The present invention relates to high-strength hot-dip galvanized steel sheets and high-strength alloyed hot-dip galvanized steel sheets suitable for press molding, and in particular, high-strength hot-dip galvanized steel sheets and high-strength alloyed hot-dip galvanized steel sheets having excellent ductility and low-temperature impact characteristics. Regarding steel plates.
Background technology
[0002]
　Today, the industrial technology field has become highly divided, and in each technology field, special and high-performance materials are required. With respect to steel sheets for automobiles, in order to improve fuel efficiency by reducing the weight of the vehicle body, it is required to increase both the yield strength and the tensile strength.
[0003]
　When a high-strength steel plate is applied to the vehicle body of an automobile, the thickness of the steel plate can be reduced to reduce the weight of the vehicle body, and at the same time, the desired strength can be imparted to the vehicle body. However, in press molding for forming the vehicle body of an automobile, the thinner the steel sheet, the more easily cracks and wrinkles are generated. Therefore, the thin steel sheet for automobiles is also required to have excellent uniform ductility and local ductility.
[0004]
　Further, in order to improve the collision safety performance of an automobile, the steel plate needs to have excellent shock absorption. From the viewpoint of shock absorption, the steel sheet for automobiles needs to have excellent local ductility in order to suppress cracking under impact load in addition to having higher strength.
[0005]
　As described above, the steel sheet for automobiles has high strength for reducing the weight of the vehicle body and improving collision safety, high uniform ductility for improving moldability, and improving moldability and collision safety. High local ductility is required. Furthermore, in order to ensure collision safety even in a low temperature environment, steel sheets for automobiles are also required to have excellent low temperature impact characteristics.
[0006]
　However, in steel sheets, improvement and high strength of uniform ductility and local ductility, and improvement and high strength of low temperature impact characteristics are both contradictory events, and it is difficult to achieve these at the same time. Further, although corrosion resistance is required for steel sheets for automobiles, maintaining the required corrosion resistance makes it more difficult to increase the strength while ensuring ductility and low temperature impact characteristics.
[0007]
　As a method for improving the ductility of a high-strength cold-rolled steel sheet, a method for adding retained austenite to a metal structure has been proposed. Steel sheets containing retained austenite show a large elongation due to transformation-induced plasticity (TRIP) developed by transforming austenite into martensite during processing.
[0008]
　In Patent Documents 1 and 2, a steel sheet containing Si and Mn is heated to a two-phase region of ferrite-austenite or a single-phase region of austenite, annealed and then cooled, and then held in the steel sheet at 350 to 500 ° C. Disclosed is a method for producing a high-strength cold-rolled steel sheet, which is subjected to an annealing treatment to stabilize austenite. According to these manufacturing methods, the strength and ductility of the cold-rolled steel sheet can be improved in a well-balanced manner.
[0009]
　Patent Document 3 states that by containing Si and Mn in a certain ratio or more with respect to the amount of C, transformation of austenite during alloying treatment is suppressed and a metal structure in which retained austenite is mixed in ferrite is formed, which is high strength. A method for producing an alloyed hot-dip galvanized steel sheet is disclosed.
[0010]
　Patent Document 4 describes high-tensile melt with excellent ductility, stretch flangeability, and fatigue resistance, in which retained austenite and low-temperature transformation formation phase are dispersed in ferrite and tempered martensite with an average crystal grain size of 10 μm or less. Galvanized steel sheets are disclosed. It is disclosed that tempered martensite is an effective phase for improving stretch flangeability and fatigue resistance, and that when tempered martensite is made into fine particles, the above-mentioned properties are further improved.
Prior art literature
Patent documents
[0011]
Patent Document 1: Japanese Patent Application Laid-Open No. 61-157625
Patent Document 2: Japanese Patent Application
Laid-Open No. 61-217529 Patent Document 3: Japanese Patent Application Laid-Open No. 11-279691
Patent Document 4: Japanese Patent Application Laid-Open No. 2001-192768
Outline of the invention
Problems to be solved by the invention
[0012]
　In the production of hot-dip galvanized steel sheets and alloyed hot-dip galvanized steel sheets, austempering cannot be sufficiently performed in a general continuous hot-dip galvanized steel sheet due to restrictions on holding temperature and holding time. And it is difficult to apply the method for producing a cold-rolled steel sheet as disclosed in 2. Further, since austenite is easily decomposed in the plating step and the alloying step, it is difficult to secure the required amount of retained austenite.
