TW201816141A - Plated steel sheet, method for producing hot-dip galvanized steel sheet, and method for producing alloyed hot-dip galvanized steel sheet having excellent properties of uniform ductility and local ductility - Google Patents

Abstract

In the plated steel sheet, it satisfies the chemical composition in terms of mass percentage and at least contains C: 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, N: 0.020% or less, the remainder consisting of iron and impurities. In addition, the metal structure contains more than 5.0% by volume of residual Austenite iron, more than 5.0% by volume of tempered Martensite iron, and the amount of C in the residual Austenite iron is 0.85% by mass or more.

Description

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

FIELD OF THE INVENTION The present invention relates to a method for manufacturing a plated steel sheet, a hot-dip galvanized steel sheet, and an alloyed hot-dip galvanized steel sheet. In particular, the present invention relates to a high-strength hot-dip galvanized steel sheet suitable for press forming, such as a car body, and having excellent uniform ductility and local ductility, and a high-strength alloyed hot-dip galvanized steel sheet and a method for manufacturing the same.

Background of the Invention Today, the industrial technology field has been highly divided, and materials used in various technical fields require special and high performance. Regarding automotive steel sheets, there is a demand for higher strength in order to improve fuel consumption by reducing the weight of the vehicle body. The so-called strength means both the drop strength and the tensile strength.

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

In addition, in order to improve the crash safety performance of automobiles, automotive steel sheets need to have excellent impact absorption. From the standpoint of impact absorption, in addition to higher strength of automotive steel sheets, in order to suppress breakage when subjected to impact loads, local ductility must be excellent.

In this way, the requirements for automotive steel sheets are (1) high strength for weight reduction and collision safety, (2) high uniform ductility to improve formability, and (3) improvement in formability and collision safety. High local ductility.

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

So far, there have been proposals for a technique of containing residual Vosstian iron in a metal structure as a method for improving the ductility of a high-tensile cold-rolled steel sheet. A steel sheet containing residual Vosstian iron has a large stretchability due to transformation induced plasticity (TRIP) caused by the transformation of Vosstian iron into Asada loose iron during processing.

Patent Documents 1 and 2 disclose a method for manufacturing a high-strength cold-rolled steel sheet by heating a steel sheet containing Si and Mn to a two-phase region of ferrous iron-Vostian iron or a single-phase region of Vostian iron. It is annealed and cooled, and is subjected to isothermal quenching treatment of Vosstian iron maintained at 350 to 500 ° C, so that Vosstian iron is stabilized. Through these technologies, strength and ductility can be improved in a balanced manner in cold-rolled steel sheets.

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

Patent Document 3 discloses a manufacturing method of the following high-strength alloyed hot-dip galvanized steel sheet: it contains Si and Mn in a certain proportion or more with respect to C, thereby suppressing the deformation of Vostian iron in the alloying treatment, and forming it in ferrous iron Includes the metal structure of the residual Vostian iron. However, no consideration was given to the problem of local ductility deterioration in a steel sheet containing a residual Vosstian iron in the metal structure.

Patent Document 4 discloses a high-tensile hot-dip galvanized steel sheet excellent in ductility, stretch flangeability, and fatigue resistance. The ferrous grain iron and the tempered Asada loose iron with an average crystal grain size of 10 μm or less are dispersed with residual ferrite. Staine iron and low temperature metamorphic phase. Tempered Asada loose iron is effective for improving the flange extension and fatigue resistance characteristics. If the tempered Asada loose iron is fine-grained, these properties will be further improved.

However, in order to obtain a metal structure containing tempered Asada iron and residual Vostian iron, a primary heat treatment to generate Asada loose iron and tempering Asada's loose iron to further obtain the second The secondary heat treatment greatly reduces productivity. Further, in the manufacturing method described in Patent Document 4, since a secondary heat treatment at a high temperature of _one point Ac, so tempered martensite excessively softened, it is difficult to obtain a high strength.

As described above, since the strength (drop strength and tensile strength) is opposite to the ductility (uniform ductility and local ductility), it is difficult in the conventional technology to manufacture steel plates that sufficiently improve both. of.

Prior Art Literature Patent Literature Patent Literature 1: Japanese Patent Laid-Open No. 61-157625 Patent Literature 2: Japanese Patent Laid-Open No. 61-271529 Patent Literature 3: Japanese Patent Laid-Open No. 11-279691 Patent Literature 4: Japanese Patent Laid-Open 2001-192768

Summary of the Invention Problems to be Solved by the Invention In view of such a technical background, the present invention is to provide a plated steel sheet having excellent uniform ductility and local ductility, high drop strength and tensile strength, and excellent formability and impact absorption. The purpose of a method for producing a hot-dip galvanized steel sheet and a method for producing an alloyed hot-dip galvanized steel sheet are the objects.

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 drop strength in a hot-dip galvanized steel sheet and an alloyed hot-dip galvanized steel sheet. As a result, the following opinions (A) to (E) were sought.

(A) If continuous low-galvanizing equipment is used to produce low-carbon hot-dip galvanized steel sheets containing Si and Mn or low-carbon alloyed hot-dip galvanized steel sheets containing Si and Mn, the uniform ductility and local ductility will decrease, and even worse In some cases, the intensity of the fall may also decrease. This is considered to be due to the fact that in the continuous molten galvanizing equipment, the isothermal quenching treatment of Vosstian iron becomes insufficient, and a metal structure containing residual Vosstian iron and hard Asada iron with low C concentration is formed.

(B) However, if this type of hot-dip galvanized steel sheet and alloyed hot-dip galvanized steel sheet having a metal structure containing residual Vostian iron and hard Asada iron with low C concentration is reheated, the hot-dip galvanized steel sheet and The uniform ductility and local ductility of the alloyed hot-dip galvanized steel sheet will be improved, and the drop strength will also be improved.

The reason for this is not clear, but it is presumed to be caused by (a) during the reheating process, the concentration of C toward Vostian Iron was increased, and the stability of Vostian Iron was improved; and (b) the hard Asada iron was returned. Fire changed into soft tempered Asada scattered iron.

(C) If the hot-dip galvanized steel sheet and the alloyed hot-dip galvanized steel sheet are subjected to quenching and tempering before the reheating treatment, the uniform and local ductility of the hot-dip galvanized steel sheet or the alloyed hot-dip galvanized steel sheet may be deteriorated. It is further improved, and the intensity of drop is further improved.

The reason for this is not clear, but it is presumed to be caused by (a) the introduction of differential rows in the Vosstian iron by temper rolling, and the concentration of C toward Vostian iron in the subsequent reheating process is promoted, while Mn is also concentrated. Enrichment, and the stability of Vostian Iron is further improved; (b) by temper rolling, a part of Vostian Iron is transformed into Asada loose iron, which is tempered in the metal structure after reheating treatment Iron is increased; and (c) Asana loose iron metamorphism that may occur during cooling after reheating treatment is suppressed, and hard Asada loose iron is reduced in the metal structure after reheating treatment.

(D) The effect of improving the properties of quenched and tempered rolling. In the metal structure of the hot-dip galvanized steel sheet and the alloyed hot-dip galvanized steel sheet, the smaller the amount of Vostian iron becomes, the larger it becomes.

The reason is not clear, but it is presumed to be due to (a) the processing strain is concentrated on Vosstian iron, the smaller the amount of Vosstian iron, the more the differential displacement introduced into Vosstian iron increases; and (b) by this, the During the heating process, the C-vossite iron enrichment and the Mn-vossfield iron enrichment, etc., further improved the stability of the Vosstian iron.

(E) In the metal structure of the hot-dip galvanized steel sheet and the alloyed hot-dip galvanized steel sheet manufactured by quenching and tempering and reheating, in addition to the residual Vosted iron and tempered Asada loose iron, If it contains polygonal ferrous iron, it will not damage the local ductility of the hot-dip galvanized steel sheet and alloyed hot-dip galvanized steel sheet, and the uniform ductility will be further improved.

The reason is not yet clear, but it is presumed that (a) the Mn concentration in the residual Vosstian iron increased and the stability of Vosstian iron improved; (b) the Mn in Vosstian iron usually hinders the concentration of C towards Vostian iron However, by quenching and tempering and reheating, the concentration of C toward Vostian iron is promoted, and the C concentration in the remaining Vostian iron is ensured.

Based on the findings of (A) to (E) above, the inventors have further discovered that after performing hot-dip galvanizing on a steel sheet (blank steel sheet) or after further performing hot-dip galvanizing and further alloying treatment, The following hot-dip galvanized steel sheet and alloyed hot-dip galvanized steel sheet can be manufactured by quenching and tempering and reheating: they have residual Vostian iron with high C and Mn concentrations, tempered Asada iron, and The metal structure of the polygonal ferrous iron is excellent in uniform ductility and local ductility, and has high drop strength and tensile strength.

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

(1) A plated steel sheet characterized by 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, 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% to 0.1000%, and Bi: 0% to 0.0500%, and the remainder is composed of iron and impurities; the metal structure contains more than 5.0% by volume of residual Vostian iron, and more than 5.0% by volume of tempered Asada powder Iron; and the amount of C in the aforementioned residual Vostian iron is 0.85 mass% or more.

(2) The plated steel sheet according to the above (1), wherein the metal structure further contains polygonal ferrous iron in an amount exceeding 2.0% by volume; and the amount of Mn in the residual Vosted iron satisfies the following formula (1). [Mn] _γ / [Mn] _ave ≧ 1.10… (1) [Mn] _γ : Mn content (mass%) in residual Vostian iron [Mn] _ave : Mn content (mass%) in the chemical composition of the steel sheet

(3) The plated steel sheet according to (1) or (2), wherein the chemical composition further includes, in mass%, a material 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% in one or more groups.

