the hot-rolled steel sheet is pickled and cold-rolled to form a cold-rolled steel sheet; and (C) the cold-rolled steel sheet is defined by the following formula (i): T (° C.) or higher and 810 ° C. or higher and held in a temperature range of 950 ° C. or lower for 5 seconds or longer and 150 seconds or shorter, and then the average cooling rate in the temperature range of 580 ° C. or higher and 800 ° C. or lower is 3 ° C./second As described above, cooling to 100 ° C./second or lower to a temperature range of 400 ° C. or higher and 560 ° C. or lower, followed by 25 seconds including the time of plating bath immersion and alloying treatment in the temperature range of 400 ° C. or higher and 580 ° C. or lower. As above, hold for 500 seconds or less, perform alloying treatment in a temperature range of 500 ° C. or more and 580 ° C. or less, and further hold in a temperature range of 300 ° C. or more and 500 ° C. or less for 20 seconds or more and 200 seconds or less until room temperature Cool to alloy and melt Continuous hot dip galvanizing process for galvanized steel sheet.
T = 910−203 × (C ^0.5 ) −15.2 × Ni + 44.7 × Si + 104 × V +
31.5 * Mo-30 * Mn-11 * Cr-20 * Cu + 700 * P +
400 × Al + 50 × Ti (i)
Here, the element symbol in a formula shows content (unit: mass%) of each element in the chemical composition of the said steel plate material.