TWI626318B - Method for producing plated steel sheet, hot-dip galvanized steel sheet, and method for producing alloyed hot-dip galvanized steel sheet - Google Patents

Abstract

In the plated steel sheet, the chemical composition is at least C: 0.03% to 0.70%, Si: 0.25% to 2.50%, Mn: 1.00% to 5.00%, P: 0.100% or less, and S: 0.010%. Hereinafter, sol.Al: 0.001% to 2.500, N: 0.020% or less, the remainder is composed of iron and impurities, and the metal structure contains more than 5.0% by volume of residual Worthite iron, and more than 5.0% by volume of tempered Ma Tiansan Iron, and the amount of C in the residual Worthite iron is 0.85 mass% or more.

Description

Method for producing plated steel sheet, hot-dip galvanized steel sheet, and method for producing alloyed hot-dip galvanized steel sheet

FIELD OF THE INVENTION The present invention relates to a plated steel sheet, a method of producing a hot-dip galvanized steel sheet, and an alloyed hot-dip galvanized steel sheet. More particularly, the present invention relates to a high-strength hot-dip galvanized steel sheet which is suitable for press forming such as an automobile body, and which is excellent in uniform ductility and local ductility, and a high-strength alloyed hot-dip galvanized steel sheet and the like.

BACKGROUND OF THE INVENTION The field of industrial technology has been highly specialized today, with special and high performance requirements for materials used in various technical fields. In order to improve the fuel consumption by using the weight of the vehicle body, the steel sheet for automobiles has a high strength. By intensity is meant both the strength of the drop and the tensile strength.

When a high-strength steel sheet is applied to an automobile body, the strength of the steel sheet can be made thin while the weight of the steel sheet is reduced, and the strength required for the vehicle body can be imparted. However, in the press forming for forming an automobile body, the thinner the steel sheet to be used, the more likely it is to be damaged or wrinkled. Therefore, the steel sheet for automobiles also needs to have excellent uniform ductility and local ductility.

In addition, in order to improve the collision safety performance of automobiles, automotive steel sheets are required to have excellent impact absorption. From the viewpoint of impact absorption, in addition to the higher strength of the steel sheet for automobiles, local ductility must be excellent in order to suppress breakage when subjected to an impact load.

In this way, the demand for automotive steel sheets (1) high strength for the weight reduction of the vehicle body and the improvement of the collision safety, (2) high uniformity for improving the formability, and (3) for improving the formability and improving the collision safety. High local ductility.

However, the uniform ductility and local ductility of the steel sheet are opposite to the high strength of the steel sheet, and it is difficult to satisfy these characteristics at the same time. In addition, there is a requirement for corrosion resistance of automotive steel sheets, but maintaining corrosion resistance makes it more difficult to achieve high ductility and high strength.

So far, proposals have been made to improve the ductility of high-tension cold-rolled steel sheets by using a technique in which metal structures contain residual Worth iron. The steel sheet containing the residual Worthite iron exhibits a large tensile property by undergoing transformation inducing plasticity (TRIP) generated by the transformation of the Worth iron into the granulated iron in the processing.

Patent Documents 1 and 2 disclose a method for producing a high-strength cold-rolled steel sheet by heating a steel sheet containing Si and Mn to a two-phase region of a ferrite-iron-Worstian iron or a single-phase region of a Worthite iron, and then performing Annealing and cooling, and austempering treatment of Worthfield iron maintained at 350 to 500 ° C, stabilizes the Worthite iron. Through these technologies, the strength and ductility can be balanced in the cold-rolled steel sheet.

However, in the production of the hot-dip galvanized steel sheet and the alloyed hot-dip galvanized steel sheet, the general continuous hot-dip galvanizing equipment cannot sufficiently perform the isothermal quenching treatment of the Worthite iron due to the limitation of the temperature and the holding time. Furthermore, since the Worthite iron is easily decomposed in the plating step and the alloying treatment step, it is difficult to ensure the required amount of residual Worth iron.

Patent Document 3 discloses a method for producing a high-strength alloyed hot-dip galvanized steel sheet containing a certain proportion or more of Si and Mn with respect to C, thereby suppressing the transformation of the Worth iron in the alloying treatment, and forming it in the ferrite iron. The metal structure of the residual Worthite iron is mixed. However, there is no consideration at all about the problem of local ductility deterioration in the steel sheet containing the residual Worthite iron in the metal structure.

Patent Document 4 discloses a high-tension hot-dip galvanized steel sheet excellent in ductility, stretch flangeability, and fatigue resistance, which is dispersed in a ferrite iron having an average crystal grain size of 10 μm or less and a tempered granulated iron. Stone and low temperature metamorphic phase. The tempered granulated iron is effective for the improvement of the stretch flangeability and the fatigue resistance. If the tempered granulated iron is finely granulated, these characteristics will be further improved.

However, in order to obtain a metal structure containing tempered granulated iron and residual Worth iron, a heat treatment is required to cause the generation of granulated iron, and tempering of the granulated iron to further obtain the residual Worth iron. Sub-heat treatment, so the productivity is greatly reduced. Further, in the production method described in Patent Document 4, since the secondary heat treatment is performed at a high temperature of Ac ₁ point or higher, the tempered granulated iron is excessively softened, and it is difficult to obtain high strength.

As described above, since strength (falling strength and tensile strength) is opposite to ductility (uniform ductility and local ductility), it is difficult to manufacture steel sheets that sufficiently increase both of them. of.

CITATION LIST Patent Literature Patent Literature 1: Japanese Patent Application Laid-Open No. Hei. No. Hei. No. Hei. Bulletin 2001-192768

SUMMARY OF THE INVENTION Problems to be Solved by the Invention In view of the above technical background, the present invention provides a plated steel sheet which is excellent in uniform ductility and local ductility, has high lodging strength and tensile strength, and is excellent in formability and impact absorbability. A method for producing a hot-dip galvanized steel sheet and a method for producing an alloyed hot-dip galvanized steel sheet are intended.

Means for Solving the Problem The present inventors have intensively studied a method for improving uniform ductility and local ductility while ensuring tensile strength and lodging strength in a hot-dip galvanized steel sheet and an alloyed hot-dip galvanized steel sheet. As a result, the following findings (A) to (E) were sought.

(A) If a low-carbon hot-dip galvanized steel sheet containing Si and Mn or a low-carbon alloyed hot-dip galvanized steel sheet containing Si and Mn is produced by continuous hot-dip galvanizing equipment, uniform ductility and local ductility may be lowered, and even more Sometimes, there is a case where the strength of the fall is also lowered. This is considered to be because, in the continuous hot-dip galvanizing facility, the isothermal quenching treatment of the Worthite iron is insufficient, and a metal structure containing the residual Worth iron having a low C concentration and the loose iron of the hard hemp field is formed.

(B) However, this type of hot-dip galvanized steel sheet and alloyed hot-dip galvanized steel sheet having a metal structure containing a residual Worstian iron having a low C concentration and a hard ramie loose iron is subjected to reheat treatment, and the hot-dip galvanized steel sheet and The uniform ductility and local ductility of the alloyed hot-dip galvanized steel sheet are improved, and the lodging strength is also improved.

The reason for this is not clear, but it is presumed that (a) in the reheating process, C is concentrated in the Vostian iron, and the stability of the Worth iron is improved; and, (b) the hard granulated iron is returned. The fire changes to a soft tempering Ma Tian loose iron.

(C) If the hot-dip galvanized steel sheet and the alloyed hot-dip galvanized steel sheet are subjected to temper rolling before the reheating treatment, the uniform ductility and local ductility of the hot-dip galvanized steel sheet or the alloyed hot-dip galvanized steel sheet Further improvement, the strength of the fall is further improved.

The reason for this is not clear, but it is presumed that (a) introduces a poor row in the Vostian iron by quenching and tempering, and promotes the concentration of C to the Vostian iron during the reheating treatment in the subsequent reheating process. Concentration, while the stability of Worth Iron is further improved; (b) By quenching and tempering, part of the Worth Iron is metamorphosed into the granulated iron, and it is tempered in the reheated metal structure. The iron is increased; and (c) the metamorphosis of the granulated iron which may occur during the cooling after the reheating treatment is suppressed, and the hard granulated iron in the metal structure after the reheating treatment is less.

(D) Through the characteristic improvement effect of the temper rolling, the smaller the amount of the Worthite iron in the metal structure of the galvanized steel sheet and the alloyed hot-dip galvanized steel sheet which are rolled and tempered.

The reason for this is not clear, but it is presumed to be due to (a) the processing strain is concentrated on the Worthite iron, the smaller the Worthian iron is, the more the differential displacement is introduced into the Worthite iron; and, (b) During the heating process, C is concentrated in the Vostian iron and the Mn is concentrated in the Vostian iron, which further improves the stability of the Worthite iron.

(E) In the metal structure of the hot-dip galvanized steel sheet and the alloyed hot-dip galvanized steel sheet produced by the temper rolling and reheating treatment, except for the Worstian iron and the tempered granulated iron If it contains polygonal ferrite, it will not damage the local ductility of the hot-dip galvanized steel sheet and the alloyed hot-dip galvanized steel sheet, and the uniform ductility will be further improved.

The reason for this is not clear, but it is presumed that it is caused by (a) the increase of the Mn concentration in the residual Worthite iron, and the stability of the Worthite iron is improved; (b) usually the Mn in the Worthite iron will hinder the C to the Worthite iron. However, through the quenching and tempering and reheating treatment, the concentration of C to the Worthite iron is promoted, and the C concentration in the residual Worthite iron is ensured.

Based on the findings of the above (A) to (E), the inventors have further found that after the hot-dip galvanizing of the steel sheet (binder steel sheet) or after the hot-dip galvanizing and further alloying treatment, The following hot-dip galvanized steel sheet and alloyed hot-dip galvanized steel sheet can be produced by temper rolling and reheating treatment: a residual Worthite iron having a high C concentration and a high Mn concentration, and a tempered granulated iron and The metal structure of the polygonal fat iron has excellent uniform ductility and local ductility, and has high lodging strength and tensile strength.

The present invention has been completed based on the above findings, and the gist thereof is as follows. Further, in the present invention, the "steel plate" includes a "steel strip".

(1) A plated steel sheet comprising: C: 0.03% to 0.70% by mass, Si: 0.25% to 2.50%, Mn: 1.00% to 5.00%, P: 0.100% or less, and S. : 0.010% or less, sol.Al: 0.001% to 2.500%, N: 0.020% or less Ti: 0% to 0.300%, Nb: 0% to 0.300%, V: 0% to 0.300%, Cr: 0% to 2.000 %, Mo: 0%~2.000%, B: 0%~0.0200%, Cu: 0%~2.000%, Ni: 0%~2.000%, Ca: 0%~0.0100%, Mg: 0%~0.0100%, REM: 0%~0.1000%, and Bi: 0%~0.0500%, and the remainder consists of iron and impurities; the metal structure contains more than 5.0% by volume of residual Worth iron, more than 5.0% by volume of tempered Ma Tiansan Iron; and the amount of C in the residual Worthite iron is 0.85 mass% or more.

(2) The plated steel sheet according to the above (1), wherein the metal structure further contains more than 2.0% by volume of polygonal ferrite iron; and the amount of Mn in the residual Worstian iron satisfies the following formula (1). [Mn] _γ /[Mn] _ave ≧1.10...(1) [Mn] _γ : Mn amount (% by mass) in the residual Worthite [Mn] _ave : Mn amount (% by mass) of the chemical composition of the steel sheet

(3) The plated steel sheet according to the above (1) or (2), wherein the chemical composition further contains, by mass%, from Ti: 0.001% to 0.300%, and Nb: 0.001% to 0.300%, And one or more of the group consisting of V: 0.001% to 0.300%.