(4) The plated steel sheet according to any one of (1) to (3), wherein the chemical composition further includes, in mass%, a material selected from the group consisting of Cr: 0.001% to 2.000%, and Mo: 0.001%. ~ 2.000% and B: One or more of the group consisting of 0.0001% ~ 0.0200%.

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

(6) The plated steel sheet according to any one of (1) to (5), wherein the chemical composition further includes, in mass%, a material selected from the group consisting of Ca: 0.0001% to 0.0100%, and Mg: 0.0001%. ~ 0.0100% and REM: 0.0001% ~ 0.1000% in one or two or more groups.

(7) The plated steel sheet according to any one of (1) to (6), wherein the chemical composition further includes Bi in a mass% of 0.0001% to 0.0500%.

(8) The plated steel sheet according to any one of (1) to (7), 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 (1) to (7), wherein the plated steel sheet is an alloyed hot-dip galvanized steel sheet whose hot-dip galvanized layer has been alloyed.

(10) A method for manufacturing a hot-dip galvanized steel sheet, comprising the following steps: a step of heating a blank steel sheet to exceed Ac ₁ point and annealing, the chemical composition of the blank steel sheet 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%, 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 consists of Consisting of iron and impurities; the first cooling step, the first cooling step is an average of 2 ° C / sec. And lower than 100 ° C / sec. In a temperature range of 650 ° C to 500 ° C after the aforementioned annealing step; The cooling rate is cooled below 500 ° C. The step of hot-dip galvanizing is to perform the hot-dip galvanizing on the billet steel sheet cooled in the first step of cooling after the first cooling step; Step, performing the hot-dip plating before After the step, the hot-dip galvanized blank steel sheet is cooled to 300 ° C at an average cooling rate of 2 ° C / sec or more in a temperature range from the plating temperature in the foregoing step of performing galvanizing to 300 ° C. ℃ or less; after the second cooling step, the quenched and tempered rolling step is subjected to the quenched and tempered rolling of 0.10% or more of the billet steel sheet cooled in the second cooling step. And, in the heat treatment step, after the aforementioned 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 more than 1 second.

(11) The method for manufacturing a hot-dip galvanized steel sheet according to the above (10), characterized in that: in the aforementioned annealing step, the aforementioned billet steel sheet is heated to a point exceeding Ac ₃ and annealed; and After the step, the average cooling rate in the temperature range from the heating temperature to (heating temperature -50 ° C) was set to 7 ° C / sec or less, and the annealed billet steel sheet was cooled.

A method for manufacturing an alloyed hot-dip galvanized steel sheet, which is characterized in that it comprises a step of heating a blank steel sheet to exceed ₁ point of Ac and annealing, and the chemical composition of the blank steel sheet includes C: 0.03% to 0.70 in 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%, N: 0.020% or less Ti: 0% to 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 caused by iron and impurities Composition; the first cooling step, after the aforementioned annealing step, cooling in a temperature region of 650 ° C to 500 ° C to an average cooling rate of 2 ° C / sec or more and less than 100 ° C / sec to 500 ° C or less; In the step of hot-dip galvanizing, after the aforementioned first cooling step, hot-dip galvanizing is performed on the blank steel sheet cooled in the above-mentioned first cooling step; in the step of alloying treatment, the aforementioned step of performing hot-dip galvanizing is performed. After the previous The galvanized blank steel sheet is subjected to alloying treatment at an alloying treatment temperature. In the second cooling step, after the aforementioned alloying treatment step, the temperature in the temperature range from the aforementioned alloying treatment temperature to 300 ° C is set. The average cooling rate is 2 ° C / sec or more, and the billet steel plate subjected to the aforementioned alloying treatment is cooled to below 300 ° C; in the step of quenching and tempering, after the step of performing the second cooling, the second step of performing the second cooling step is performed. In the cooling step, the cooled billet steel sheet is subjected to quenched and tempered rolling with a tensile ratio of 0.10% or more; and, in the heat treatment step, after the quenched and tempered rolling step, the tempered and rolled steel sheet is tempered and rolled. The blank steel sheet is heated to a temperature range of 200 ° C to 600 ° C and maintained at the temperature for more than 1 second.

(13) The method for manufacturing 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; and After the annealing step, the average cooling rate in the temperature range from the heating temperature to (heating temperature -50 ° C) is set to 7 ° C / sec or less, and the annealed billet steel sheet is cooled.

ADVANTAGE OF THE INVENTION According to this invention, the hot-dip galvanized steel sheet and alloyed hot-dip galvanized steel sheet which are excellent in uniform ductility and local ductility, have high drop strength and tensile strength, and are excellent in formability and impact absorption are provided.

Forms for Carrying Out the Invention The plated steel sheet of 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 Vostian iron, and more than 5.0% by volume Tempered Asada loose iron; and the amount of C in the aforementioned residual Vosda iron is 0.85 mass% or more.

The plated steel sheet of the present invention is characterized by being 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 whose hot-dip galvanized layer is alloyed.

The method for manufacturing a hot-dip galvanized steel sheet according to the present invention is characterized by having the following steps: a step of heating a blank steel sheet to exceed Ac ₁ point and annealing; the aforementioned blank steel sheet has a chemical composition of 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 rest is made of iron and Constituted by impurities; the first cooling step is performed, and the first cooling is after the foregoing annealing step, so that the average cooling rate in a temperature range of 650 ° C to 500 ° C is 2 ° C / sec or more and less than 100 ° C / Sec, cooling to below 500 ° C; after the first cooling step, the galvanized steel sheet cooled in the first cooling step is subjected to the hot-dip galvanizing step; the second cooling step is performed, The second cooling is first After the step of performing hot-dip galvanizing, the average cooling rate in the temperature range from the plating temperature in the step of performing hot-dip galvanizing to 300 ° C is 2 ° C / second or more, and the blank that has been subjected to the above-mentioned hot-dip galvanizing is provided. The steel sheet is cooled to below 300 ° C; after the aforementioned second cooling step, the billet steel sheet cooled in the aforementioned second cooling step is subjected to a quenched and tempered rolling step of 0.10% or more; and The heat treatment step is performed. The aforementioned heat treatment is after the aforementioned tempering and rolling steps, the billet steel plate that has undergone the aforementioned tempering and rolling is heated to a temperature range of 200 ° C to 600 ° C, and is maintained at the temperature for 1 second. the above.

The method for manufacturing an alloyed hot-dip galvanized steel sheet according to the present invention is characterized by having the following steps: a step of heating a blank steel sheet to exceed Ac ₁ point and annealing; the aforementioned blank 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%, 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 It is composed of iron and impurities. The first cooling step is the first cooling step after the aforementioned annealing step, in a temperature range of 650 ° C to 500 ° C, at a temperature of 2 ° C / sec or more and less than 100 ° C / sec. The average cooling rate is cooled below 500 ° C. In the hot-dip galvanizing step, after the aforementioned first cooling step, hot-dip galvanizing is performed on the billet steel sheet cooled in the aforementioned first cooling step; After the zinc step, The step of performing alloying treatment on the hot-dip galvanized billet steel sheet at the alloying treatment temperature; the second cooling step, after the aforementioned alloying treatment step, from the aforementioned alloying treatment temperature to 300 ° C In the temperature zone up to the average cooling rate of 2 ° C / sec or more, the billet steel plate subjected to the aforementioned alloying treatment is cooled to 300 ° C or less; the step of temper rolling, after the second cooling step, The billet steel sheet cooled in the aforementioned second cooling step is subjected to quenched and tempered rolling with a stretch ratio of 0.10% or more; and the heat treatment step is performed after the quenched and tempered rolling step described above. The billet steel sheet that has been tempered and rolled is heated to a temperature range of 200 ° C. to 600 ° C. and maintained at the temperature for more than 1 second.

Hereinafter, the hot-dip galvanized steel sheet, alloyed hot-dip galvanized steel sheet, and manufacturing methods thereof according to this embodiment will be described. In the following description, unless otherwise specified, the steel sheet finally produced according to the manufacturing method of this embodiment will be referred to as a "hot-dip galvanized steel sheet" or "alloyed hot-dip galvanized steel sheet" or " Steel plate ", and the steel plate in the process of production is called" blank steel plate ".

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

[C: 0.03% to 0.70%] C is an effective element to obtain residual Vosstian iron. If the C content is less than 0.03%, the metal structure containing the residual Vosstian iron and tempered Asada iron will not be obtained, so the C content must be 0.03% or more. The C content is desirably 0.10% or more, preferably 0.13% or more, and more preferably 0.16% or more.

On the other hand, when the C content exceeds 0.70%, the weldability of the steel sheet is significantly reduced, so the C content should be made 0.70% or less. The C content is desirably 0.30% or less, preferably 0.26% or less, and more preferably 0.24% or less.

[Si: 0.25% to 2.50%] Si is an element that plays a role in suppressing the precipitation of cis-carbon iron and promoting the generation of residual Vostian iron. In addition, Si is an element that plays a role in preventing excessive softening of tempered Asada loose iron and maintaining strength. In the case where the Si content is less than 0.25%, the effect will not be fully exhibited. Therefore, the Si content should be more than 0.25%, and the Si content should preferably be more than 0.60%, preferably more than 1.00%, and more preferably More than 1.45%.