[0013]
　In Patent Document 3, no consideration is given to deterioration of local ductility and low temperature impact characteristics, which are problems in steel sheets in which retained austenite is mixed in the metal structure.
[0014]
　Regarding Patent Document 4, in order to obtain a metallographic structure containing tempered martensite and retained austenite, a primary heat treatment for producing martensite and a secondary heat treatment for tempering martensite and further obtaining retained austenite are performed. The productivity of the steel sheet manufacturing method of Patent Document 4 is significantly low. Further, in the method for producing a steel sheet of Patent Document 4, since the secondary heat treatment is performed at a high temperature of one or more points of Ac , the tempered martensite is excessively softened, and it is difficult to obtain high strength.
[0015]
　As mentioned above, the improvement of strength (yield strength and tensile strength) and the improvement of ductility (uniform ductility and local ductility) and low temperature impact characteristics are contradictory events. It is difficult to do with the prior art.
[0016]
　In view of the prior art, in the hot-dip galvanized steel sheet and the alloyed hot-dip galvanized steel sheet, the hot-dip galvanized steel sheet having improved uniform ductility and local ductility, low-temperature impact characteristics, yield strength and tensile strength are all improved. An object of the present invention is to provide an alloyed hot-dip galvanized steel sheet.
Means to solve problems
[0017]
　The present inventors have diligently studied a method for solving the above problems. As a result, the following findings (A) to (D) have been obtained.
[0018]
　(A) When a low-carbon hot-dip galvanized steel sheet containing Si and Mn or a low-carbon alloyed hot-dip galvanized steel sheet containing Si and Mn is manufactured by a continuous hot-dip galvanizing facility, uniform ductility and local ductility are reduced. However, the yield strength may also decrease. It is considered that this is because the austemper treatment is insufficient in the continuous hot-dip galvanizing facility, and a metal structure containing retained austenite having a low C concentration and hard martensite is formed.
[0019]
　(B) However, when the hot-dip galvanized steel sheet and the alloyed hot-dip galvanized steel sheet having a metal structure containing retained austenite and hard martensite having a low C concentration are reheated, uniform ductility and local ductility are improved. In addition, the yield strength also increases.
[0020]
　It is presumed that this is due to the concentration of C in austenite during the reheat treatment, which enhances the stability of austenite, and the tempering of hard martensite to soft tempered martensite. ..
[0021]
　(C) Further, when the hot-dip galvanized steel sheet and the alloyed hot-dip galvanized steel sheet are temper-rolled before the reheating treatment, the uniform ductility and the local ductility are further improved, and the yield strength is further increased. do.
[0022]
　This is because the temper rolling introduces rearrangements into the austenite, and the subsequent reheating treatment promotes the concentration of C in the austenite, further improving the stability of the austenite. The temper rolling further improves the stability of the austenite. A part of the austenite transformation occurs, and the tempered austenite increases in the metal structure after the reheat treatment, and the martensitic transformation that occurs during cooling after the reheat treatment is suppressed, and the metal after the reheat treatment is suppressed. It is presumed to be due to the reduction of hard martensites in the tissue.
[0023]
　(D) In ​​the heating stage of the reheating treatment, when the low temperature region is heated at a slow temperature rise rate and the high temperature region is heated at a high temperature rise rate, the low temperature impact characteristics of the hot-dip galvanized steel sheet and the alloyed hot-dip galvanized steel sheet are exhibited. improves.
[0024]
　This is because when the former austenite grain boundaries are strengthened, brittle fracture starting from the former austenite grain boundaries is suppressed, the low-temperature impact characteristics are improved, and by slowly heating the low-temperature region, the former austenite grain boundaries are C. It is presumed that this is due to the fact that the grain boundaries are strengthened by segregation of and B, and that the segregation of P to the former austenite grain boundaries is suppressed and the grain boundaries are strengthened by rapidly heating the high temperature region. NS.