(4) The plated steel sheet according to any one of (1) to (3), wherein the chemical composition further contains, by mass%, from 0.001% to 2.000%, and Mo: 0.001%. One or two or more of the groups consisting of ~2.000% and B: 0.0001% to 0.0200%.

(5) The plated steel sheet according to any one of (1) to (4), wherein the chemical composition further contains, by mass%, Cu: 0.001% to 2.000%, and Ni: 0.001. One or two of the groups consisting of %~2.000%.

(6) The plated steel sheet according to any one of (1) to (5), wherein the chemical composition further contains, by mass%, from 0.0001% to 0.0100%, and Mg: 0.0001%. One or two or more of the groups consisting of ~0.0100% and REM: 0.0001% to 0.1000%.

(7) The plated steel sheet according to any one of the above (1) to (6), wherein the chemical composition further contains Bi: 0.0001% to 0.0500% by mass%.

(8) The plated steel sheet according to any one of the above (1), wherein the plated steel sheet is a hot-dip galvanized steel sheet including a hot-dip galvanized layer.

(9) The plated steel sheet according to any one of the above (1), wherein the plated steel sheet is an alloyed hot-dip galvanized steel sheet in which a hot-dip galvanized layer is alloyed.

(10) A method for producing a hot-dip galvanized steel sheet, comprising the steps of: heating a billet steel sheet to a point exceeding Ac ₁ and performing annealing, wherein the raw material steel sheet has a chemical composition containing C in mass%: 0.03% to 0.70%, Si: 0.25% to 2.50%, Mn: 1.00% to 5.00%, P: 0.100% or less, S: 0.010% or less, sol. Al: 0.001% to 2.500%, and N: 0.020% or less Ti : 0%~0.300%, Nb: 0%~0.300%, V: 0%~0.300%, Cr: 0%~2.000%, Mo: 0%~2.000%, B: 0%~0.0200%, Cu: 0 %~2.000%, Ni: 0%~2.000%, Ca: 0%~0.0100%, Mg: 0%~0.0100%, REM: 0%~0.1000%, and Bi: 0%~0.0500%, and the rest is In the first cooling step, the first cooling is performed at a temperature of 650 ° C to 500 ° C in an average temperature of 2 ° C / sec or more and less than 100 ° C / sec after the annealing step. Cooling rate, cooling to 500 ° C or less; molten galvanizing step, after the step of performing the first cooling, performing hot-dip galvanizing on the billet steel sheet cooled in the step of performing the first cooling; second cooling Step of performing the melt plating in the foregoing After the step, the hot-dip galvanized billet steel sheet is cooled to 300 in a temperature range from the plating temperature in the step of performing the hot-dip galvanizing to 300 ° C at an average cooling rate of 2 ° C /sec or more. °C or less; a step of temper rolling: after the step of performing the second cooling, the billet rolling of the blank steel sheet which has been cooled in the step of performing the second cooling is performed at a rate of 0.10% or more. And, in the step of heat treatment, after the step of performing the temper rolling, the billet steel sheet subjected to the temper rolling is heated to a temperature range of 200 ° C to 600 ° C, and maintained at the temperature for 1 second or more.

(11) The method for producing a hot-dip galvanized steel sheet according to the above (10), wherein in the step of performing annealing, the billet steel sheet is heated to a point exceeding Ac ₃ and annealed; and annealing is performed as described above. After the step, the average annealing rate in the temperature range from the heating temperature to (heating temperature - 50 ° C) is 7 ° C / sec or less, and the annealed billet steel sheet is cooled.

A method for producing an alloyed hot-dip galvanized steel sheet, comprising the step of heating a billet steel sheet to a point exceeding Ac ₁ and annealing, wherein the billet steel sheet has a chemical composition containing C: 0.03% to 0.70 by mass% %, Si: 0.25% to 2.50%, Mn: 1.00% to 5.00%, P: 0.100% or less, S: 0.010% or less, sol. Al: 0.001% to 2.500%, and N: 0.020% or less Ti: 0%~ 0.300%, Nb: 0%~0.300%, V: 0%~0.300%, Cr: 0%~2.000%, Mo: 0%~2.000%, B: 0%~0.0200%, Cu: 0%~2.000% , Ni: 0%~2.000%, Ca: 0%~0.0100%, Mg: 0%~0.0100%, REM: 0%~0.1000%, and Bi: 0%~0.0500%, and the rest is made of iron and impurities. The first cooling step is performed after the annealing step, and is cooled to 500 ° C or lower at an average cooling rate of 2 ° C / sec or more and less than 100 ° C / sec in a temperature range of 650 ° C to 500 ° C; In the step of hot-dip galvanizing, after the step of performing the first cooling, the raw billet steel sheet cooled in the first cooling step is subjected to hot-dip galvanizing; the step of alloying treatment, the step of performing the hot-dip galvanizing step After the aforementioned The galvanized billet steel sheet is alloyed at an alloying treatment temperature; and the second cooling step is performed in the temperature region from the foregoing alloying treatment temperature to 300 ° C after the step of alloying treatment described above The average cooling rate is 2° C./sec or more, and the billet steel sheet subjected to the alloying treatment is cooled to 300° C. or less. The step of temper rolling is performed after the second cooling step described above. The cooled billet steel sheet in the cooling step is subjected to temper rolling and rolling at a tensile ratio of 0.10% or more; and the heat treatment step is carried out after the step of temper rolling and rolling The billet steel sheet is heated to a temperature range of 200 ° C to 600 ° C and maintained at this temperature for 1 second or more.

(13) The method for producing an alloyed hot-dip galvanized steel sheet according to the above (12), characterized in that in the step of performing annealing, the billet steel sheet is heated to a point exceeding Ac ₃ and annealed; After the annealing step, the average cooling rate in the temperature range from the heating temperature to (heating temperature - 50 ° C) is 7 ° C / sec or less, and the annealed billet steel sheet is cooled.

According to the present invention, it is possible to manufacture and provide a hot-dip galvanized steel sheet and an alloyed hot-dip galvanized steel sheet which are excellent in uniform ductility and local ductility, have high lodging strength and tensile strength, and are excellent in formability and impact absorbability.

The plated steel sheet according to the present invention is characterized in that the chemical composition contains, by mass%, C: 0.03% to 0.70%, Si: 0.25% to 2.50%, Mn: 1.00% to 5.00%, and P: 0.100. % or less, S: 0.010% or less, sol. Al: 0.001% to 2.500%, N: 0.020% or less Ti: 0% to 0.300%, Nb: 0% to 0.300%, V: 0% to 0.300%, Cr: 0%~2.000%, Mo: 0%~2.000%, B: 0%~0.0200%, Cu: 0%~2.000%, Ni: 0%~2.000%, Ca: 0%~0.0100%, Mg: 0% ~0.0100%, REM: 0%~0.1000%, and Bi: 0%~0.0500%, and the remainder is composed of iron and impurities; the metal structure contains more than 5.0% by volume of residual Worth iron, more than 5.0% by volume The amount of C in the residual Worthite iron is 0.85 mass% or more.

The plated steel sheet of the present invention is characterized in that it is a hot-dip galvanized steel sheet containing a hot-dip galvanized layer.

The plated steel sheet of the present invention is characterized in that it is an alloyed hot-dip galvanized steel sheet in which a hot-dip galvanized layer is alloyed.

A method for producing a hot-dip galvanized steel sheet according to the present invention is characterized by comprising the steps of: heating a billet steel sheet to a point exceeding Ac ₁ and performing annealing, wherein the billet steel sheet has a chemical composition containing C: 0.03% by mass% ~0.70%, Si: 0.25% to 2.50%, Mn: 1.00% to 5.00%, P: 0.100% or less, S: 0.010% or less, sol. Al: 0.001% to 2.500%, N: 0.020% or less Ti: 0 %~0.300%, Nb: 0%~0.300%, V: 0%~0.300%, Cr: 0%~2.000%, Mo: 0%~2.000%, B: 0%~0.0200%, Cu: 0%~ 2.000%, Ni: 0%~2.000%, Ca: 0%~0.0100%, Mg: 0%~0.0100%, REM: 0%~0.1000%, and Bi: 0%~0.0500%, and the remainder is made of iron and a step of performing the first cooling, wherein the first cooling is performed after the annealing step, and the average cooling rate in the temperature range of 650 ° C to 500 ° C is 2 ° C / sec or more and less than 100 ° C /second, cooling to 500 ° C or lower; after the step of performing the first cooling, the step of performing hot-dip galvanizing on the billet steel sheet cooled in the step of performing the first cooling; and performing the second cooling step, The second cooling is before After the step of performing the hot-dip galvanizing, the average cooling rate in the temperature range from the plating temperature in the step of performing the hot-dip galvanizing to 300 ° C is 2 ° C / sec or more, and the billet which has been subjected to the aforementioned hot-dip galvanizing The steel sheet is cooled to 300° C. or less; and after the step of performing the second cooling, the billet having the elongation rate of 0.10% or more is subjected to a step of rolling and rolling the billet steel sheet cooled in the second cooling step; a heat treatment step of heating the billet steel sheet subjected to the temper rolling and rolling to a temperature range of 200 ° C to 600 ° C after the step of performing the temper rolling step, and maintaining the temperature at the temperature for 1 second. the above.

The method for producing an alloyed hot-dip galvanized steel sheet according to the present invention is characterized by comprising the steps of: heating a billet steel sheet to a point exceeding Ac ₁ and annealing, and the chemical composition of the billet steel sheet contains C in mass% : 0.03% to 0.70%, Si: 0.25% to 2.50%, Mn: 1.00% to 5.00%, P: 0.100% or less, S: 0.010% or less, sol. Al: 0.001% to 2.500%, and N: 0.020% or less Ti: 0% to 0.300%, Nb: 0% to 0.300%, V: 0% to 0.300%, Cr: 0% to 2.000%, Mo: 0% to 2.000%, B: 0% to 0.0200%, Cu: 0%~2.000%, Ni: 0%~2.000%, Ca: 0%~0.0100%, Mg: 0%~0.0100%, REM: 0%~0.1000%, and Bi: 0%~0.0500%, and the rest The first cooling step is a step of annealing in the temperature range of 650 ° C to 500 ° C and 2 ° C / sec or more and less than 100 ° C / sec. The average cooling rate is cooled to 500° C. or less; in the step of performing the first cooling, after the step of performing the first cooling, the billet steel sheet cooled in the step of performing the first cooling is subjected to hot-dip galvanizing; After the zinc step, a step of alloying the billet steel sheet on which the hot-dip galvanizing has been performed at an alloying treatment temperature; and a second cooling step, after the step of performing the alloying treatment, from the foregoing alloying treatment temperature to 300 ° C The raw material steel sheet subjected to the alloying treatment is cooled to 300 ° C or lower at an average cooling rate of 2 ° C /sec or more in the temperature zone up to the temperature zone; the step of temper rolling is performed after the step of performing the second cooling The billet having a tensile ratio of 0.10% or more is subjected to the step of performing the second cooling step, and the step of heat treatment is performed after the step of performing the temper rolling step. The billet steel sheet which has been subjected to the above quenching and tempering is heated to a temperature range of 200 ° C to 600 ° C and maintained at this temperature for 1 second or more.

Hereinafter, the hot-dip galvanized steel sheet, the alloyed hot-dip galvanized steel sheet, and the method for producing the same according to the present embodiment will be described. In the following description, unless otherwise specified, the steel sheet finally obtained according to the production method of the present embodiment will be referred to as "melted galvanized steel sheet" or "alloyed hot-dip galvanized steel sheet" or referred to as " Steel plate, and the steel plate in the middle of production is called "binder steel plate".

(A) Chemical composition First, the reason for limiting the chemical composition of the billet steel sheet used in the hot-dip galvanized steel sheet, the alloyed hot-dip galvanized steel sheet, and the manufacturing method of the present embodiment will be described. Hereinafter, the % with respect to the chemical composition means the mass%.