On the other hand, when the Si content exceeds 2.50%, the plating properties of the steel sheet are significantly reduced, and at the same time, the weldability of the steel sheet is reduced. Therefore, the Si content must be made 2.50% or less. The Si content is preferably 2.30% or less, preferably 2.10% or less, and 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 Vosstian iron and tempered loosening iron, which will be described later. If the Mn content is less than 1.00%, these effects are not sufficiently exhibited, so the Mn content should be made 1.00% or more. The Mn content is preferably more than 1.50%, preferably more than 2.00%, and more preferably more than 2.50%. On the other hand, when the Mn content exceeds 5.00%, the weldability of the steel sheet is reduced, so the Mn content must be 5.00% or less. The Mn content is desirably 4.00% or less, preferably 3.50% or less, and more preferably 3.00% or less.

[P: 0.100% or less] P is an impurity element and will segregate at the grain boundaries to make the steel sheet brittle, so the less the better the element. If the P content exceeds 0.100%, the embrittlement of the steel sheet will become obvious, so the P content must be 0.100% or less. The P content is preferably less than 0.020%, preferably less than 0.015%, and 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. Therefore, on a practical steel plate, the practical lower limit of the P content is 0.0001%.

[S: 0.010% or less] S is an impurity element and forms sulfide-based inclusions in the steel, which deteriorates the local ductility of the steel sheet, so the less the better the element. When the S content exceeds 0.010%, the deterioration of the local ductility of the steel sheet becomes obvious, so the S content should be made 0.010% or less. The S content is preferably 0.005% or less, and 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. Therefore, on a practical steel plate, the actual lower limit of the S content is 0.0001%.

[sol.Al: 0.001% to 2.500%] Al is an element that deoxidizes molten steel. If the content of sol.Al is less than 0.001%, the effect will not be fully exhibited, so the content of sol.Al must be 0.001% or more. The sol.Al content is preferably 0.015% or more, preferably 0.025% or more, and more preferably 0.045% or more. In addition, Al, like Si, plays a role in promoting the formation of residual Vostian iron, and is an effective element in obtaining a metal structure containing residual Vostian iron and tempered Asada iron, which will be described later. From this point of view, the sol.Al content should preferably be 0.050% or more. The sol.Al content is preferably 0.055% or more, and 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 ) will be generated, and surface defects caused by alumina are prone to occur. It is 2.500% or less. In addition, if the sol.Al content is above 0.080%, the abnormal point increases sharply, which makes it difficult to anneal in a temperature region exceeding the Ac ₃ point, so the sol.Al content should be less than 0.080%. The sol.Al content is preferably 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 nitrides that cause the damage of the steel billet will be formed in the continuous casting of steel, so the less the better the element. If the N content exceeds 0.020%, the doubt about the damage of the steel billet will increase, so the N content should be made 0.020% or less. The N content is desirably 0.010% or less, preferably less than 0.008%, and 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. Therefore, in practical steel plates, the practical lower limit of the N content is 0.0001%.

Furthermore, in order to improve the characteristics, in addition to the above-mentioned elements, it may be made to contain the elements described below.

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

When Ti, Nb, and V are excessive, the recrystallization temperature during annealing increases, the metal structure after annealing becomes non-uniform, and local ductility may be impaired. Therefore, the Ti content should be less than 0.080%, preferably 0.035% or less; the Nb content should be less than 0.050%, preferably 0.030% or less; the V content should be 0.200% or less, and preferably less than 0.100%. .

Although the lower limit of the content of Ti, Nb, and V includes 0%, in order to ensure the effect, the content of any one of Ti, Nb, and V should be 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 0.020% or more. As described above, in order to obtain the aforementioned effects, it is desirable 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 2 or more.

[Cr: 0% ~ 2.000%] [Mo: 0% ~ 2.000%] [B: 0% ~ 0.0200%] Cr, Mo, and B are used to improve the hardenability of steel, and in order to obtain the residual Vostian iron and Tempered Asada loose iron is an effective element in the metal structure.

However, when the content of Cr and Mo exceeds 2.000%, or the content of B exceeds 0.0200%, the effect is saturated and the manufacturing cost increases. Therefore, both the Cr content and the Mo content should be 2.000% or less, and the B content should be 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% of any element, but in order to obtain the effect, the content of Cr and Mo should be more than 0.001%, and the content of B should be more than 0.0001%. The Cr content is preferably 0.100% or more, the Mo content is 0.050% or more, and the B content is preferably 0.0010% or more. As described above, in order to obtain the aforementioned effects, it is desirable 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 2 or more.

[Cu: 0% to 2.000%] [Ni: 0% to 2.000%] Cu and Ni are elements that contribute to the improvement in drop strength and tensile strength. However, if the Cu content and Ni content exceed 2.000%, the effect will be saturated and the manufacturing cost will increase. Therefore, both the Cu content and the Ni content must be 2.000% or less. The Cu content and the Ni content are both preferably 0.800% or less.

Although the lower limits of the Cu content and the Ni content include 0%, in order to ensure the effect, the Cu content and the Ni content should preferably be 0.001% or more. The content of any element is preferably at least 0.010%. As described above, in order to obtain the aforementioned effects, it is preferable to contain one or two selected from the group consisting of Cu: 0.001% to 2.000% and Ni: 0.001% to 2.000%.

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

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

The lower limits of the content of Ca, Mg, and REM all include 0%, but in order to obtain the effect, it is desirable to make the content of Ca, Mg, and REM all be 0.0001% or more. It is preferable that the content of any element is 0.0005% or more. As described above, in order to obtain the aforementioned effects, it is desirable to contain one or more 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.

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

[Bi: 0% ~ 0.0500%] Bi is an element that helps to refine the solidified structure and improve local ductility. However, if the Bi content exceeds 0.0500%, the effect will be saturated and the manufacturing cost will increase. Therefore, the Bi content must be 0.0500% or less. The Bi content is desirably 0.0100% or less, and preferably 0.0050% or less. Although the lower limit of the Bi content includes 0%, in order to obtain the effect, the Bi content should preferably be 0.0001% or more. The Bi content is preferably 0.0003% or more. As shown above, in order to obtain the aforementioned effect, it is preferable to contain Bi: 0.0001% to 0.0500%.

The remainder of the chemical composition of the hot-dip galvanized steel sheet, the alloyed hot-dip galvanized steel sheet, and the blank steel sheet used in these manufacturing methods is iron and impurities. Impurities are elements that are mixed in the manufacturing process of steel, such as ore or scrap, or billet, or are mixed in for various reasons during the manufacturing process. These elements are allowed to be contained within a range that does not interfere with the characteristics of the present invention.

(B) Metal structure Next, the metal structure of the hot-dip galvanized steel sheet and alloyed hot-dip galvanized steel sheet according to this embodiment will be described. In order to maintain the drop strength and tensile strength while improving uniform ductility and local ductility, the metal structure of the hot-dip galvanized steel sheet and the alloyed hot-dip galvanized steel sheet of this embodiment is characterized in that it contains more than 5.0% of residual by volume% The Vostian iron contains more than 5.0% tempered Asada iron, and the amount of C in the remaining Vostian iron is 0.85 mass% or more. In addition, the metal structure is more preferably characterized by further containing polygonal ferrous iron in an amount of more than 2.0%, and the amount of Mn in the residual Vostian iron satisfies the following formula (1). In addition, the amount of C in the residual Vosstian iron means the C concentration in the Vosstian iron phase, and the amount of Mn in the residual Vossda iron means the Mn concentration in the Vosstian iron phase.

[Mn] _γ / [Mn] _ave ≧ 1.10… (1) [Mn] _γ : Mn content (mass%) in residual Vostian iron [Mn] _ave : Mn content (mass%) in the chemical composition of the steel sheet

The following describes the organizational requirements in order.

[Residual Vastfield Iron: More than 5.0% by volume] In order to improve uniform ductility, the volume ratio of the residual Vastfield Iron is more than 5.0%. The volume ratio of the residual Vostian iron is preferably more than 6.0%, preferably more than 8.0%, and more preferably more than 10.0%.

However, when excessive residual Vastfield iron is present, local ductility will be deteriorated, so the volume ratio of residual Vastfield iron should be less than 30.0%. The volume ratio of the residual Vosstian iron is preferably less than 20.0%, and more preferably less than 15.0%.

[Tempered Asada scattered iron: more than 5.0% by volume] In order to maintain the drop strength and tensile strength and improve local ductility, the volume ratio of tempered Asada scattered iron should be more than 5.0%. Ideally, the volume ratio of tempered Asada loose iron is more than 16.0%, preferably the volume ratio of tempered Asada loose iron is more than 30.0%, more preferably more than 40.0%, and particularly preferably more than 50.0%.

However, when there is an excessive amount of tempered Mata loose iron, the uniform ductility will be deteriorated, so the volume ratio of tempered Mata loose iron should be 70.0% or less. The volume ratio of the tempered Asada scattered iron is preferably 60.0% or less.

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

However, when there is an excessive amount of polygonal ferrous iron, the drop strength and tensile strength are reduced, and even more, the local ductility is reduced. Therefore, the volume ratio of polygonal ferrous iron should be less than 35.0%. The volume ratio of polygonal ferrous iron is preferably less than 30.0%, more preferably less than 25.0%, and particularly preferably less than 20.0%.

[Amount of C in Residual Vosstian Iron: 0.85 mass% or more] Among the residual Vosstian iron in the metal structure of the hot-dip galvanized steel sheet and alloyed hot-dip galvanized steel sheet according to the present embodiment, the Vostian iron remains to stabilize and improve uniformity Ductility and local ductility. The amount of C in the residual Vosted iron is 0.85 mass% or more.