[C: 0.03%~0.70%] C is an effective element for obtaining residual Worth Iron. When the C content is less than 0.03%, the metal structure containing the residual Worthite iron and the tempered Ueda iron will not be obtained later, so the C content is made 0.03% or more. The C content is preferably 0.10% or more, preferably 0.13% or more, more preferably 0.16% or more.

On the other hand, when the C content exceeds 0.70%, the weldability of the steel sheet is remarkably lowered, so that the C content is made 0.70% or less. The C content is preferably 0.30% or less, preferably 0.26% or less, more preferably 0.24% or less.

[Si: 0.25% to 2.50%] Si is an element that suppresses the precipitation of ferritic carbon and promotes the formation of residual Worth iron. In addition, Si is an element that prevents excessive softening of the tempered granules and maintains strength. In the case where the Si content is less than 0.25%, the effect is not sufficiently exhibited, so the Si content is made 0.25% or more, and desirably, the Si content is more than 0.60%, preferably more than 1.00%, more preferably More than 1.45%.

On the other hand, when the Si content exceeds 2.50%, the plating property of the steel sheet is remarkably lowered, and the weldability of the steel sheet is lowered, so that the Si content is 2.50% or less. The Si content is preferably 2.30% or less, preferably 2.10% or less, more preferably 1.90% or less.

[Mn: 1.00% to 5.00%] Mn has an effect of improving the hardenability of steel, and is an effective element for obtaining a metal structure containing residual Worthite iron and tempered granita iron which will be described later. When the Mn content is less than 1.00%, these effects are not sufficiently exhibited, so that the Mn content is 1.00% or more. It is desirable that the Mn content is more than 1.50%, preferably more than 2.00%, more preferably more than 2.50%. On the other hand, when the Mn content exceeds 5.00%, the weldability of the steel sheet is lowered, so that the Mn content is made 5.00% or less. The Mn content is preferably 4.00% or less, preferably 3.50% or less, more preferably 3.00% or less.

[P: 0.100% or less] P is an impurity element and segregates at the grain boundary to make the steel plate embrittled, so that the less the better the element. When the P content exceeds 0.100%, the embrittlement of the steel sheet becomes conspicuous, so the P content is made 0.100% or less. It is desirable that the P content is less than 0.020%, preferably less than 0.015%, more preferably less than 0.010%. Although the lower limit of the P content includes 0%, when the P content is reduced to less than 0.0001%, the manufacturing cost is greatly increased, so the practical lower limit of the P content on the practical steel sheet is 0.0001%.

[S: 0.010% or less] S is an impurity element, and sulfide-based inclusions are formed in the steel, so that the local ductility of the steel sheet is deteriorated, so that the element is less and better. Since the deterioration of the local ductility of the steel sheet becomes remarkable when the S content exceeds 0.010%, the S content is made 0.010% or less. The S content is preferably 0.005% or less, preferably 0.0012% or less. Although the lower limit of the S content includes 0%, when the S content is reduced to less than 0.0001%, the manufacturing cost is greatly increased, so the practical lower limit of the S content is 0.0001% on the practical steel sheet.

[sol. Al: 0.001% to 2.500%] Al is an element that deoxidizes molten steel. When the sol. Al content is less than 0.001%, the effect is not sufficiently exhibited, so that the sol. Al content is 0.001% or more. The content of sol. Al is preferably 0.015% or more, preferably 0.025% or more, more preferably 0.045% or more. In addition, Al acts as an effective element for obtaining the formation of residual Worth iron in the same manner as Si, and is obtained on a metal structure containing residual Worthite iron and tempered Ueda iron. From this point of view, it is desirable to have a sol. Al content of 0.050% or more. Preferably, the sol. Al content is 0.055% or more, more preferably 0.060% or more.

On the other hand, if the sol.Al content exceeds 2.500%, an excessive amount of alumina (Al ₂ O ₃ ) is formed, and surface enthalpy due to alumina is liable to occur, so that the sol. Al content is required. It is 2.500% or less. Further, when the sol. Al content is 0.080% or more, the deformation point is greatly increased, and annealing at a temperature exceeding the Ac ₃ point becomes difficult, so that the sol. Al content should be less than 0.080%. Preferably, the sol. Al content is 0.075% or less, more preferably 0.070% or less, and particularly preferably less than 0.065%.

N: 0.020% or less N is an impurity element, and in the continuous casting of steel, a nitride causing damage to the steel embryo is formed, so that the element is less and better. When the N content exceeds 0.020%, the doubt that the steel embryo is broken is large, so the N content is made 0.020% or less. The N content is preferably 0.010% or less, preferably less than 0.008%, more preferably 0.005% or less. Although the lower limit of the N content includes 0%, when the N content is reduced to less than 0.0001%, the manufacturing cost is greatly increased, so the practical lower limit of the N content on the practical steel sheet is 0.0001%.

Furthermore, in order to improve the characteristics, in addition to the above elements, the elements described below may be included.

[Ti:0%~0.300%] [Nb:0%~0.300%] [V:0%~0.300%] Ti, Nb and V are elements that help to refine the metal structure and improve strength and ductility. . However, if the content of these elements exceeds 0.300%, the effects are saturated and the manufacturing cost is increased. Therefore, the content of any of Ti, Nb, and V is 0.300% or less.

When Ti, Nb, and V are excessive, the recrystallization temperature at the time of annealing increases, and the metal structure after annealing becomes uneven, and local ductility is impaired. Therefore, the Ti content is preferably less than 0.080% or less, preferably 0.035% or less; the Nb content is preferably less than 0.050%, preferably 0.030% or less; and the V content is preferably 0.200% or less, preferably less than 0.100%. .

Although the lower limit of the content of Ti, Nb, and V is 0%, in order to obtain an effect, it is preferable that the content of any of Ti, Nb, and V is 0.001% or more. The Ti content is preferably 0.005% or more, more preferably 0.010% or more; the Nb content is preferably 0.005% or more, more preferably 0.010% or more, particularly preferably 0.015% or more; and the V content is preferably 0.010% or more, more preferably It is 0.020% or more. As described above, in order to obtain the above effects, it is preferable to contain one selected from the group consisting of Ti: 0.001% to 0.300%, Nb: 0.001% to 0.300%, and V: 0.001% to 0.300%. Or two or more.

[Cr: 0%~2.000%] [Mo: 0%~2.000%] [B: 0%~0.0200%] Cr, Mo, and B improve the hardenability of steel, and in order to obtain the residual Worthite iron and The metal structure of the tempered granulated iron is an effective element.

However, when the Cr content and the Mo content exceed 2.000%, or when the B content exceeds 0.0200%, the effect is saturated and the manufacturing cost increases. Therefore, the Cr content and the content of Mo are both 2.000% or less, and the B content is 0.0200% or less. The Cr content is preferably 1.000% or less, the Mo content is 0.500% or less, and the B content is 0.0030% or less.

The lower limit of the content of Cr, Mo and B is 0% in any of the elements. However, in order to obtain an effect, the Cr content and the Mo content are preferably 0.001% or more, and the B content is preferably 0.0001% or more. Preferably, the Cr content is 0.100% or more, the Mo content is 0.050% or more, and the B content is 0.0010% or more. As described above, in order to obtain the above effects, it is preferable to contain one selected from the group consisting of Cr: 0.001% to 2.000%, Mo: 0.001% to 2.000%, and B: 0.0001% to 0.0200%. Or two or more.

[Cu: 0% to 2.000%] [Ni: 0% to 2.000%] Cu and Ni are elements that contribute to the improvement of the tensile strength and tensile strength. However, when the Cu content and the Ni content exceed 2.00%, the effect is saturated and the production cost is increased. Therefore, the Cu content and the Ni content are both 2.000% or less. It is desirable that the Cu content and the Ni content are both 0.800% or less.

Although the lower limit of the Cu content and the Ni content is 0%, in order to obtain an effect, the Cu content and the Ni content are preferably 0.001% or more. Preferably, the content of any element is above 0.010%. As described above, in order to obtain the above-described effects, one or two types selected from the group consisting of Cu: 0.001% to 2.000%, and Ni: 0.001% to 2.000% are preferably contained.

[Ca: 0%~0.0100%] [Mg: 0%~0.0100%] [REM: 0%~0.1000%] Ca, Mg, and REM are elements that help to adjust the shape of inclusions and enhance local ductility. .

However, when the Ca content and the Mg content exceed 0.0100%, or when the REM content exceeds 0.1000%, the effect is saturated and the manufacturing cost increases. Therefore, it is necessary to set both the Ca content and the Mg content to be 0.0100% or less, and to make the REM content 0.1000% or less. The Ca content and the Mg content are preferably 0.0020% or less, and the REM content is 0.0100% or less.

The lower limit of Ca, Mg, and REM content is 0%, but in order to obtain an effect, the contents of Ca, Mg, and REM should be 0.0001% or more. It is preferred that the content of any of the elements is 0.0005% or more. As described above, in order to obtain the above effects, it is preferable to contain one selected from the group consisting of Ca: 0.0001% to 0.0100%, Mg: 0.0001% to 0.0100%, and REM: 0.0001% to 0.1000%. 2 or more types.

Here, REM is a general term for a total of 17 elements of Sc, Y, and lanthanoid elements. Lanthanides are industrially added in the form of mixed rare earth alloys. Further, in the present invention, the REM content means the total amount of these elements.

[Bi: 0% to 0.0500%] Bi is an element that contributes to refining the solidified structure and improving local ductility. However, when the Bi content exceeds 0.0500%, the effect is saturated and the production cost is increased, so the Bi content is made 0.0500% or less. The Bi content is preferably 0.0100% or less, preferably 0.0050% or less. Although the lower limit of the Bi content includes 0%, in order to surely obtain an effect, the Bi content is preferably 0.0001% or more. Preferably, the Bi content is 0.0003% or more. As described above, in order to obtain the above effects, it is preferable to contain Bi: 0.0001% to 0.0500%.

The remaining portion of the chemical composition of the hot-dip galvanized steel sheet, the alloyed hot-dip galvanized steel sheet, and the billet steel sheet used in the production methods of the present embodiment is iron and impurities. The impurities are steel billets such as ore or scrap when industrially producing steel, or elements mixed for various reasons in the manufacturing steps. These elements are tolerated insofar as they do not impair the characteristics of the present invention.

(B) Metal structure Next, the metal structure of the hot-dip galvanized steel sheet and the alloyed hot-dip galvanized steel sheet according to the present embodiment will be described. The metal structure of the hot-dip galvanized steel sheet and the alloyed hot-dip galvanized steel sheet according to the present embodiment is characterized by containing more than 5.0% by volume in terms of volume %, in order to maintain the tensile strength and the tensile strength while improving the uniform ductility and the local ductility. Vostian Iron, which contains more than 5.0% tempered granita iron, in addition, the amount of C in the residual Worth iron is above 0.85 mass%. Further, the metal structure is preferably characterized by further containing more than 2.0% of polygonal ferrite iron, and the amount of Mn remaining in the Worstian iron satisfies the following formula (1). Further, the amount of C in the residual Worthite iron means the C concentration in the iron phase of the Vostian, and the amount of Mn in the residual Worthite iron means the Mn concentration in the iron phase of the Vostian.

[Mn] _γ /[Mn] _ave ≧1.10...(1) [Mn] _γ : Mn amount (% by mass) in the residual Worthite [Mn] _ave : Mn amount (% by mass) of the chemical composition of the steel sheet

In the following, the organizational requirements are described in order.

[Residual Worth Iron: more than 5.0% by volume] In order to improve uniform ductility, the volume fraction of the residual Worthite iron is more than 5.0%. The volume fraction of the residual Worthite iron is preferably more than 6.0%, preferably more than 8.0%, more preferably more than 10.0%.

However, when the residual Worstian iron is excessively present, the local ductility is deteriorated, so the volume fraction of the remaining Worthite iron is preferably less than 30.0%. Preferably, the volume fraction of the residual Worthite iron is less than 20.0%, more preferably less than 15.0%.

[Returning 麻田散铁: more than 5.0% by volume] In order to maintain the strength of the fall and the tensile strength, and to improve the local ductility, the volume fraction of the tempered granulated iron is more than 5.0%. Preferably, the volume fraction of the tempered granules is more than 16.0%, preferably the volume fraction of the tempered granules is more than 30.0%, more preferably more than 40.0%, and particularly preferably more than 50.0%.

However, when the tempered granulated iron is excessively present, the uniform ductility may be deteriorated, so the volume fraction of the tempered granulated iron is preferably 70.0% or less. Preferably, the volume fraction of the tempered granulated iron is 60.0% or less.

[Polygonal ferrite iron: more than 2.0% by volume] In order to further improve uniform ductility, the volume fraction of polygonal ferrite iron should be more than 2.0%. Preferably, the volume fraction of the polygonal ferrite is more than 6.0%, more preferably more than 8.0%, and particularly preferably more than 13.0%.

However, when the polygon ferrite is excessively present, the drop strength and tensile strength are lowered, and even more, the local ductility is also lowered, so the volume fraction of the polygonal ferrite is preferably less than 35.0%. Preferably, the volume fraction of the polygonal ferrite is less than 30.0%, more preferably less than 25.0%, and particularly preferably less than 20.0%.

[Amount of C in the residual Worthite iron: 0.85 mass% or more] In the residual Worthite iron of the metal structure of the hot-dip galvanized steel sheet and the alloyed hot-dip galvanized steel sheet according to the present embodiment, the Worstian iron is left to be stabilized and uniformized. The ductility and local ductility, the amount of C in the residual Worthite iron is 0.85 mass% or more.

In order to further improve the uniform ductility, the amount of C in the residual Worth iron is preferably 0.87 mass% or more, preferably 0.89 mass% or more. On the other hand, when the amount of C in the residual Worthite iron is too large, the TRIP effect cannot be obtained and the uniform ductility is deteriorated, so the amount of C remaining in the Worthite iron is preferably less than 1.50% by mass. The amount of C in the residual Worthite iron is preferably less than 1.20% by mass, more preferably less than 1.10% by mass.

[Amount of Mn in the residual Worthite iron: the following formula (1)] [Mn] _γ / [Mn] _ave ≧ 1.10 (1) [Mn] _γ : Mn amount (% by mass) in the residual Worthite [Mn] _Ave : Mn amount (mass%) of the chemical composition of the steel sheet The above formula (1) is a formula that defines the relationship between [Mn] _γ and [Mn] _ave . In the molten Worstian iron of the hot-dip galvanized steel sheet and the alloyed hot-dip galvanized steel sheet according to the present embodiment, it is preferable to concentrate Mn to a desired amount. Similarly to C, Mn effectively functions to stabilize the residual Worthite iron and improve uniform ductility and local ductility.

In order to maximize the utilization of its function, it is preferable to set [Mn] _γ /[Mn] _ave to 1.10 or more, more preferably 1.15 or more. [Mn] The upper limit of _γ /[Mn] _ave is not particularly limited, but is substantially 2.00. From the viewpoint of productivity, [Mn] _γ /[Mn] _{ave is} preferably 1.35 or less, preferably 1.25 or less.

[Mitano Iron] In the hot-dip galvanized steel sheet and the alloyed hot-dip galvanized steel sheet according to the present embodiment, in order to maintain the lodging strength and further improve the local ductility, it is necessary to suppress the amount of the iron in the field. Here, the so-called Ma Tian loose iron refers to the granulated iron of the Ma Tian that has not been tempered, that is, the new Ma Tian loose iron. The volume fraction of the granulated iron should be less than 5.0%. Preferably, the volume fraction of the granulated iron is less than 2.0%, more preferably less than 1.0%.

[Remaining tissue] The remaining part of the metal structure is a low-temperature metamorphic structure such as needle-shaped ferrite iron and toughened iron. It may also contain Borne iron or precipitates such as ferritic carbon iron. The remaining part of the structure does not necessarily have to contain a low temperature metamorphosis product, a ferrite and a precipitate, so the lower limit of the volume ratio of each of the low temperature metamorphosis product, the ferrite and the precipitate is 0% by volume.

The upper limit of the volume ratio of each of the low temperature metamorphosis product, the ferrite, and the precipitate is not particularly limited. However, when the low temperature metamorphosis product, the ferritic iron, and the precipitate are excessively present, the drop strength and the tensile strength are lowered, so that the total volume ratio of the low temperature metamorphosis product, the pulverized iron, and the precipitate is preferably 40.0% or less. . The total volume ratio of the tissues is preferably 20.0% or less, more preferably 10.0% or less.

When the excess iron is present, the strength of the drop and the tensile strength are lowered, and even more, the uniform ductility is also lowered, so the volume fraction of the ferrite is preferably less than 10.0%. Preferably, the volume fraction of the ferrite is less than 5.0%, more preferably less than 3.0%.

The metal structure of the hot-dip galvanized steel sheet and the alloyed hot-dip galvanized steel according to the present embodiment is measured as shown below. A test piece is taken from any position of the steel sheet, and a longitudinal section parallel to the rolling direction is polished, and scanning electrons are used in a range of 1/4 depth from the boundary between the steel sheet and the plating layer of the substrate to the thickness of the steel sheet of the substrate. The metal structure was observed under a microscope (SEM), and the area ratio of each tissue was measured by image processing. The area ratio is equal to the volume rate, and the measured area ratio is the volume rate.

The tempered granulated iron is a point that can be used for the presence of iron carbides in the tissue to extend in multiple directions to distinguish it from toughened iron. The polygonal ferrite iron can be distinguished from the needle-shaped ferrite by using the point of the block shape and the point of low difference density.

The volume fraction of the residual Worthite iron and the amount of C in the residual Worthite iron are obtained by taking a test piece from any position of the steel sheet, and the thickness of the steel sheet from the boundary between the steel sheet and the plating layer of the substrate to the substrate. At 1/4 depth position, the rolling surface was chemically ground, and an X-ray diffraction device (XRD) was used to measure the X-ray diffraction intensity and the diffraction peak position.

The amount of Mn ([Mn] _γ ) in the residual Worthite iron was measured in the following manner. A test piece is taken from any position of the steel sheet, and an SEM having an electron beam backscatter pattern analysis device (EBSP) is used in a range of 1/4 depth from the boundary between the steel sheet and the plating layer of the substrate to the thickness of the steel sheet of the substrate. To observe the metal structure, confirm the residual Worthfield iron particles.

Next, the Mn concentration of the above-mentioned residual Worthite iron particles was measured using an SEM equipped with an electron micro-analyzer (EPMA). The average value of the measured Mn amount was [Mn] _γ for the measurement of EMPA by using 10 or more residual Worstian iron particles.

In the measurement by EPMA, since the electron beam is irradiated to the residual Worthfield iron particles by a beam diameter smaller than the particle diameter of the residual Worthite iron, it is preferable to use a field emission type electron microprobe. SEM of the analyzer (FE-EPMA).

The hot-dip galvanized layer and the alloyed hot-dip galvanized layer may be a plating layer formed by general plating conditions and an alloyed plating layer. However, when the Fe concentration of the alloyed hot-dip galvanized layer is less than 7% by mass, the weldability and the slidability may be insufficient, so that the Fe concentration of the alloyed hot-dip galvanized layer is preferably 7% by mass or more. .

The upper limit of the Fe concentration of the alloyed hot-dip galvanizing layer is preferably 20% by mass or less, preferably 15% by mass or less, from the viewpoint of pulverization resistance. The amount of Fe in the plating layer can be adjusted by controlling the processing conditions in the alloying treatment after the hot-dip galvanizing.

(C) Mechanical properties The mechanical properties of the hot-dip galvanized steel sheet and the alloyed hot-dip galvanized steel sheet according to the present embodiment are not limited by specific mechanical properties.

However, the uniform elongation in the direction orthogonal to the rolling direction is defined as UE1 (Uniform Elongation). Then, according to the following formula (2), the total elongation (TEl ₀ ) in the direction orthogonal to the rolling direction is converted into a total elongation corresponding to a plate thickness of 1.2 mm, and the value obtained by the above conversion is defined as TE1 ( Total Elongation). Further, the local elongation corresponding to the plate thickness of 1.2 mm and the direction orthogonal to the rolling direction is defined as LE1 (Local Elongation) according to the following formula (3). In the hot-dip galvanized steel sheet and the alloyed hot-dip galvanized steel sheet according to the present embodiment, the value of TS×UE1 is preferably 10000 MPa·% or more from the viewpoint of press formability, and the value of TS×LEl is preferably 5,000 MPa·%. the above.

The value of TS × UE1 is large as both the tensile strength and the uniform elongation are excellent, and is used as an index for evaluating uniform ductility. The value of TS × LEl is large when both tensile strength and local elongation are excellent, and is used as an index for evaluating local ductility.

It is preferable that the value of TS × UE1 is 11,000 MPa% or more and the value of TS × LE1 is 6000 MPa% or more. It is more preferable if the value of TS × UE1 is 12,000 MPa% or more and the value of TS × LE1 is 7000 MPa% or more. TEl=TEl ₀ ×(1.2/t ₀ ) ^0.2 (2) LEl=TEl-UEl (3)

Here, TE1 ₀ in the formula (2) is an actual measurement value of the total elongation obtained by using a JIS No. 5 tensile test piece, and t ₀ is the thickness of the JIS No. 5 tensile test piece provided for measurement. Further, TEl and LE1 are conversion values corresponding to the total elongation and the local elongation at a plate thickness of 1.2 mm, respectively. UE1 is a measured value of uniform elongation obtained by measurement using a JIS No. 5 tensile test piece.

In order to improve the impact absorption of the steel sheet, the tensile strength (TS) of the steel sheet is preferably 780 MPa or more. The steel sheet preferably has a tensile strength (TS) of 980 MPa or more, more preferably 1180 MPa or more. The steel plate has a drop ratio (YR) of preferably 0.59 or more. Preferably, the steel sheet has a drop ratio (YR) of 0.66 or more, more preferably 0.72 or more.

Since the local ductility is higher, the damage at the time of impact load is suppressed and the absorbed energy is increased. Therefore, from the viewpoint of suppressing damage, the value of TS × LE1 is preferably 5500 MPa·% or more. Preferably, the value of TS × LEl is 6500 MPa·% or more.

(D) Manufacturing method Next, a method for producing a hot-dip galvanized steel sheet and a method for producing an alloyed hot-dip galvanized steel sheet according to the present embodiment will be described.

The method for producing a hot-dip galvanized steel sheet according to the present embodiment includes the steps of: heating a billet steel sheet having the chemical composition to a point exceeding Ac ₁ and annealing; and performing a first cooling step in the annealing step. Thereafter, the average cooling rate in the temperature range of 650 ° C to 500 ° C is 2 ° C / sec or more and less than 100 ° C / sec, and is cooled to 500 ° C or less; the step of hot-dip galvanizing, the first cooling is performed After the step, the billet steel sheet that has been cooled in the step of performing the first cooling is subjected to hot-dip galvanizing; in the second cooling step, the second cooling is performed after the step of performing the hot-dip galvanizing step The plating temperature in the step of performing the hot-dip galvanizing is carried out in a temperature range of 300 ° C to an average cooling rate of 2 ° C /sec or more, and the billet steel sheet subjected to the hot-dip galvanizing is cooled to 300 ° C or less; In the step of rolling, after the step of performing the second cooling, the billet steel sheet which has been cooled in the step of performing the second cooling is subjected to temper rolling and rolling at an elongation of 0.10% or more; and heat treatment Step, before After the step of temper rolling, the billet steel sheet subjected to the above-mentioned temper rolling is heated to a temperature range of 200 ° C to 600 ° C and maintained at this temperature for 1 second or longer.