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

[Amount of Mn in Residual Vastian Iron: The following formula (1)] [Mn] _γ / [Mn] _ave ≧ 1.10 ... (1) [Mn] _γ : Amount of Mn in Residual Vastian Iron (mass%) [Mn] _ave : Mn content (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 residual Vosstian iron of the hot-dip galvanized steel sheet and the alloyed hot-dip galvanized steel sheet according to this embodiment, Mn is preferably concentrated to a desired amount. Like C, Mn also functions effectively in stabilizing the residual Vostian iron and improving uniform ductility and local ductility.

In order to make full use of its function, it is desirable to set [Mn] _γ / [Mn] _ave to 1.10 or more, and more preferably 1.15 or more. The upper limit of [Mn] _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, and more preferably 1.25 or less.

[Matian loose iron] In the hot-dip galvanized steel sheet and alloyed hot-dip galvanized steel sheet according to this embodiment, in order to maintain the drop strength and further improve the local ductility, it is necessary to try to suppress the amount of Asada loose iron. Here, the so-called Asada loose iron refers to the Asada loose iron that has not been tempered, that is, the newly-born Asada loose iron. The volume ratio of Asada loose iron is preferably less than 5.0%. The volume ratio of Asada scattered iron is preferably less than 2.0%, and more preferably less than 1.0%.

[Remaining Structure] The remaining structure of the metal structure is a low-temperature metamorphic structure such as acicular ferrous iron and toughened iron, and may also contain boron iron, and precipitates such as cis carbon iron. The remaining tissue does not necessarily contain low-temperature metamorphic products, boron iron and precipitates, so the lower limit of the volume ratio of each of the low-temperature metamorphic products, boron iron and precipitates is 0% by volume.

The upper limit of the volume ratio of each of the low-temperature metamorphic product, boron iron, and precipitates is not particularly limited. However, when the low-temperature metamorphic products, boron iron and precipitates are excessively present, the dropout strength and tensile strength will be reduced. Therefore, the total volume ratio of the low-temperature metamorphic products, boron iron and precipitates should be less than 40.0%. . The total volume ratio of these tissues is preferably 20.0% or less, and more preferably 10.0% or less.

When excessive Plei iron is present, the drop strength and tensile strength are reduced, and even, the uniform ductility is reduced. Therefore, the volume ratio of Plei iron should be less than 10.0%. The volume ratio of boron iron is preferably less than 5.0%, and more preferably less than 3.0%.

The metal structures of the hot-dip galvanized steel sheet and the alloyed hot-dip galvanized steel of this embodiment are measured in the following manner. Take a test piece from an arbitrary position of the steel plate, grind the longitudinal section parallel to the rolling direction, and use scanning electrons in the position range of 1/4 depth from the boundary between the steel plate of the base material and the coating to the thickness of the steel plate of the base material. The metal structure was observed under a microscope (SEM), and the area ratio of each structure was measured by image processing. The area ratio is equal to the volume ratio, and the measured area ratio is the volume ratio.

Tempered Asada loose iron can be used for the point that the iron carbides existing inside the tissue extend in multiple directions to distinguish it from toughened iron. Polygonal ferrous iron can be distinguished from acicular ferrous iron by the point of showing a block-like shape and the point of low differential density.

The volume ratio of the residual Vosstian iron and the amount of C in the residual Vosstian iron were obtained by taking a test piece from an arbitrary position of the steel sheet, and measuring the thickness from the boundary between the steel plate of the substrate and the coating to the thickness of the steel plate of the substrate The rolled surface was chemically polished at a 1/4 depth position, and the X-ray diffraction intensity and the diffraction peak position were measured using an X-ray diffraction device (XRD).

The amount of Mn ([Mn] _γ ) in the residual Vosstian iron was measured in the following manner. A test piece was taken from an arbitrary position of the steel plate, and an SEM equipped with an electron beam backscattering pattern analysis device (EBSP) was used in a position range from the boundary between the steel plate of the substrate and the plating layer to the depth of 1/4 of the plate thickness of the substrate. Let's observe the metal structure and confirm the presence of iron particles.

Next, the Mn concentration of the residual Vosted iron particles was measured using an SEM equipped with an electron microprobe analyzer (EPMA). The measurement by EMPA was performed on 10 or more residual Vostian iron particles, and the average value of the measured Mn amount was [Mn] _γ .

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

The hot-dip galvanized layer and alloyed hot-dip galvanized layer may be a plated layer and an alloyed plated layer formed under ordinary plating conditions. However, when the Fe concentration of the alloyed hot-dip galvanized layer is less than 7% by mass, the weldability and sliding properties may become insufficient. Therefore, the Fe concentration of the alloyed hot-dip galvanized layer is preferably 7% by mass or more. .

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

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

However, the uniform elongation in a 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 a direction orthogonal to the rolling direction is converted into a total elongation equivalent to a plate thickness of 1.2 mm, and the value obtained by the foregoing conversion is defined as TEl ( Total Elongation). Further, a local elongation in a direction corresponding to a plate thickness of 1.2 mm and a direction orthogonal to the rolling direction is defined as LEl (Local Elongation) according to the following formula (3). In the hot-dip galvanized steel sheet and alloyed hot-dip galvanized steel sheet of this embodiment, from the viewpoint of press formability, the value of TS × UEl is preferably 10,000 MPa ·% or more, and the value of TS × LEl is preferably 5,000 MPa ·%. the above.

The value of TS × UE1 becomes large if both the tensile strength and the uniform elongation are excellent, so it is used as an index for evaluating uniform ductility. The value of TS × LE1 becomes large if both the tensile strength and the local elongation are excellent, so it is used as an index for evaluating local ductility.

It is preferable that the value of TS × UE1 is 11000 MPa% or more and the value of TS × LE1 is 6000 MPa% or more. It is more preferable that 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, TEl ₀ (2) used in the formula for the number Found JIS5 tensile test pieces obtained by measuring the total elongation, t ₀ is provided to the determination of the number JIS5 tensile test piece thickness. In addition, TEl and LEl are conversion values equivalent to the total elongation and local elongation at a plate thickness of 1.2 mm, respectively. UE1 is an actual measured value of a uniform elongation measured 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 should be above 780 MPa. The tensile strength (TS) of the steel sheet is preferably 980 MPa or more, and more preferably 1180 MPa or more. The reduction ratio (YR) of the steel plate is preferably 0.59 or more. The reduction ratio (YR) of the steel sheet is preferably 0.66 or more, and more preferably 0.72 or more.

The higher the local ductility, the more the breakage under impact load is suppressed and the absorbed energy increases. Therefore, from the viewpoint of suppressing breakage, the value of TS × LEl should be 5500 MPa ·% or more. The value of TS × LE1 is preferably 6500 MPa ·% or more.

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

The method of manufacturing a hot-dip galvanized steel sheet according to the present embodiment includes the following steps: a billet having the chemical composition of the steel sheet is heated to above Ac ₁ point, and the step of annealing; a first cooling step, the step of annealing of the aforementioned After that, 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 the temperature is cooled to 500 ° C or less. In the step of hot-dip galvanizing, the first cooling step is performed as described above. After the step, the galvanized steel sheet cooled in the first cooling step is subjected to hot-dip galvanizing; in the second cooling step, the second cooling is performed after the hot-dip galvanizing step is performed. The plating temperature in the step of performing hot-dip galvanizing is to cool the blank steel sheet that has been subjected to the above-mentioned hot-dip galvanizing to a temperature of 300 ° C or lower at an average cooling rate of 2 ° C / sec or more; In the rolling step, after the aforementioned second cooling step, the billet steel sheet cooled in the aforementioned second cooling step is subjected to quenched and tempered rolling with an elongation of 0.10% or more; and Steps, before After the step of tempering and rolling is described, the billet steel sheet that has undergone the aforementioned tempering and rolling is heated to a temperature range of 200 ° C. to 600 ° C. and maintained at the temperature for more than 1 second.

Among them, as shown in FIG. 1, the ideal manufacturing method of the method for manufacturing the hot-dip galvanized steel sheet according to this embodiment is provided with the following steps: the step of heating the billet steel sheet having the aforementioned chemical composition to exceed Ac ₃ points, and performing annealing; In the first cooling step, after the aforementioned annealing step, the annealed blank is set at an average cooling rate of 7 ° C / sec or lower in a temperature range from the heating temperature to (the aforementioned heating temperature -50 ° C). The steel sheet is cooled, and further, the temperature range of 650 ° C to 500 ° C is cooled to 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 is as described above. After the first cooling step, the galvanized steel sheet cooled in the first cooling step is subjected to hot-dip galvanizing; in the second cooling step, after the hot-dip galvanizing step is performed, the hot-dip galvanizing is performed. In the galvanizing step, the hot-dip galvanized steel sheet is cooled below 300 ° C at an average cooling rate of 2 ° C / sec or higher in a temperature range from 300 ° C to the plating temperature; ,in After the aforementioned second cooling step, the billet steel plate cooled in the aforementioned second cooling step is subjected to quenched and tempered rolling at a stretch ratio of 0.10% or more; and, in the heat treatment step, tempered rolling is performed before the aforementioned step. After the step of temper rolling, the billet steel plate subjected to the aforementioned temper rolling is heated to a temperature range of 200 ° C. to 600 ° C. and maintained at the temperature for more than 1 second.