As shown in FIG. 1 , an ideal manufacturing method of the method for producing a hot-dip galvanized steel sheet according to the present embodiment includes the steps of: heating a billet steel sheet having the chemical composition above a point of Ac ₃ and annealing; In the first cooling step, after the annealing step, the annealed billet is obtained at an average cooling rate of 7 ° C /sec or less in a temperature range from a heating temperature to (the heating temperature of -50 ° C) The steel sheet is cooled, and further, in a temperature range of 650 ° C to 500 ° C, the average cooling rate of 2 ° C / sec or more and less than 100 ° C / sec, and cooled to 500 ° C or less; the step of hot-dip galvanizing, in the foregoing After the first cooling step, the billet steel sheet cooled in the step of performing the first cooling is subjected to hot-dip galvanizing; and the second cooling step is performed after the step of performing the hot-dip galvanizing step. In the temperature range from 300° C. in the galvanizing step to an average cooling rate of 2° C./sec or more, the hot-dip galvanized blank steel sheet is cooled to 300° C. or less; ,in After the second cooling step, the billet steel sheet cooled in the second cooling step is subjected to a temper rolling of 0.10% or more; and the heat treatment step is performed in the above-described manner. After the step of rolling, the billet steel sheet subjected to the above-mentioned quenching and temper rolling is heated to a temperature range of 200 ° C to 600 ° C and maintained at this temperature for 1 second or more.

The method for producing an alloyed hot-dip galvanized steel sheet according to the present embodiment includes a step of heating a billet steel sheet having the chemical composition above Ac ₁ and annealing, and a first cooling step in which the first cooling is performed. After the annealing step, the temperature is cooled in the temperature range of 650 ° C to 500 ° C at an average cooling rate of 2 ° C / sec or more and less than 100 ° C / sec to 500 ° C or less; the step of hot-dip galvanizing, the first step is performed in the foregoing After the step of cooling, the billet plate which has been cooled in the step of performing the first cooling is subjected to hot-dip galvanizing; the step of alloying, after the step of performing the hot-dip galvanizing, the embryo which has been subjected to the aforementioned hot-dip galvanizing The steel sheet is alloyed at the alloying treatment temperature; and the second cooling step is performed at a temperature of from 2 ° C / sec in the temperature range from the alloying treatment temperature to 300 ° C after the step of alloying treatment described above. The average cooling rate is to cool the billet steel sheet subjected to the alloying treatment to 300 ° C or lower; the step of quenching and tempering, after the step of performing the second cooling, and performing the second cooling step The cooled billet steel sheet is subjected to quenching and rolling at a stretching ratio of 0.10% or more; and the heat treatment step is performed after the step of performing the quenching and tempering step after the step of quenching and tempering The billet steel sheet is heated to a temperature range of 200 ° C to 600 ° C and maintained at this temperature for 1 second or more.

As shown in FIG. 2, the preferred method for producing the alloyed hot-dip galvanized steel sheet according to the present embodiment comprises the steps of: heating a billet steel sheet having the chemical composition above a point of Ac ₃ and annealing; 1 step of cooling, after the step of annealing, the temperature range from the heating temperature to (heating temperature - 50 ° C) is the average cooling rate of 7 ° C / sec or less, the previously annealed billet steel sheet Cooling, and further cooling to 500 ° C or lower in an average temperature of 2 ° C / sec or more and less than 100 ° C / sec in a temperature range of 650 ° C to 500 ° C; a step of hot-dip galvanizing, performing the first cooling described above After the step, the billet plate cooled by the step of performing the first cooling is subjected to hot-dip galvanizing; the step of alloying, after the step of performing the hot-dip galvanizing, the billet which has been subjected to the aforementioned hot-dip galvanizing The steel sheet is alloyed at the alloying treatment temperature; the second cooling step is performed at a temperature of 2 ° C / sec in a temperature range from the foregoing alloying treatment temperature to 300 ° C after the step of alloying treatment described above. The average cooling rate is obtained, and the billet steel sheet subjected to the alloying treatment is cooled to 300° C. or less; and the step of quenching and tempering is performed in the step of performing the second cooling in the step of performing the second cooling step. The cooled billet steel sheet is subjected to temper rolling and rolling at a stretching ratio of 0.10% or more; and the heat treatment step is a billet which has been subjected to the above-mentioned quenching and tempering after the step of performing the temper rolling step The steel sheet is heated to a temperature range of 200 ° C to 600 ° C and maintained at this temperature for more than 1 second.

The method for producing the billet steel sheet to be used in the method for producing a hot-dip galvanized steel sheet according to the present embodiment and the method for producing a molten hot-dip galvanized steel sheet is not limited to a specific production method. For example, a steel preform having the aforementioned chemical composition is produced by casting, heated to a temperature region of less than 1250 ° C, and after heating, at a finishing rolling temperature of Ar ₃ or more and over 850 ° C, Hot rolling. Next, the coiling is performed at a coiling temperature of 500 ° C or more and less than 700 ° C, and cold rolling is performed at a rolling reduction ratio of 40% or more and less than 70% to produce a billet steel sheet.

The casting method of the steel blank is not limited to a specific casting method, but a continuous casting method is preferred, and a steel block cast by another casting method may be formed into a steel sheet by block rolling or the like. In the continuous casting step, in order to suppress the occurrence of surface defects due to inclusions, it is preferable to cause the molten steel to flow in the mold by electromagnetic stirring or the like. The steel block in the high temperature state after continuous casting or the steel sheet in the high temperature state after the block rolling may be temporarily cooled and then heated and supplied to the hot rolling.

Further, the steel block in a high-temperature state after continuous casting or the steel sheet in a high-temperature state after the block rolling may be directly supplied to the hot rolling without reheating, or may be supplied to the heat after auxiliary heating. Rolling. Further, the steel block and the steel sheet supplied to the hot rolling are collectively referred to as "steel embryos".

In order to prevent the coarsening of the Worthite iron, the temperature of the steel blank supplied to the hot rolling is preferably lower than 1,250 °C. Preferably, the steel embryo temperature is 1200 ° C or lower. The lower limit of the temperature of the steel preform supplied to the hot rolling is not particularly limited, but it is preferably a temperature at which the hot rolling can be completed at Ar ₃ or more.

The condition of the hot rolling is not particularly limited. However, if the completion temperature of the hot rolling is too low, a coarse low-temperature metamorphic structure extending in the rolling direction occurs in the metal structure of the hot-rolled steel sheet, and the uniform ductility is hindered. And the local ductility, so the completion temperature of the hot rolling is preferably Ar ₃ points or more and more than 850 ° C. The completion temperature of the hot rolling is preferably Ar ₃ or more and more than 880 ° C, more preferably Ar ₃ or more and more than 900 ° C. The upper limit of the completion temperature of the hot rolling is not particularly limited, but is preferably 1000 ° C or less at the point of fine-graining the metal structure of the hot-rolled steel sheet.

Moreover, when the hot rolling is composed of rough rolling and finishing rolling, in order to complete the finishing rolling in the above temperature range, the rough rolling may be performed between the rough rolling and the finishing rolling. Material heating. At this time, the rough rolling material is heated, so that the rear end of the rough rolling material becomes higher than the front end of the rough rolling material, and the overall length of the rough rolling material at the start of finishing rolling is suitable. The deviation is suppressed below 140 °C. By this temperature suppression, the property uniformity in the coil of the coiled hot-rolled steel sheet can be improved.

The heating of the rough rolled web may be carried out by a known means. For example, an electromagnetic (solenoid type) induction heating device may be provided between the rough rolling extension and the finishing rolling mill, according to the longitudinal direction temperature of the rough rolled material in the upstream side of the induction heating device. Distribution, etc., controlling the amount of heating and heating caused by a solenoid type induction heating device.

The condition from the end of the hot rolling to the start of the coiling is not particularly limited. However, in order to improve the cold rolling ductility of the hot-rolled steel sheet by softening the hot-rolled steel sheet, it is preferable to make the coiling temperature (when the coiling is started) The temperature is 600 ° C or more. The coiling temperature is preferably 640 ° C or higher, more preferably 680 ° C or higher. If the coiling temperature is too high, the pickling property of the hot-rolled steel sheet may be impaired, so the coiling temperature is preferably 750 ° C or lower, preferably lower than 720 ° C. Preferably, after the coiling, the temperature range from the coiling temperature to (winding temperature - 50 ° C) is cooled at an average cooling rate exceeding 15 ° C / hour. Thereby, the productivity is improved, and at the same time, in the annealing step described later, the dissolution of the carbide can be promoted.

The hot-rolled steel sheet is cold-rolled in accordance with a general method to form a cold-rolled steel sheet. Descaling can also be carried out by pickling or the like before cold rolling. In order to promote recrystallization, the metal structure after cold rolling and annealing is homogenized, and the local ductility is further improved, and the rolling reduction ratio of the cold rolling is preferably 40% or more. If the rolling reduction ratio is too high, the rolling load will increase and the rolling will become difficult. Therefore, the rolling reduction is preferably less than 70%, preferably less than 60%.

Next, the steps of the method for producing a hot-dip galvanized steel sheet and the method for producing the alloyed hot-dip galvanized steel sheet according to the present embodiment will be described.

[Step of Annealing] (Heating Temperature: Exceeding Ac ₁ Point) In the step of annealing the billet steel sheet, the billet steel sheet is heated. In order to generate the Worthite iron during heating, the heating temperature is made to exceed Ac ₁ point. The Ac ₁ point is the temperature at which the Worthite iron starts to be formed in the metal structure when the billet steel sheet is heated. In order to improve the local ductility of the hot-dip galvanized steel sheet and the alloyed hot-dip galvanized steel sheet by homogenizing the metal structure, it is preferable to heat the billet steel sheet to a point exceeding Ac ₃ and perform annealing. The Ac ₃ point is the temperature at which the ferrite iron disappears in the metal structure when the billet steel sheet is heated.

By heating the billet steel sheet to the above temperature range, that is, heating to the Worthite iron region, the carbide is dissolved, and in the metal structure of the hot-dip galvanized steel sheet and the alloyed hot-dip galvanized steel sheet, the amount of Worthite iron remains. The amount of C in the residual Worthfield iron will increase.

The upper limit of the heating temperature is not particularly limited. However, if the heating temperature is too high, the Worthite iron will be coarsened and the local ductility will be impaired, so the heating temperature should be (Ac _{3 +} 100) ° C or less, preferably ( Ac ₃ points +50) °C or less. The heating time is not particularly limited, and the heating time is not particularly limited. However, in order to homogenize the metal structure in the coil, the holding time is preferably 10 seconds or more, and the point of suppressing the coarsening of the Worthite iron is Should be maintained for more than 240 seconds.

[Step of performing first cooling] (average cooling rate in a temperature range from heating temperature to (heating temperature - 50 ° C): 7 ° C / sec or less) After heating the billet steel sheet to a point exceeding Ac ₃ and annealing In the case of performing the first cooling, the average cooling rate in the temperature range from the heating temperature to (heating temperature - 50 ° C) is preferably 7 ° C / sec or less. By this cooling, in the metal structure of the hot-dip galvanized steel sheet and the alloyed hot-dip galvanized steel sheet, the amount of Mn remaining in the Worthite iron increases, and the polygonal ferrite iron is formed, and the uniform ductility and local ductility are improved.

When the average cooling rate in the temperature range from the heating temperature to (heating temperature - 50 ° C) exceeds 7 ° C / sec, the amount of Mn in the residual Worth iron decreases, and at the same time, the polygonal ferrite iron is reduced, and uniform ductility And local ductility will be impaired. Therefore, it is preferred that the average cooling rate in the above temperature zone be 7 ° C / sec or less. Preferably, the average cooling rate in the above temperature zone is 5 ° C / sec or less. The lower limit of the average cooling rate is not particularly limited, but from the viewpoint of productivity, it is preferably 1 ° C / sec or more.

Further, if the temperature region for cooling at an average cooling rate of 7 ° C /sec or less is wider, the amount of Mn in the residual Worth iron is increased, and the amount of polygonal ferrite is increased. Therefore, it is preferred to cool the billet steel sheet at a temperature ranging from the heating temperature to (heating temperature - 100 ° C) at an average cooling rate of 7 ° C / sec or less, and preferably from the heating temperature to (heating temperature - In the temperature zone up to 150 ° C), the billet steel sheet is cooled at an average cooling rate of 7 ° C / sec or less.

(Average cooling rate in a temperature range of 650 ° C to 500 ° C: 2 ° C / sec or more and less than 100 ° C / sec) In the first cooling step, the average in the temperature range of 650 ° C to 500 ° C The cooling rate was 2 ° C / sec or more and less than 100 ° C / sec, and isothermal was not maintained on the way until the billet steel sheet was cooled to 500 ° C or lower.

If the average cooling rate in the temperature range of 650 ° C to 500 ° C is less than 2 ° C / sec, the polygonal ferrite and the ferritic iron will be excessively formed, and the drop strength and tensile strength will be lowered. Therefore, the average cooling rate in the above temperature zone is required to be 2 ° C / sec or more. It is desirable that the average cooling rate in the above temperature zone is 3 ° C / sec or more, preferably 4 ° C / sec or more, more preferably 5 ° C / sec or more.

On the other hand, if the average cooling rate in the temperature range of 650 ° C to 500 ° C is 100 ° C / sec or more, the shape of the steel sheet may be impaired, so that the average cooling rate in the above temperature zone is less than 100 ° C / second. It is desirable that the average cooling rate in the above temperature range is 50 ° C / sec or less, preferably 30 ° C / sec or less, more preferably 20 ° C / sec.

(Cooling stop temperature: 500 ° C or less) The billet steel sheet cooled at the desired average cooling rate is continuously cooled to 500 ° C or lower. The cooling condition in the temperature range of 500 ° C or lower is not particularly limited, but it is preferable to maintain the billet steel sheet at a temperature region of 500 ° C or lower and 460 ° C or higher for 4 seconds to 45 seconds. It is preferably maintained for 10 seconds to 35 seconds. By this maintenance, in the metal structure formed in the second cooling step described later, the volume fraction of the remaining Worthite iron and the amount of C remaining in the Worstian iron can be appropriately adjusted to further improve uniform ductility and locality. Extensibility, and even worse, also increases the strength of the fall.

[Step of Performing Melt Galvanizing] After the step of performing the first cooling, the billet steel sheet is subjected to hot-dip galvanizing. At least between the step of performing the first cooling and the step of performing the hot-dip galvanizing, the billet steel sheet may be cooled and heated as needed.

The plating bath temperature and the plating bath composition of the hot-dip galvanizing are generally not limited. The amount of plating adhesion is not particularly limited and may be within the usual range. For example, each single side of the billet steel sheet is preferably an adhesion amount of 20 g/m ² to 80 g/m ² . The plating temperature is not particularly limited, but is usually 460 ° C to 470 ° C.

[Step of alloying treatment] When manufacturing the alloyed hot-dip galvanized steel sheet, after the step of performing the hot-dip galvanizing, the hot-dip galvanized billet steel sheet is heated to be required for alloying the hot-dip galvanizing alloy The temperature (alloying treatment temperature) is alloyed.

The alloying treatment is preferably carried out under the conditions that the Fe concentration in the plating layer is 7% by mass or more. For example, it is desirable to carry out the alloying treatment under the conditions of an alloying treatment temperature of 470 ° C to 560 ° C and an alloying treatment time of 5 seconds to 60 seconds.

[Step of performing second cooling] (Average cooling rate in a temperature range from a plating temperature or an alloying treatment temperature to 300 ° C: 2 ° C / sec or more) (Cooling stop temperature: 300 ° C or less) In the cooling after the step of zinc or after the step of alloying, the average cooling rate in the temperature range from the plating temperature to 300 ° C or the temperature range from the alloying treatment temperature to 300 ° C is 2 After cooling at °C/sec or more, it is cooled to 300 ° C or lower.

When the average cooling rate in the second cooling step is less than 2 ° C / sec, the excessively generated Boron iron, the lodging strength and the tensile strength are lowered, and the amount of residual Worstian iron is reduced, and the uniform ductility is impaired. Therefore, the average cooling rate in the above temperature range is made 2 ° C / sec or more. It is desirable that the average cooling rate in the above temperature zone is 3 ° C / sec or more, preferably more than 5 ° C / sec, more preferably more than 10 ° C / sec.

The upper limit of the average cooling rate in the step of performing the second cooling is not particularly limited, but is preferably 500 ° C / sec or less from the viewpoint of economy. Further, in order to efficiently perform the temper rolling as described later, the cooling stop temperature is preferably room temperature.

The billet steel sheet after the second cooling step preferably contains the residual Worthite iron in an amount of 5.0% or more and 35.0% or less by volume, and has a metal structure in which the amount of C in the residual Worth iron is less than 0.85 mass%. Thereby, in the step of performing heat treatment described later, C concentration and C Mn concentration toward the residual Worthite iron can be promoted, uniform ductility and local ductility are improved, and the lodging strength is also increased.

The volume fraction of the residual Worthite iron is preferably 10.0% or more and 30.0% or less, more preferably 15.0% or more and 25.0% or less. The amount of C in the residual Worthite iron is preferably less than 0.80% by mass, more preferably less than 0.75% by mass, and particularly preferably less than 0.70% by mass. The lower limit of the amount of C in the residual Worthite iron is not particularly limited, but about 0.50% by mass is a substantially lower limit value.

[Step of temper rolling] (stretching ratio: 0.10% or more) After the second cooling step, the billet was subjected to a temper rolling of 0.10% or more. By the tempering and rolling, in the heat treatment step described later, C concentration and C Mn concentration in the Worstian iron can be promoted, and in the metal structure of the hot-dip galvanized steel sheet and the alloyed hot-dip galvanized steel sheet. The amount of C and the amount of Mn in the residual Worthite iron are increased, the uniform ductility and local ductility are improved, and the lodging strength is also increased.

When the elongation is less than 0.10%, the above effects in the subsequent heat treatment step cannot be obtained, so that the elongation is 0.10% or more. The stretching ratio is preferably 0.30% or more, preferably 0.50% or more. Although the upper limit of the stretching ratio is not particularly limited, if the stretching ratio is too high, the rolling load is increased, so the stretching ratio is preferably 2.00% or less. Preferably, the stretch ratio is less than 1.50%, more preferably less than 1.00%.

The temperature at which the temper rolling is performed is not particularly limited. However, in order to effectively impart a processing strain to the Worth iron, the temperature at which the temper rolling is performed is preferably a low temperature, and the starting temperature of the temper rolling is preferably room temperature. In addition, the temper rolling is preferably carried out by surface temper rolling.

[Step of heat treatment] (heating temperature: 200 ° C to 600 ° C) (maintenance time: 1 second or more) After the step of temper rolling, the billet steel sheet is heated to a temperature range of 200 ° C to 600 ° C, and Maintain at this temperature for 1 second or longer.

When the heat treatment temperature (maximum heating temperature) is lower than 200 ° C, C concentration in C and Mn toward the Vostian iron and Mn concentration become insufficient, and uniform ductility is impaired. In addition, when the heat treatment temperature (the highest heating temperature) is lower than 200 ° C, there is a hard granulated loose iron, the local ductility is impaired, and the lodging strength is also lowered. Therefore, the heat treatment temperature is required to be 200 ° C or higher. The heat treatment temperature is preferably 240 ° C or higher, preferably 260 ° C or higher, more preferably 280 ° C or higher.

On the other hand, when the heat treatment temperature exceeds 600 ° C, the amount of residual Worth iron is insufficient, the uniform ductility is impaired, and the tempered granulated iron is excessively softened, and the drop strength and tensile strength are lowered. In addition, if the heat treatment temperature exceeds 600 ° C, a hard new ramification iron is formed, so the local ductility is impaired and the lodging strength is also lowered. Therefore, the heat treatment temperature should be 600 ° C or less. The heat treatment temperature is preferably 550 ° C or lower, preferably 500 ° C or lower, more preferably 450 ° C or lower.

When the heat treatment time (the maintenance time at the highest heating temperature) is less than 1 second, C concentration and Mn concentration in C and Mn toward the Vostian iron become insufficient, and uniform ductility is impaired. In addition, if the heat treatment time is less than 1 second, a hard granulated iron is left, and the local ductility is impaired, and the lodging strength is also lowered. Therefore, the heat treatment time is required to be 1 second or longer. It is desirable that the heat treatment time is more than 5 seconds, preferably more than 10 seconds, more preferably more than 15 seconds.

On the other hand, if the heat treatment time is too long, the amount of iron in the remaining Worth is reduced, the uniform ductility is impaired, and the tempered granulated iron is excessively softened, and the lodging strength and tensile strength are lowered. In addition, if the heat treatment time is too long, there will be a hard new generation of granulated iron, which will be damaged in local ductility and the lodging strength will also decrease. Therefore, the upper limit of the heat treatment time is preferably 5760 minutes or less. The heat treatment time is preferably 2880 minutes or less, more preferably 1440 minutes or less.

The heat treatment time should be moderately adjusted according to the heat treatment temperature. For example, when the heat treatment temperature is 200 ° C or more and 300 ° C or less, the heat treatment time is preferably more than 3 minutes, preferably more than 10 minutes, more preferably more than 20 minutes.

When the heat treatment temperature is 400 ° C or more and 600 ° C or less, the heat treatment time is preferably 20 minutes or shorter, preferably 6 minutes or shorter, more preferably less than 3 minutes. From the viewpoint of productivity, the heat treatment temperature should preferably exceed 400 ° C, and the heat treatment time should be 20 minutes or less.

After the heat treatment step, in order to correct the billet steel sheet to a flat shape, the billet steel sheet may be subjected to temper rolling, and an oil-coated or lubricated film may be applied to the billet steel sheet.

The thickness of the hot-dip galvanized steel sheet and the alloyed hot-dip galvanized steel sheet according to the present embodiment is not limited to a specific thickness. However, the method for producing a hot-dip galvanized steel sheet and an alloyed hot-dip galvanized steel sheet according to the present embodiment is suitable for production. Steel plate with a thickness of 0.8mm~2.3mm.

[Examples] Next, the examples of the present invention will be described, but the conditions in the examples are only a conditional example used to confirm the workability and effects of the present invention, and the present invention is not limited to this condition. example. The present invention can be obtained by various conditions as long as the object of the present invention can be achieved without departing from the gist of the present invention.

(Example 1) A molten steel having a chemical composition shown in Table 1 was cast using a vacuum melting furnace, and steels A to S were produced. The Ac ₁ point and the Ac ₃ point in Table 1 were obtained by changing the thermal expansion when the cold-rolled steel sheets of steels A to S were heated at 2 ° C / sec. After the steels A to S were heated to 1200 ° C and maintained for 60 minutes, hot rolling was carried out under the conditions shown in Table 2.