A method for producing alloyed hot-dip galvanized steel sheet according to the present embodiment comprises of: billet having the chemical composition of the steel sheet is heated to above Ac ₁ point, and the step of annealing; a first step of cooling, the cooling in the first After the annealing step, the temperature in the temperature range of 650 ° C to 500 ° C is cooled to an average cooling rate of 2 ° C / sec or more and less than 100 ° C / sec to 500 ° C or less. In the step of hot-dip galvanizing, the first step is performed as described above. After the cooling step, the galvanized steel sheet cooled in the first cooling step is subjected to hot-dip galvanizing; in the alloying step, after the aforementioned hot-dip galvanizing step, the hot-dip galvanized embryo is subjected to the hot-dip galvanizing. The steel sheet is subjected to alloying treatment at an alloying treatment temperature. In the second cooling step, after the aforementioned alloying treatment step, the temperature range from the aforementioned alloying treatment temperature to 300 ° C is 2 ° C / sec or more. The average cooling rate is to cool the billet steel plate after the alloying treatment to below 300 ° C; in the step of temper rolling, after the second cooling step, the second cooling step is performed. The billet steel plate that has been cooled in the process is subjected to quenched and tempered rolling with a tensile ratio of 0.10% or more; and the heat treatment step is performed after the quenched and tempered rolling step described above. The blank steel sheet is heated to a temperature range of 200 ° C to 600 ° C, and maintained at the temperature for more than 1 second.

Among them, as shown in FIG. 2, the preferred manufacturing method of the alloyed hot-dip galvanized steel sheet according to this embodiment includes the following steps: the step of heating the billet steel sheet having the aforementioned chemical composition to exceed Ac ₃ points, and annealing; 1 cooling step, after the aforementioned annealing step, in the temperature range from the heating temperature to (heating temperature -50 ° C), the annealed billet steel sheet is an average cooling rate of 7 ° C / sec or less Cooling, and further cooling in a temperature range of 650 ° C to 500 ° C to an average cooling rate of 2 ° C / sec or more and less than 100 ° C / sec to 500 ° C or less; in the step of hot-dip galvanizing, the first cooling is performed as described above. After the step, the galvanized steel sheet cooled in the first cooling step is subjected to hot-dip galvanizing; in the step of alloying treatment, after the aforementioned hot-dip galvanizing step is performed, the hot-dip galvanized blank is performed. The steel sheet is subjected to alloying treatment at the alloying treatment temperature; the second cooling step is, after the aforementioned alloying treatment step, in a temperature range from the aforementioned alloying treatment temperature to 300 ° C at a temperature of 2 ° C / sec or more The above average cooling rate is to cool the billet steel plate after the alloying treatment to below 300 ° C; in the tempering and rolling step, after the second cooling step, the second cooling step is performed. The cooled billet steel sheet is subjected to quenched and tempered rolling with a stretch rate of 0.10% or more; and the heat treatment step is to perform the quenched and tempered billet after the aforementioned quenched and tempered step. The steel sheet is heated to a temperature range of 200 ° C to 600 ° C and maintained at the temperature for more than 1 second.

The method for manufacturing the blank steel sheet and the method for manufacturing the hot-dip galvanized steel sheet and the method for manufacturing the alloyed hot-dip galvanized steel sheet according to this embodiment are not limited to a specific manufacturing method. For example, a steel blank with the aforementioned chemical composition is manufactured by casting, and it is heated to a temperature region lower than 1250 ° C. After heating, the finishing rolling temperature is higher than Ar ₃ points and higher than 850 ° C. Hot rolled. Next, coiling is performed at a coiling temperature of 500 ° C. or higher and lower than 700 ° C., and cold rolling is performed at a rolling reduction ratio of 40% or higher and lower than 70% to produce a billet steel sheet.

The casting method of the steel slab is not limited to a specific casting method, but a continuous casting method is suitable, and a steel block obtained by other casting methods may be rolled into pieces by block rolling or the like. In the continuous casting step, in order to suppress the occurrence of surface defects caused by inclusions, it is desirable to flow the molten steel in the mold by electromagnetic stirring or the like. After being continuously cast, the steel block in the high temperature state 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.

In addition, high-temperature steel blocks after continuous casting or high-temperature steel sheets after block rolling can also be directly supplied to the hot rolling without reheating, or auxiliary heating can be performed before supplying the heat Rolling. The steel ingots and steel sheets supplied to the hot rolling are collectively referred to as "steel billets".

In order to prevent coarsening of the Vosstian iron, the temperature of the hot-rolled steel billet should be lower than 1250 ° C. Preferably, the temperature of the steel billet is 1200 ° C or lower. The lower limit of the temperature to be supplied to the hot-rolled steel billet is not particularly limited, but is preferably a temperature at which the hot-rolling can be completed at Ar ₃ or more.

The conditions of the hot rolling are not particularly limited, but if the completion temperature of the hot rolling is too low, in the metal structure of the hot-rolled steel sheet, a coarse low-temperature deformed structure extending in the rolling direction will be generated, which will prevent uniform ductility. Because of the possibility of local ductility, the completion temperature of the hot rolling is preferably more than Ar ₃ points and more than 850 ° C. The completion temperature of the hot rolling is preferably Ar ₃ points or more and exceeding 880 ° C, and more preferably Ar ₃ points or more and exceeding 900 ° C. The upper limit of the completion temperature of the hot rolling is not particularly limited, but it is preferably 1000 ° C. or lower in terms of fine graining the metal structure of the hot rolled steel sheet.

In addition, when the hot rolling is composed of rough rolling and finishing rolling, in order to complete the finishing rolling within the above-mentioned temperature range, the rough rolling can also be performed between the rough rolling and the finishing rolling.材 热。 Material heating. At this time, the rough rolled material is heated so that the rear end of the rough rolled material becomes higher temperature than the front end of the rough rolled material, and the temperature of the entire length of the rough rolled material at the beginning of the finishing rolling should preferably be reached. The deviation is suppressed below 140 ° C. By this temperature suppression, the uniformity of the characteristics in the coil of the rolled hot-rolled steel sheet can be improved.

The heating of the rough-rolled rolled material may be performed by a known method. For example, an electromagnetic type (solenoid type) induction heating device may be installed between the rough rolling mill and the finishing rolling mill. According to the temperature in the longitudinal direction of the rough rolled material on the upstream side of the induction heating apparatus, Distribution, etc., to control the heating temperature increase caused by the electromagnetic type (solenoid type) induction heating device.

The conditions from the end of hot rolling to the start of coiling are not particularly limited, but in order to improve the cold rolling ductility of the hot rolled steel sheet by softening the hot rolled steel sheet, the coiling temperature (when coiling is started) Temperature) is 600 ° C or higher. The winding temperature is preferably 640 ° C or higher, and more preferably 680 ° C or higher. If the coiling temperature is too high, the pickling property of the hot-rolled steel sheet may be damaged. Therefore, the coiling temperature should be 750 ° C or lower, and preferably less than 720 ° C. After the winding, the temperature range from the winding temperature to (winding temperature -50 ° C) is preferably cooled at an average cooling rate exceeding 15 ° C / hour. This improves the productivity and promotes the dissolution of carbides in the annealing step described later.

The hot-rolled steel sheet is cold-rolled in accordance with a general method to make a cold-rolled steel sheet. Before cold rolling, descaling can also be performed by pickling. In order to promote recrystallization, to homogenize the metal structure after cold rolling and annealing, and further improve the local ductility, the rolling reduction of cold rolling should be more than 40%. If the rolling reduction is too high, the rolling load will increase and rolling will become difficult. Therefore, the rolling reduction should be less than 70%, and preferably less than 60%.

Next, the process conditions in the manufacturing method of the hot-dip galvanized steel sheet and the manufacturing method of the alloyed hot-dip galvanized steel sheet of this embodiment are demonstrated.

[Step for annealing] (Heating temperature: ₁ point exceeding Ac) In the step of annealing the billet steel sheet, the billet steel sheet is heated. In order to generate Vosstian iron during heating, the heating temperature must be higher than Ac ₁ point. The so-called Ac ₁ point is the temperature at which the Vosted iron is generated 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 desirable to heat the blank steel sheet to exceed Ac ₃ points and perform annealing. The Ac ₃ point is the temperature at which the ferrous iron disappears in the metal structure when the billet steel sheet is heated.

By heating the blank steel sheet to the above temperature range, that is, heating to the Vosstian iron zone, carbides will be dissolved, and in the metal structure of the hot-dip galvanized steel sheet and the alloyed hot-dip galvanized steel sheet, the amount of iron and The amount of C in the residual Vastian iron will increase.

The upper limit of the heating temperature is not particularly limited, but if the heating temperature is too high, the Vostian iron will coarsen and local ductility will be impaired. Therefore, the heating temperature should be below (Ac ₃ point +100) ℃, preferably ( Ac ₃ points +50) ℃ or less. Regardless of the heating temperature, the holding time at the heating temperature is not particularly limited, but in order to make the metal structure in the coil uniform, the holding time should be 10 seconds or more. In order to suppress the coarsening of the Vosted iron, 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 plate to exceed Ac ₃ points and annealing In this case, in the step of performing the first cooling, the average cooling rate in the temperature range from the heating temperature to (heating temperature -50 ° C) should preferably be 7 ° C / second or less. As a result of 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 in the residual Vostian iron increases, and at the same time, polygonal ferrous iron is generated, and uniform ductility and local ductility are improved.

If the average cooling rate in the temperature range from the heating temperature to (heating temperature -50 ° C) exceeds 7 ° C / s, the amount of Mn in the residual Vostian iron will decrease, and the polygonal ferrous iron will decrease, and the ductility will be uniform. And local ductility will be impaired. Therefore, it is desirable to make the average cooling rate in the above-mentioned temperature range 7 ° C / sec or less. The average cooling rate in the temperature range is preferably 5 ° C./second 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.