Specifically, in the temperature zone of Ar ₃ or more, the steel A to S is rolled 10 times to obtain a hot-rolled steel sheet having a thickness of 2.5 mm to 3.0 mm. After the hot rolling, the hot-rolled steel sheet is cooled to 550 ° C to 680 ° C by spraying water, and the cooling end temperature is taken as the coiling temperature, and the hot-rolled steel sheet is placed in an electric heating furnace maintained at the coiling temperature, and maintained. 60 minutes. Thereafter, the hot rolled steel sheet was furnace-cooled to room temperature at a cooling rate of 20 ° C / hour to simulate the cold cooling after coiling.

The cold-rolled hot-rolled steel sheet is pickled and used as a base material for cold rolling, and is cold-rolled at a rolling reduction ratio of 47 to 52% to obtain a cold-rolled steel sheet having a thickness of 1.2 mm to 1.6 mm. Steel plate). The billet steel sheet was heated to 650 ° C at a heating rate of 10 ° C / sec using a hot-dip galvanizing simulator, and then heated at a heating rate of 2 ° C / sec until the temperature shown in Table 2, and soaking for 30 to 90 seconds.

Thereafter, the billet steel sheet was cooled to 460 ° C under the cooling conditions shown in Table 2, and the billet steel sheet was immersed in a hot-dip galvanizing bath maintained at 460 ° C, and the billet steel sheet was subjected to hot-dip galvanizing. For a part of the billet steel sheet, it was heated to 520 ° C after hot-dip galvanizing, and alloying treatment was performed.

The billet steel sheet was subjected to secondary cooling (second cooling) from the plating temperature (meaning the plating bath temperature) or the alloying treatment temperature under the cooling conditions shown in Table 2. After subjecting the secondary-cooled billet steel sheet to temper rolling with a draw ratio of 0.50%, heat treatment is performed under the heat treatment conditions shown in Table 2 to obtain a hot-dip galvanized steel sheet or an alloyed hot-dip galvanized steel sheet (hereinafter, The hot-dip galvanized steel sheet and the alloyed hot-dip galvanized steel sheet are collectively referred to as "plated steel sheets".

When the stop temperature of the secondary cooling was 100 ° C, the surface temper rolling was performed without cooling to room temperature after the secondary cooling was stopped, and then heat treatment was performed under the heat treatment conditions shown in Table 2 without cooling to room temperature. For a part of the billet steel sheet, surface temper rolling or heat treatment is omitted.

The "thickness after rolling" in the hot rolling conditions described in Table 2 is the thickness of the obtained hot rolled steel sheet. The "residence time in the temperature range of 500 to 460 ° C" in the annealing conditions described in Table 2 means the residence time in the temperature range of 500 to 460 ° C in the step of performing the first cooling. Regarding the "with or without alloying treatment" in the annealing conditions described in Table 2, the symbol "Yes" indicates that the alloying treatment was performed after the hot-dip galvanizing, and the symbol "None" indicates that the alloy was not alloyed after the hot-dip galvanizing. Processing. The "secondary cooling rate" in the annealing conditions described in Table 2 means that the alloying treatment means an average cooling rate in a temperature range from the alloying temperature to 300 ° C, and if it is not alloyed. In this case, it means the average cooling rate in the temperature range from the plating temperature to 300 °C. In Table 2, the mark "RT" indicates room temperature. Regarding the "with or without quenching and rolling" in Table 2, the symbol "Yes" indicates that the quenching and tempering was carried out in the step of performing the quenching and tempering, and the symbol "None" indicates that the quenching and rolling were not performed. In the column labeled "heat treatment conditions" in Table 2, the mark "-" indicates that heat treatment has not been performed.

〔Table 1〕

〔Table 2〕

A test piece for XRD measurement was taken from the plated steel sheet and the billet steel sheet after the completion of the secondary cooling, and the rolled surface of the test piece was chemically ground from the boundary between the steel sheet and the plating layer to a position of 1/4 depth of the thickness of the steel sheet. . The X-ray diffraction test was carried out on the rolling surface, and the volume fraction of the residual Worthite iron and the amount of C in the remaining Worthite iron were measured.

Specifically, the Mo-Kα line is incident on the test piece, and the integrated intensity of the α phase (200) and (211) diffraction peaks and the integrated intensity of the γ phase (200), (220), and (311) diffraction peaks are measured. The volume fraction of the residual Worthite iron is obtained.

In addition, the incident Fe-Kα line is obtained, and the lattice constant (a _γ ) of the Worthite iron is obtained from the positions of the γ phase (200), (220), and (311) diffraction peaks, and a _γ (Å) is used. A relational expression of =3.578 + 0.033 × C _γ (% by mass) is used to calculate the amount of C (C _γ ) in the residual Worth iron.

Further, a test piece for SEM observation was taken from the plated steel sheet, and a longitudinal section parallel to the rolling direction of the test piece was polished, and then the longitudinal section was subjected to natal corrosion and LEPERA corrosion. The liquid was corroded and the metal structure in the range of 1/4 depth from the boundary of the steel sheet and the plating layer to the thickness of the steel sheet was observed. By image processing, the volume fraction of the tempered granulated iron, the polygonal ferrite iron, the new granulated iron, and the remaining part of the tissue were measured.

The volume fraction of the new Ma Tian loose iron is determined by the sum of the volume ratios of the residual Worthite iron and the new Ma Tian loose iron measured by Lepera corrosion, minus the measurement by the above XRD measurement. The volume fraction of the remaining Worth iron.

The tensile stress (YS), tensile strength (TS), and uniform elongation (UE1) were taken from the plated steel sheet in the direction of the rolling direction, and the tensile test piece was applied to the test piece. And ask for it.

The stretching speed was set to 1 mm/min until reaching the point of undulation, and was set to 10 mm/min thereafter. The drop ratio (YR) is obtained by dividing YS by TS. The total elongation (TEl) and the local elongation (LEl) are tensile tests of JIS No. 5 tensile test pieces taken along the direction orthogonal to the rolling direction, using the measured values of total elongation (TEl ₀ ) and The measured value (UE1) of the uniform elongation was calculated based on the above formula (2) and formula (3), and the converted value corresponding to a plate thickness of 1.2 mm was obtained.

When the value of YR is 0.59 or more, and the value of TS×UE1 is 10000 MPa·% or more, and the value of TS×LEl is 5,000 MPa·% or more, it is judged to be a good characteristic. In addition, when the value of TS × UE1 is 12,000 MPa·% or more, and the value of TS × LE1 is 6,000 MPa % or more, it is judged to be particularly excellent.

Table 3 shows the results of observing the metal structure after the completion of the secondary cooling, the results of observing the metal structure of the plated steel sheet, and the results of evaluating the mechanical properties of the plated steel sheet.

In the column labeled "Metal structure after completion of secondary cooling" in Table 3, the symbol "-" indicates that metal structure observation was not performed. In the column labeled "C amount (% by mass) in the residual Worthite iron" in Table 3, the symbol "-" indicates that the amount of C in the residual Worthite iron was not measured. In Table 3, the column labeled "TEl" indicates that the total elongation has been converted to a thickness equivalent to 1.2 mm, and the column labeled "UE1" indicates the uniform elongation and is labeled "LEl". The column indicates that the local elongation has been converted to a thickness equivalent to 1.2 mm.

In the remark column of Table 3, the sample with "○" is an example of the present invention, and the sample with "x" is a comparative example. Further, in Tables 1 to 3, the numerical value or symbol attached to the bottom line means that it is outside the scope of the present invention.

〔table 3〕

The invention examples with the ○ mark in the remark column (test numbers A1 to A3, A9, A11, A13, A14, A19, A21, A23, A26, A28 to A37, and A40 to A45) are all TS × UE1 at 10000. Above, TS × LE1 is above 5000 and shows good uniform ductility and local ductility. In addition, YR shows a higher value of 0.59 or more. In particular, for the test numbers A11, A21, A26, A28, A30, A31, A34, etc., the tempered granulated iron is more than 16%, and the polygonal ferrite is more than 2.0%, and the TS × UE1 is above 12,000. TS × LE1 is above 6000, showing particularly good uniform ductility and local ductility.

On the other hand, the test results outside the scope of the present invention for the chemical composition or the step conditions (test numbers A4 to A8, A10, A12, A15 to A18, A20, A22, A24, A25, A27, A20, A22, A24, A25, A27, A38, and A39) are inferior in any or all of the aspect ratio, uniform ductility, and local ductility.

Specifically, although steels C, E, and N having chemical compositions within the scope of the present invention were used, TS × UE1 and TS × LE1 were lower in test numbers A15, A24, and A38 where surface temper rolling was not performed. . The test using steels A and C (test numbers A10 and A20) was not heat-treated, so the value of YR and TS × LEl was lower in test number A10. In test number A20, YR, TS × UE1 and TS The value of ×LEl is lower.

Tests using steels A, C, E, and N (test numbers A4, A16, A25, and A39) have a low value of YR and TS × LEl in test number A4 because of the low heat treatment temperature, at test number A16, In A25 and A39, the values of YR, TS×UE1 and TS×LEl are low. Further, in the tests using the steels A, C, and F (test numbers A5, A17, and A27), since the heat treatment temperature was too high, YR, TS × UE1, and TS × LE1 were low.

Although steel A having a chemical composition within the scope of the present invention was used, in Test No. A6 in which the soaking temperature was too low in the annealing step, TS × UE1 was low. In the test using steel A (test number A7), since the average cooling rate in the temperature range of 650 to 500 ° C was too low in the first cooling step, YR and TS × LE1 were low. In the test using the steels A and C (test numbers A8 and A18), since the average cooling rate (secondary cooling rate) in the temperature range of the alloying temperature of -300 ° C is too low in the second cooling step, In the test number A8, the values of YR and TS × LE1 are low, and the values of TS × UE1 and TS × LE1 are lower in the test number A18.

In the test number A12 using steel B, since the amount of Si in the steel is small, YR, TS × UE1, and TS × LE1 are low. In the test No. A22 using steel D, since the amount of Mn in the steel was small, YR and TS × LE1 were low.

(Example 2) An experiment was conducted in the same manner as in Example 1, and a plated steel sheet was produced under the conditions shown in Table 4 for the steels A to S shown in Table 1. The results are shown in Table 5. Further, the measurement procedure is the same as in the first embodiment.

In addition, the amount of Mn in the residual Worthite iron is obtained by taking an EBSP measurement test piece from a plated steel sheet, and performing electrolytic polishing on a longitudinal section parallel to the rolling direction, and then observing the thickness from the boundary between the steel sheet and the plating layer to the steel sheet. The metal structure in the 1/4 depth position, and the distribution of the residual Worthite iron was confirmed by image processing. Next, using a SEM equipped with FE-EPMA, the metal structure in the same field of view was observed, and EMPA measurement was performed on 10 or more residual Worthfield iron particles, and the amount of Mn in the residual Worthite iron was measured. The average value of the measured Mn amount was determined, and the average value was obtained as the amount of Mn ([Mn] _γ ) in the residual Worthite iron. The amount of Mn of the steel sheet of the base material was [Mn] _ave , and [Mn] _γ / [Mn] _{ave was} calculated.

When the value of YR is 0.59 or more, and the value of TS × UE1 is 10000 MPa% or more, and the value of TS × LE1 is 5,000 MPa % or more, it is judged to be a good characteristic. In addition, when the value of TS × UE1 is 12,000 MPa·% or more, and the value of TS × LE1 is 6,000 MPa % or more, it is judged to be particularly excellent. In addition, the description of Table 4 and Table 5 is the same as Table 2 and Table 3, respectively. Further, in the column labeled "[Mn] _γ /[Mn] _ave ", the symbol "-" indicates that the amount of Mn in the residual Worthite iron was not measured.

〔Table 4〕

〔table 5〕

The invention examples with the ○ mark in the remark column (test numbers B1, B2, B5, B6, B11, B13, B14, B18, B21~B23, B25~B35, and B38~B42) are all TS × UE1 at 10000 Above, TS × LE1 is above 5000 and shows good uniform ductility and local ductility. In addition, YR shows a higher value of 0.59 or more.

In particular, the test numbers B1, B5, B6, B11, B18, B23, B26, B27, B29, B30, B32~B35, B38, and B39 have a heating temperature exceeding Ac ₃ and are in the first cooling step. The average cooling rate in the temperature range from the heating temperature to (heating temperature - 50 ° C) is 7 ° C / sec or less, so the volume fraction of the further polygonal ferrite is 2.0% or more, [Mn] _γ / [Mn] _Ave is 1.10 or higher. As a result, the samples of these test numbers were TS × UE1 of 12,000 or more, and TS × LE1 of 6000 or more, showing particularly good uniform ductility and local ductility.

On the other hand, the test results of the steel sheets outside the range of the present invention for the chemical composition or the step conditions (test numbers B3, B4, B7 to B10, B12, B15 to B17, B19, B20 of the mark of the remark column) B24, B36, and B37), either or both of the drop ratio, uniform ductility, and local ductility are poor.

Specifically, although steels C, E, and N having a chemical composition within the range of the present invention were used, the amount of C in the Worthite iron and [Mn) were left in the test numbers B7, B19, and B36 where the temper rolling was not performed. ] _γ /[Mn] _{ave is} low, and TS × UE1 and TS × LEl are low. The test number B12 using steel C was not heat-treated, so the volume fraction of tempered granulated iron, the amount of C in the residual Worthite iron, and [Mn] _γ /[Mn] _{ave were} lower, and YR, TS × UE1 and TS × LEl is lower.

Although steels C, E, and N having chemical compositions within the scope of the present invention were used, in the test numbers B8, B20, and B37 where the heat treatment temperature was too low, the volume ratio of the tempered granules in the granulated iron was retained in the residual Worthite iron. The amount of C and [Mn] _γ /[Mn] _{ave are} low, and YR, TS × UE1 and TS × LEl are low. In the test using the steels C and F (test numbers B9 and B24), since the heat treatment temperature is too high, the volume fraction of the remaining Worthite iron and the amount of C in the residual Worthite iron are low, and YR, TS×UEl And TS × LEl is lower.

Although steel C having a chemical composition within the scope of the present invention was used, in the test number B16 in which the soaking temperature was too low in the annealing step, the volume fraction of the remaining Worthite iron and the volume ratio of the tempered sesame loose iron were low. And TS × UEl is lower. In the test using the steels A and C (test numbers B3 and B15), since the average cooling rate in the temperature range of 650 to 500 ° C in the first cooling step was too low, in the test number B3, the residue was left. The volume fraction of the Wostian iron, the volume fraction of the tempered granules and the [Mn] _γ /[Mn] _{ave are} lower, and the YR and TS×LEl are lower. In test number B15, the residual Worthite volume fraction and [Mn] _γ /[Mn] _{ave were} lower, and YR, TS × UE1 and TS × LEl were lower. Although the steel C having the chemical composition within the range of the present invention is used, the test number B10 in which the average cooling rate (secondary cooling rate) in the temperature range of the alloying treatment temperature to 300 ° C is too low in the second cooling step is used. Among them, the volume fraction of the residual Worthfield iron and the amount of C in the residual Worthfield iron are low, and TS×UE1 and TS×LEl are low.

In the test number B4 using steel B, since the amount of Si in the steel is small, the volume fraction of the remaining Worthite iron and the amount of C in the residual Worthite iron are low, and YR, TS × UE1 and TS × LEl are low. . In the test No. B17 using the steel D, since the amount of Mn in the steel is small, the volume fraction of the remaining Worstian iron and [Mn] _γ / [Mn] _{ave are} low, and YR and TS × LE1 are low.

INDUSTRIAL APPLICABILITY As described above, according to the present invention, it is possible to manufacture and provide a hot-dip galvanized steel sheet and an alloyed hot-dip galvanized steel sheet which are excellent in uniform ductility and local ductility, and moreover, lodging strength and pulling It has high tensile strength and excellent formability and impact absorption. The hot-dip galvanized steel sheet and the alloyed hot-dip galvanized steel sheet produced by the present invention are most suitable for structural parts of automobile bodies such as members and pillars, and therefore the present invention has high industrial applicability.

(no)

Fig. 1 is a view for explaining a method of manufacturing a hot-dip galvanized steel sheet. Fig. 2 is a view for explaining a method of producing an alloyed hot-dip galvanized steel sheet.

Claims (13)

A plated steel sheet characterized by having a chemical composition containing C: 0.03% to 0.70%, Si: 0.25% to 2.50%, Mn: 1.00% to 5.00%, P: 0.100% or less, and S: 0.010% by mass%. Hereinafter, sol.Al: 0.001% to 2.500%, N: 0.020% or less Ti: 0% to 0.300%, Nb: 0% to 0.300%, V: 0% to 0.300%, Cr: 0% to 2.000%, Mo : 0%~2.000%, B: 0%~0.0200%, Cu: 0%~2.000%, Ni: 0%~2.000%, Ca: 0%~0.0100%, Mg: 0%~0.0100%, REM: 0 %~0.1000%, and Bi: 0%~0.0500%, and the remainder consists of iron and impurities; the metal structure contains more than 8.0% by volume of residual Worthite iron, more than 5.0% by volume of tempered 麻田散铁; The amount of C in the residual Worthite iron is 0.85 mass% or more; and the amount of Mn in the above-mentioned residual Worthite iron satisfies the following formula (1): [Mn] _γ / [Mn] _ave ≧ 1.10 (1) [Mn _γ : Mn amount (% by mass) in the residual Worth iron [Mn] _ave : Mn amount (% by mass) of the chemical composition of the steel sheet. The plated steel sheet according to claim 1, wherein the aforementioned metal structure further contains more than 2.0% by volume of polygonal ferrite. The plated steel sheet according to claim 1 or 2, wherein the chemical composition further contains, in mass%, selected from the group consisting of Ti: 0.001% to 0.300%, Nb: 0.001% to 0.300%, and V: 0.001% to 0.300%. One or two or more of the groups formed. The plated steel sheet according to claim 1 or 2, wherein the chemical composition further contains, in mass%, selected from the group consisting of Cr: 0.001% to 2.000%, Mo: 0.001% to 2.000%, and B: 0.0001% to 0.0200%. One or two or more of the groups formed. The plated steel sheet according to claim 1 or 2, wherein the chemical composition further contains, in mass%, selected from Cu: 0.001% to 2.000%, and Ni: One or two of the groups consisting of 0.001% to 2.000%. The plated steel sheet according to claim 1 or 2, wherein the chemical composition further contains, in mass%, from Ca: 0.0001% to 0.0100%, Mg: 0.0001% to 0.0100%, and REM: 0.0001% to 0.1000%. One or two or more of the groups formed. The plated steel sheet according to claim 1 or 2, wherein the chemical composition further contains Bi: 0.0001% to 0.0500% by mass%. The plated steel sheet according to claim 1 or 2, wherein the plated steel sheet is a hot-dip galvanized steel sheet comprising a hot-dip galvanized layer. The plated steel sheet according to claim 1 or 2, wherein the plated steel sheet is an alloyed hot-dip galvanized steel sheet in which a hot-dip galvanized layer has been alloyed. A method for producing a hot-dip galvanized steel sheet, comprising the steps of: heating a billet steel sheet to a point exceeding Ac ₁ and performing annealing, wherein the chemical composition of the billet steel sheet contains C: 0.03% by mass% 0.70%, Si: 0.25% to 2.50%, Mn: 1.00% to 5.00%, P: 0.100% or less, S: 0.010% or less, sol. Al: 0.001% to 2.500%, N: 0.020% or less, Ti: 0% ~0.300%, Nb: 0%~0.300%, V: 0%~0.300%, Cr: 0%~2.000%, Mo: 0%~2.000%, B: 0%~0.0200%, Cu: 0%~2.000 %, Ni: 0%~2.000%, Ca: 0%~0.0100%, Mg: 0%~0.0100%, REM: 0%~0.1000%, and Bi: 0%~0.0500%, and the remainder is made of iron and impurities The first cooling step is performed by cooling to 500 at an average cooling rate of 2 ° C /sec or more and less than 100 ° C / sec in a temperature range of 650 ° C to 500 ° C after the annealing step. °C or less; the step of hot-dip galvanizing, after the step of performing the first cooling, performing hot-dip galvanizing on the billet steel sheet cooled in the step of performing the first cooling; and the second cooling step, performing the melting in the foregoing After the galvanizing step, the melting is performed from the foregoing In the temperature range from 300° C. in the galvanizing step to an average cooling rate of 2° C./sec or more, the hot-dip galvanized billet steel sheet is cooled to below 300° C.; In the step of performing the second cooling step, the billet rolling of the billet having a cooling rate of 0.10% or more in the step of performing the second cooling step; and the step of heat treatment are performed in the foregoing After the step of temper rolling, the billet steel sheet subjected to the above-mentioned quenching and tempering is heated to a temperature range of 200 ° C to 600 ° C, and maintained at this temperature for 1 second or more. The method for producing a hot-dip galvanized steel sheet according to claim 10, wherein in the step of annealing, the billet steel sheet is heated to a point exceeding Ac ₃ and annealed; and after the annealing step, from the heating temperature to ( The annealed billet steel sheet is cooled at an average cooling rate of 7 ° C /sec or less in a temperature range up to a heating temperature of -50 ° C. A method for producing an alloyed hot-dip galvanized steel sheet, comprising the steps of: heating a billet steel sheet to a point exceeding Ac ₁ and performing annealing, wherein the raw material steel sheet has a chemical composition containing C: 0.03 by mass% %~0.70%, Si: 0.25%~2.50%, Mn: 1.00%~5.00%, P: 0.100% or less, S: 0.010% or less, sol.Al: 0.001%~2.500%, N: 0.020% or less Ti: 0%~0.300%, Nb: 0%~0.300%, V: 0%~0.300%, Cr: 0%~2.000%, Mo: 0%~2.000%, B: 0%~0.0200%, Cu: 0% ~2.000%, Ni: 0%~2.000%, Ca: 0%~0.0100%, Mg: 0%~0.0100%, REM: 0%~0.1000%, and Bi: 0%~0.0500%, and the rest is made of iron And the first cooling step, after the step of annealing, cooling to 500 in a temperature range of 650 ° C to 500 ° C at an average cooling rate of 2 ° C / sec or more and less than 100 ° C / sec. °C or less; the step of hot-dip galvanizing, after the step of performing the first cooling, performing hot-dip galvanizing on the billet steel sheet cooled in the step of performing the first cooling; and the step of alloying, melting in the foregoing After the galvanizing step, the pair has been The hot-dip galvanized blank steel sheet is alloyed at an alloying treatment temperature; and the second cooling step is performed in the temperature region from the foregoing alloying treatment temperature to 300 ° C after the step of alloying treatment described above The billet steel sheet subjected to the alloying treatment is cooled to 300 ° C or lower at an average cooling rate of 2 ° C /sec or more; the step of quenching and tempering is carried out, and after the second cooling step, the second step is performed. The cooled billet steel sheet in the cooling step is subjected to temper rolling and rolling at a stretching ratio of 0.10% or more; and a heat treatment step, after the step of temper rolling and rolling, the embryo which has been subjected to the quenching and tempering The steel sheet is heated to a temperature range of 200 ° C to 600 ° C and maintained at this temperature for more than 1 second. The method for producing an alloyed hot-dip galvanized steel sheet according to claim 12, wherein in the step of performing annealing, the billet steel sheet is heated to a point exceeding Ac ₃ and annealed; and after the step of annealing is performed, The annealed billet steel sheet is cooled at an average cooling rate of 7 ° C /sec or less in a temperature range from the heating temperature to (heating temperature - 50 ° C).