In addition, the wider the temperature range for cooling at an average cooling rate of 7 ° C / sec or less, the more the amount of Mn in the residual Vostian iron will increase, and the amount of polygonal fertilized iron will increase. Therefore, it is preferable to cool the blank steel plate at an average cooling rate of 7 ° C / sec or less in a temperature range from the heating temperature to (heating temperature -100 ° C), and preferably from the heating temperature to (heating temperature- In the temperature range 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 temperature in the temperature range of 650 ° C to 500 ° C is set. The cooling rate is 2 ° C./sec or more and less than 100 ° C./sec. The isothermal temperature is not maintained on the way until the blank steel sheet is cooled to 500 ° C. or less.

If the average cooling rate in the temperature range of 650 ° C to 500 ° C is lower than 2 ° C / sec, polygonal fertilized iron and slag iron will be excessively generated, which will cause the drop-down strength and tensile strength to decrease. Therefore, it is necessary to set the average cooling rate in the above temperature range to 2 ° C / sec or more. The average cooling rate in the above temperature range is preferably 3 ° C / second or more, preferably 4 ° C / second or more, and more preferably 5 ° C / second 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 plate will be damaged, so the average cooling rate in the above temperature range should be less than 100 ° C / second. The average cooling rate in the temperature range is preferably 50 ° C / sec or less, preferably 30 ° C / sec or less, and more preferably 20 ° C / sec.

(Cooling stop temperature: 500 ° C. or less) The billet steel sheet cooled at a desired average cooling rate is continuously cooled to 500 ° C. or less. The cooling conditions in the temperature range of 500 ° C or lower are not particularly limited, but it is desirable to maintain the blank steel sheet in a temperature range of 500 ° C or lower and 460 ° C or higher for 4 seconds to 45 seconds. It is preferably maintained for 10 to 35 seconds. Through this maintenance, in the metal structure formed in the second cooling step described later, the volume ratio of the residual Vosstian iron and the amount of C in the residual Vosstian iron can be adjusted appropriately, thereby improving uniform ductility and locality. Ductility, and even more, also improves the drop intensity.

[Step of performing hot-dip galvanizing] After the first cooling step, hot-dip galvanizing is performed on the blank steel sheet. Between the step of performing the first cooling and the step of performing the hot-dip galvanizing, at least one of cooling and heating may be performed on the blank steel sheet as necessary.

The plating bath temperature and composition of the hot-dip galvanizing may be ordinary, and there is no particular limitation. The amount of plating deposit is not particularly limited, and it may be within a normal range. For example, each side of the blank steel sheet should preferably have an adhesion amount of 20g / m ² to 80g / m ² . Although the plating temperature is not particularly limited, it is usually 460 ° C to 470 ° C.

[Step of performing alloying treatment] When manufacturing the alloyed hot-dip galvanized steel sheet, after the step of performing hot-dip galvanizing, the hot-dip galvanized steel sheet is heated to a temperature required to alloy the hot-dip galvanized steel. At the temperature (alloying treatment temperature), an alloying treatment is performed.

The alloying treatment is preferably performed under conditions where the Fe concentration in the plating layer is 7 mass% or more. For example, it is desirable to perform the alloying treatment under 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 the 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) The molten plating is performed During the cooling after the zinc step or after the alloying step, the average cooling rate in the temperature range from the plating temperature to 300 ° C or the temperature range from the alloying temperature to 300 ° C is 2 C / sec or higher, and cooled to 300 ° C or lower.

If the average cooling rate in the second cooling step is lower than 2 ° C / sec, the excessive generation of boron iron will reduce the undulation strength and tensile strength, and the amount of residual Vostian iron will decrease, and the uniform ductility will be impaired. Therefore, the average cooling rate in the above-mentioned temperature range is set to be 2 ° C / second or more. The average cooling rate in the above temperature range is preferably 3 ° C / second or more, preferably more than 5 ° C / second, and more preferably more than 10 ° C / second.

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

The blank steel sheet after the second cooling step should preferably contain a residual Vosstian iron in a volume ratio of 5.0% to 35.0%, and has a metal structure with a C content in the residual Vosstian iron of less than 0.85% by mass. Thereby, in the heat treatment step described below, C and Mn can be promoted toward C concentration and Mn concentration of the residual Vosted iron, uniform ductility and local ductility are improved, and drop strength is also increased.

The volume ratio of the residual Vosstian iron is preferably 10.0% or more and 30.0% or less, and more preferably 15.0% or more and 25.0% or less. The amount of C in the residual Worsted 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. Although the lower limit of the amount of C in the residual Worsted iron is not particularly limited, about 0.50% by mass is a substantial lower limit.

[Step of Tempering and Rolling] (Stretch: 0.10% or more) After the second cooling step, temper rolling is performed on the billet steel sheet with a stretch ratio of 0.10% or more. By this tempering and rolling, in the heat treatment step described below, C and Mn are promoted toward C concentration and Mn concentration in Vosstian Iron, and in the metal structure of the hot-dip galvanized steel sheet and alloyed hot-dip galvanized steel sheet , The amount of C and Mn in the residual Vostian iron is increased, the uniform ductility and local ductility are improved, and the yield strength is also increased.

If the elongation is less than 0.10%, the above-mentioned effects in the subsequent step of performing heat treatment cannot be obtained, so the elongation is required to be 0.10% or more. The elongation is preferably 0.30% or more, and more preferably 0.50% or more. Although the upper limit of the elongation is not particularly limited, if the elongation is too high, the rolling load will increase, so the elongation should be less than 2.00%. The elongation is preferably less than 1.50%, and more preferably less than 1.00%.

Although the tempering rolling temperature is not particularly limited, in order to effectively impart processing strain to Vosstian iron, the tempering rolling temperature should be a low temperature, and the starting temperature of the temper rolling should be room temperature. In addition, the temper rolling is preferably performed by surface rolling.

[Steps for performing heat treatment] (Heating temperature: 200 ° C to 600 ° C) (Holding time: 1 second or more) After the step of temper rolling, the billet steel plate is heated to a temperature range of 200 ° C to 600 ° C, and Maintain at this temperature for more than 1 second.

When the heat treatment temperature (highest heating temperature) is lower than 200 ° C., the C concentration and the Mn concentration in the C and Mn-oriented Vosstian iron become insufficient, and uniform ductility is impaired. In addition, when the heat treatment temperature (highest heating temperature) is lower than 200 ° C, hard Asada iron is left, local ductility is impaired, and the drop strength is also reduced. Therefore, the heat treatment temperature should be 200 ° C or higher. The heat treatment temperature is preferably 240 ° C or higher, preferably 260 ° C or higher, and more preferably 280 ° C or higher.

On the other hand, if the heat treatment temperature exceeds 600 ° C, the amount of residual Vostian iron will be insufficient, the uniform ductility will be impaired, and the tempered Asada loose iron will be excessively softened, thereby reducing the drop strength and tensile strength. In addition, if the heat treatment temperature exceeds 600 ° C, hard fresh Asada loose iron is generated, so local ductility is impaired, and the drop strength is also reduced. Therefore, the heat treatment temperature should be 600 ° C or lower. The heat treatment temperature is preferably 550 ° C or lower, preferably 500 ° C or lower, and more preferably 450 ° C or lower.

If the heat treatment time (maintaining time at the maximum heating temperature) is less than 1 second, the C concentration and the Mn concentration in the C and Mn facing Vostian iron become insufficient, and uniform ductility is impaired. In addition, if the heat treatment time is less than 1 second, hard Asada loose iron remains, and local ductility is impaired, and the drop strength is also reduced. Therefore, the heat treatment time should be 1 second or more. The heat treatment time is preferably more than 5 seconds, preferably more than 10 seconds, and more preferably more than 15 seconds.

On the other hand, if the heat treatment time is too long, the amount of residual Vastian iron is reduced, and uniform ductility is impaired. In addition, tempered Asada loose iron is excessively softened, thereby reducing the drop strength and tensile strength. In addition, if the heat treatment time is too long, hard new Asada loose iron will be generated, the local ductility will be damaged, and the drop strength will also be reduced. 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, and more preferably 1440 minutes or less.

The heat treatment time should be appropriately 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, and 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 less, preferably 6 minutes or less, and 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 less than 20 minutes.

After the heat treatment step, in order to straighten the blank steel plate, the blank steel plate may be quenched and tempered, and the blank steel plate may be coated with an oil or a lubricating film.

The thickness of the hot-dip galvanized steel sheet and the alloyed hot-dip galvanized steel sheet according to this embodiment is not limited to a specific thickness, but the manufacturing method of the hot-dip galvanized steel sheet and the alloyed hot-dip galvanized steel sheet according to this embodiment is more suitable for manufacturing. Steel plates with a thickness of 0.8mm ~ 2.3mm.

[Examples] Next, the examples of the present invention will be described. However, the conditions in the examples are only examples of the conditions adopted to confirm the feasibility and effect of the present invention, and the present invention is not limited to this condition. example. As long as the purpose of the present invention can be achieved without departing from the spirit of the present invention, the present invention can be obtained using various conditions.

(Example 1) A molten steel having a chemical composition shown in Table 1 was cast using a vacuum furnace, and steels A to S were produced. The Ac ₁ point and Ac ₃ point in Table 1 were obtained from the thermal expansion change 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 for 60 minutes, hot rolling was performed under the conditions shown in Table 2.

Specifically, in the temperature range of Ar ₃ or more, the steels A to S were rolled ten times, thereby producing a hot-rolled steel sheet having a thickness of 2.5 mm to 3.0 mm. After hot rolling, the hot-rolled steel sheet is cooled to 550 ° C to 680 ° C by water spraying, and the cooling end temperature is the coiling temperature. The hot-rolled steel sheet is placed in an electric heating furnace maintained at the coiling temperature and maintained. 60 minutes. After that, the hot-rolled steel sheet was furnace-cooled to room temperature at a cooling rate of 20 ° C./hour to simulate the slow cooling after coiling.

The cold-rolled hot-rolled steel sheet is pickled and used as the base material for cold-rolling, and cold-rolled at a reduction rate of 47 to 52% to obtain a cold-rolled steel sheet (blank material) with a thickness of 1.2 mm to 1.6 mm. Steel plate). Using a hot-dip galvanizing simulation device, the billet steel sheet was heated at a heating rate of 10 ° C / sec to 650 ° C, and then heated at a heating rate of 2 ° C / sec to the temperature shown in Table 2, and then soaked 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. A part of the billet steel sheet is heated to 520 ° C. after hot-dip galvanizing, and is subjected to alloying treatment.

Under the cooling conditions shown in Table 2, the blank steel sheet was subjected to secondary cooling (second cooling) from the plating temperature (meaning a plating bath temperature) or the self-alloying temperature. After the secondary-cooled billet steel sheet is surface-rolled with a stretch 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 alloyed hot-dip galvanized steel sheet are collectively referred to as "plated steel sheet.").

When the secondary cooling stop temperature was set to 100 ° C., after the secondary cooling was stopped, the surface calendering was performed without cooling to room temperature, and thereafter, the 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 rolling or heat treatment is omitted.

The "thickness after rolling" in the hot rolling conditions described in Table 2 indicates 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 "presence or absence of alloying treatment" in the annealing conditions described in Table 2, the symbol "is present" indicates that alloying treatment was performed after hot-dip galvanizing, and the symbol "no" indicates that alloying was not performed after hot-dip galvanizing化 处理。 Processed. The "secondary cooling rate" in the annealing conditions described in Table 2 means the average cooling rate in the temperature range from the alloying temperature to 300 ° C if the alloying is performed, and if the alloying is not performed, , Means the average cooling rate in the temperature range from the plating temperature to 300 ° C. In Table 2, the "RT" mark indicates room temperature. Regarding the presence or absence of quenching and rolling in Table 2, the symbol “Yes” indicates that quenching and rolling was performed in the step of quenching and rolling, and the symbol “No” indicates that quenching and rolling is not performed. In the column labeled "Heat treatment conditions" in Table 2, a "-" mark indicates that no heat treatment has been performed.

〔Table 1〕

〔Table 2〕

XRD measurement test specimens were taken from the plated steel sheet and the blank steel sheet after the above secondary cooling, and the rolled surface of the test piece was chemically polished from the boundary between the steel sheet and the plating layer to a position of 1/4 of the thickness of the steel sheet. . An X-ray diffraction test was performed on the rolled surface, and the volume ratio of the residual Vastfield iron and the amount of C in the residual Vastfield iron were measured.

Specifically, the Mo-Kα line was incident on the test piece to measure the integrated intensity of the diffraction peaks of the α-phase (200) and (211) and the integrated intensity of the diffraction peaks of the γ-phase (200), (220), and (311). Find the volume ratio of the residual Vosstian iron.

In addition, the incident Fe-Kα line was used to obtain the lattice constant (a _γ ) of Vostian iron from the positions of the diffraction peaks of the γ phases (200), (220), and (311), and a _γ (Å) was used. = 3.578 + 0.033 × C _γ (mass%) is used to calculate the amount of C (C _γ ) in the residual Vosted iron.

In addition, a test piece for SEM observation was taken from a plated steel plate, and a longitudinal section parallel to the rolling direction of the test piece was ground. After that, the longitudinal section was subjected to natal corrosion and LEPERA corrosion. Liquid corrosion was observed, and the metal structure was observed in the range from the boundary between the steel plate and the plating layer to a depth of 1/4 of the steel plate thickness. Through image processing, the volume ratios of tempered Asada loose iron, polygonal ferrous iron, fresh Asada loose iron, and the rest of the tissue were measured.

The volume ratio of fresh Asada loose iron is obtained by subtracting the volume ratio measured by the XRD measurement from the sum of the volume ratios of the residual Vostian iron and fresh Asada loose iron measured by Lepera corrosion. Volume ratio of residual Vosstian iron.

The yield stress (YS), tensile strength (TS), and uniform elongation (UEl) are obtained by taking a JIS No. 5 tensile test piece from a plated steel plate in a direction straight in the rolling direction, and performing a tensile test on the test piece And find it.

The stretching speed was set to 1 mm / minute until the drop point was reached, and thereafter was set to 10 mm / minute. The step-down ratio (YR) is obtained by dividing YS by TS. The total elongation (TEl) and local elongation (LEl) are tensile tests performed on a JIS No. 5 tensile test piece taken in a direction orthogonal to the rolling direction. The measured values of the total elongation (TEl ₀ ) and The measured value (UE1) of the uniform elongation was calculated based on the above-mentioned formulas (2) and (3), which is equivalent to a plate thickness of 1.2 mm.

If the value of YR is 0.59 or more, the value of TS × UEl is 10000 MPa ·% or more, and the value of TS × LEl is 5,000 MPa ·% or more, it is judged as good characteristics. In addition, if the value of TS × UEl is 12000 MPa ·% or more and the value of TS × LEl is 6000 MPa ·% or more, it is judged to be particularly good characteristics.

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 no observation of the metal structure was performed. In the column labeled "Residual Vostian Iron C Content (mass%)" in Table 3, the symbol "-" indicates that the measurement of the C content in Residual Vostian Iron was not performed. 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 “UEl” 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 remarks column of Table 3, the samples with "○" are examples of the present invention, and the samples with "X" are comparative examples. In Tables 1 to 3, the underlined values or symbols mean those outside the scope of the present invention.

〔table 3〕

Examples of inventions marked with a ○ mark in the remarks column (test numbers A1 to A3, A9, A11, A13, A14, A19, A21, A23, A26, A28 to A37, and A40 to A45) are all TS × UEl at 10000 Above, TS × LEl is above 5000, and shows good uniform ductility and local ductility. In addition, YR shows a high value of 0.59 or more. In particular, for test numbers A11, A21, A26, A28, A30, A31, A34, which contain tempered Asada loose iron above 16%, polygonal ferrous iron above 2.0%, and TS × UEl above 12000, and TS × LEl is above 6000, showing particularly good uniform ductility and local ductility.

On the other hand, the test results for chemical compositions or step conditions that deviate from the scope of the present invention (the test number of the mark x in the remarks column A4 to A8, A10, A12, A15 to A18, A20, A22, A24, A25, A27, A38, and A39), any or all of the drop ratio, uniform ductility, and local ductility are inferior.

Specifically, although steels C, E, and N having a chemical composition within the scope of the present invention were used, TS × UEl and TS × LEl were lower in test numbers A15, A24, and A38 without surface rolling. . The tests using steels A and C (test numbers A10 and A20) were not heat treated. Therefore, in test number A10, the values of YR and TS × LEl were low. In test number A20, YR, TS × UEl, and TS × LEl has a low value.

The tests using steels A, C, E, and N (test numbers A4, A16, A25, A39) are too low in heat treatment temperature. Therefore, in test number A4, the values of YR and TS × LEl are lower, and in test numbers A16, In A25 and A39, the values of YR, TS × UEl, and TS × LE1 are lower. In addition, in the tests (test numbers A5, A17, and A27) using steels A, C, and F, YR, TS × UEl, and TS × LEl were low because the heat treatment temperature was too high.

Although Steel A having a chemical composition within the scope of the present invention was used, in the test number 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), the average cooling rate in the temperature range of 650 to 500 ° C. in the first cooling step was too low, so YR and TS × LEl were low. In the tests (test numbers A8 and A18) using steels A and C, since the average cooling rate (secondary cooling rate) in the temperature range of the alloying treatment temperature to 300 ° C was too low in the second cooling step, In test number A8, the values of YR and TS × LEl were low, and in test number A18, the values of TS × UEl and TS × LEl were low.

In test number A12 using steel B, YR, TS × UEl, and TS × LE1 were low because the amount of Si in the steel was small. In test number A22 using steel D, since the amount of Mn in the steel was small, YR and TS × LEl were low.

(Example 2) An experiment was performed in the same procedure as in Example 1, and a steel plate A to S shown in Table 1 was produced under the conditions shown in Table 4. The results are shown in Table 5. The measurement procedure is the same as in Example 1.

In addition, regarding the amount of Mn in the residual Vostian iron, a test piece for EBSP measurement was taken from a plated steel plate, and a longitudinal section parallel to the rolling direction was electrolytically polished. Then, the thickness from the boundary between the steel plate and the plating layer to the steel plate was observed. The metal structure in the 1/4 depth position was confirmed by image processing to determine the distribution of residual Vosstian iron. 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 Vosstian iron particles to measure the amount of Mn in the residual Vosstian iron. An average value of the measured amounts of Mn was obtained, and the average value was made to be the amount of Mn ([Mn] _γ ) in the residual Vosted iron. Let the amount of Mn of the steel plate of the base material be [Mn] _ave and calculate [Mn] _γ / [Mn] _ave .

If the value of YR is 0.59 or more, the value of TS × UEl is 10000 MPa ·% or more, and the value of TS × LEl is 5,000 MPa ·% or more, it is judged as good characteristics. In addition, if the value of TS × UEl is 12000 MPa ·% or more and the value of TS × LEl is 6000 MPa ·% or more, it is judged to be particularly good characteristics. The descriptions of Tables 4 and 5 are the same as those of Tables 2 and 3, respectively. In addition, in the column labeled "[Mn] _γ / [Mn] _ave ", the symbol "-" indicates that the measurement of the amount of Mn in the residual Vostian iron was not performed.

〔Table 4〕

〔table 5〕

Examples of inventions marked with ○ in the remarks column (test numbers B1, B2, B5, B6, B11, B13, B14, B18, B21 ~ B23, B25 ~ B35, and B38 ~ B42) are all TS × UEl at 10000 Above, TS × LEl is above 5000, and shows good uniform ductility and local ductility. In addition, YR shows a high value of 0.59 or more.

In particular, the test numbers B1, B5, B6, B11, B18, B23, B26, B27, B29, B30, B32 to B35, B38, and B39, because the heating temperature exceeds the Ac ₃ point, and in the first cooling step The average cooling rate in the temperature range from heating temperature to (heating temperature -50 ° C) is 7 ° C / sec or less, so the volume ratio of further polygonal ferrous iron is 2.0% or more, [Mn] _γ / [Mn] _ave is 1.10 or more. As a result, the samples of these test numbers are TS × UEl above 12000 and TS × LEl above 6000, showing particularly good uniform ductility and local ductility.

On the other hand, the test results for steel sheets whose chemical composition or step conditions are outside the scope of the present invention (× marked test numbers B3, B4, B7 to B10, B12, B15 to B17, B19, B20, B20, B24, B36, and B37), any or all of the drop ratio, uniform ductility, and local ductility are inferior.

Specifically, although steels C, E, and N having a chemical composition within the scope of the present invention were used, in test numbers B7, B19, and B36 that were not subjected to surface rolling, the amount of C and [Mn ] _γ / [Mn] _{ave is} low, and TS × UEl and TS × LEl are low. Since test number B12 using steel C was not heat-treated, the volume ratio of tempered Asada loose iron, the amount of C in residual Vostian iron, and [Mn] _γ / [Mn] _{ave were} low, and YR, TS × UEl, and TS × LEl is low.

Although steels C, E, and N having a chemical composition within the scope of the present invention were used, in test numbers B8, B20, and B37 where the heat treatment temperature was too low, the volume ratio of loose Asada iron and the residual Wasfield iron were tempered. The amount of C and [Mn] _γ / [Mn] _{ave were} lower, and YR, TS × UEl, and TS × LEl were lower. In the tests using steels C and F (test numbers B9 and B24), because the heat treatment temperature was too high, the volume ratio of residual Vosstian iron and the amount of C in the residual Vosstian iron were 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 test number B16, where the soaking temperature was too low during the annealing step, the residual Vosstian iron volume fraction and tempered Asada bulk iron volume fraction were low. And TS × UEl is low. In the test (test numbers B3 and B15) using steels A and C, the average cooling rate in the temperature range of 650 to 500 ° C in the first cooling step was too low, so in test number B3, the residual The Vostian iron volume ratio, tempered Asada loose iron volume ratio and [Mn] _γ / [Mn] _{ave were} lower, and YR and TS × LEl were lower. In test number B15, the residual Vosstian iron volume fraction and [Mn] _γ / [Mn] _{ave were} lower, and YR, TS × UEl, and TS × LEl were lower. Although the steel C having the chemical composition within the scope of the present invention was used, in the second cooling step, the average cooling rate (secondary cooling rate) in the temperature range of the alloying treatment temperature to 300 ° C was too low. Test No. B10 Among them, the volume ratio of residual Vastian iron and the amount of C in residual Vastian iron are low, and TS × UEl and TS × LEl are low.

In test number B4 using steel B, because the amount of Si in the steel is small, the volume fraction of residual Vosstian iron and the amount of C in residual Vosstian iron are low, and YR, TS × UEl, and TS × LEl are low. . In test number B17 using steel D, because the amount of Mn in the steel is small, the residual Vossian iron volume fraction and [Mn] _γ / [Mn] _{ave are} low, and YR and TS × LEl are low.

INDUSTRIAL APPLICABILITY As described above, according to the present invention, a hot-dip galvanized steel sheet and an alloyed hot-dip galvanized steel sheet can be manufactured and provided, which are excellent in uniform ductility and local ductility, and moreover, reduced strength and tensile strength. The tensile strength is high, and the moldability and impact absorption are excellent. The hot-dip galvanized steel sheet and 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 thus the present invention has high industrial applicability.

(no)

FIG. 1 is a diagram for explaining a method of manufacturing a hot-dip galvanized steel sheet. FIG. 2 is a diagram for explaining a method of manufacturing an alloyed hot-dip galvanized steel sheet.

Claims (13)

A plated steel plate characterized in that the chemical composition contains 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% in mass%. Below, 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 is composed of iron and impurities; the metal structure contains more than 5.0% by volume of residual Vostian iron, and more than 5.0% by volume of tempered Asada loose iron; and The amount of C in the aforementioned residual Vosstian iron is 0.85 mass% or more. The plated steel sheet according to claim 1, wherein the metal structure further contains polygonal ferrous iron in an amount exceeding 2.0% by volume; and the Mn content in the residual Vostian iron satisfies the following formula (1); [Mn] _γ / [Mn ] _ave ≧≧ 1.10 ... (1) [Mn] _γ : Mn content (mass%) in the residual Vosstian iron [Mn] _ave : Mn content (mass%) in the chemical composition of the steel sheet. The coated steel sheet according to claim 1 or 2, wherein the aforementioned chemical composition in mass% further contains a material 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 formed groups. For example, the plated steel sheet according to claim 1 or 2, wherein the foregoing chemical composition is further contained in mass% and is 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 formed groups. The coated steel sheet according to claim 1 or 2, wherein the aforementioned chemical composition further includes, in mass%, one selected from the group consisting of Cu: 0.001% to 2.000% and Ni: 0.001% to 2.000%. Or 2 kinds. The coated steel sheet according to claim 1 or 2, wherein the aforementioned chemical composition in mass% further contains a material 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%. One or two or more of the formed groups. For example, the plated steel sheet according to claim 1 or 2, wherein the aforementioned chemical composition further includes Bi in terms of mass%: 0.0001% to 0.0500%. The plated steel sheet according to claim 1 or 2, wherein the aforementioned plated steel sheet is a hot-dip galvanized steel sheet including a hot-dip galvanized layer. The plated steel sheet according to claim 1 or 2, wherein the aforementioned plated steel sheet is an alloyed hot-dip galvanized steel sheet whose hot-dip galvanized layer has been alloyed. A method for manufacturing a galvanized steel sheet to melt, characterized by comprising the steps of: heating the steel billet to above Ac ₁ point and the annealing step, the steel billet having a chemical composition containing, by mass%, C: 0.03% ~ 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 Structure: The first cooling step, after the foregoing annealing step, is cooled to 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. Below ℃; In the step of hot-dip galvanizing, after the aforementioned first cooling step, hot-dip galvanizing is performed on the billet steel sheet cooled in the aforementioned first cooling step; in the second cooling step, the aforementioned melting step is performed. After the galvanizing step, Cooling the hot-dip galvanized billet steel plate to a temperature below 300 ° C at an average cooling rate of 2 ° C / sec or more from a temperature range from the plating temperature to 300 ° C in the step of performing galvanizing; In the rolling step, after the aforementioned second cooling step, the billet steel sheet cooled in the aforementioned second cooling step is subjected to quenched and tempered rolling at a stretch rate of 0.10% or more; and In the step, after the aforementioned tempering and rolling step, the billet steel sheet subjected to the tempering and rolling is heated to a temperature range of 200 ° C. to 600 ° C. and maintained at the temperature for more than 1 second. For example, the method for manufacturing a hot-dip galvanized steel sheet according to claim 10, wherein in the foregoing annealing step, the blank steel sheet is heated to a point above Ac ₃ and annealed; and after the foregoing annealing step, the temperature from the heating temperature to ( The annealed billet steel sheet is cooled in a temperature range up to -50 ° C) at an average cooling rate of 7 ° C / sec or less. A method for manufacturing an alloyed hot-dip galvanized steel sheet, which is characterized by having the following steps: a step of heating a blank steel sheet to exceed Ac ₁ point and annealing; the aforementioned blank steel sheet has a chemical composition of C: 0.03 by mass% % 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%, 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, after the aforementioned annealing step, 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 Below ℃; In the step of hot-dip galvanizing, after the aforementioned first cooling step, hot-dip galvanizing is performed on the blank steel sheet cooled in the aforementioned first cooling step; in the step of alloying treatment, the aforementioned melting step is performed. Steps of galvanizing After that, the blank steel sheet that has been hot-dip galvanized is subjected to alloying treatment at an alloying treatment temperature; a second cooling step, after the aforementioned alloying treatment step, from the aforementioned alloying treatment temperature to 300 ° C In the temperature range of 2 ° C / sec, the alloyed blank steel sheet is cooled to below 300 ° C at an average cooling rate of 2 ° C / sec or more. After the second cooling step, The cooled billet steel sheet in the aforementioned second cooling step is subjected to quenched and tempered rolling with a stretching rate of 0.10% or more; and the step of heat treatment is subjected to the quenched and tempered rolling after the aforementioned quenched and tempered rolling step. The drawn blank steel sheet is heated to a temperature range of 200 ° C to 600 ° C and maintained at the temperature for more than 1 second. For example, the method for manufacturing an alloyed hot-dip galvanized steel sheet according to claim 12, wherein in the aforementioned annealing step, the aforementioned blank steel sheet is heated to exceed Ac ₃ points and annealed; and after the aforementioned annealing step, the In the temperature range from the heating temperature to (heating temperature -50 ° C), the annealed billet steel sheet is cooled at an average cooling rate of 7 ° C / sec or less.