[0025]
　Then, based on the findings of (A) to (D), the present inventors perform temper rolling on the hot-dip galvanized steel sheet or the alloyed hot-dip galvanized steel sheet, and then reheat by two-stage heating. When treated, it has a retained austenite with a high C concentration and a metallographic structure containing tempered martensite, is excellent in uniform ductility, local ductility, and low temperature impact characteristics, and has high yield strength and tensile strength, and is melted. It was found that zinc-plated steel sheets and alloyed hot-dip galvanized steel sheets can be manufactured.
[0026]
　The present invention has been made based on the above findings, and the gist thereof is as follows. In the present invention, the "steel plate" includes a "steel strip".
[0027]
　(1) A hot-dip galvanized steel sheet having a hot-dip galvanized layer on the surface of the steel sheet, wherein the component composition of the steel sheet is C: 0.03 to 0.70%, Si: 0.25 to 2 in mass%. .50%, Mn: 1.00 to 5.00%, P: 0.0005 to 0.100%, S: 0.010% or less, sol. Al: 0.001 to 2.500%, N: 0.020% or less, B: 0 to 0.0200%, Ti: 0 to 0.30%, Nb: 0 to 0.30%, V: 0 to 0.30%, Cr: 0 to 2.00%, Mo: 0 to 2.00%, Cu: 0 to 2.00%, Ni: 0 to 2.00%, Ca: 0 to 0.010%, It contains Mg: 0 to 0.010%, REM: 0 to 0.10%, and Bi: 0 to 0.050%, the balance is Fe and unavoidable impurities, and the metal structure of the steel sheet is volume%. The retained austenite contains more than 5.0% and the tempered martensite: more than 5.0%, and the retained austenite contains C: 0.85% by mass or more, and the old in the metal structure of the steel plate. Ratio of C segregation amount (number of atoms / nm 2 ): [C] γgb and P segregation amount (number of atoms / nm 2 ): [P] γgb at the austenite grain boundary : [C] γgb / [P] γgb is 4 A hot-dip zinc-plated steel sheet having a value of .0 or more.
[0028]
　(2) The composition of the steel plate is, in mass%, B: 0.0002 to 0.0200%, Ti: 0.001 to 0.30%, Nb: 0.001 to 0.30%, V: 0. .001 to 0.30%, Cr: 0.001 to 2.00%, Mo: 0.001 to 2.00%, Cu: 0.001 to 2.00%, Ni: 0.001 to 2.00 %, Ca: 0.0001 to 0.010%, Mg: 0.0001 to 0.010%, REM: 0.0001 to 0.10%, and Bi: 0.0001 to 0.050%. The hot-dip zinc-plated steel sheet of the above (1), which is characterized by containing.
[0029]
　(3) In the composition of the steel sheet, the content of B is 0.0002% or more, and the segregation amount of B at the former austenite grain boundary in the metal structure of the steel sheet (atomic number / nm 2 ): [B] γgb The ratio of P segregation amount (atomic number / nm 2 ): [P] γgb : [B] γgb / [P] γgb is 4.0 or more. Hot-dip galvanized steel sheet.
[0030]
　(4) In the hot-dip galvanized steel sheet according to any one of (1) to (3) above, the hot-dip galvanized steel sheet is an alloyed hot-dip galvanized steel sheet, wherein the hot-dip galvanized layer is an alloyed hot-dip galvanized steel sheet.
[0031]
　(5) A manufacturing method for producing a hot-dip zinc-plated steel sheet according to any one of (1) to (3) above, wherein the material steel sheet having the component composition of (1) or (2) above has a temperature exceeding one Ac point. After the annealing step and the annealing step of heating to a region and annealing, the material steel sheet is cooled to 500 ° C. or lower with an average cooling rate of 2 ° C./sec or more and less than 100 ° C./sec in the temperature range of 650 to 500 ° C. 1 After the cooling step and the first cooling step, the material steel sheet is subjected to hot-dip zinc plating. After the second cooling step of cooling to less than 300 ° C., the annealing rolling step of subjecting the material steel sheet to temper rolling with an elongation of 0.10% or more, and after the annealing rolling step, The material steel sheet is heated to 300 ° C. with an average heating rate of less than 10 ° C./sec in a temperature range up to 300 ° C., and then 300 ° C. with an average heating rate of more than 10 ° C./sec in a temperature range exceeding 300 ° C. A method for producing a hot-dip zinc-plated steel sheet, which comprises a two-stage heat treatment step of heating to a temperature range of ultra-600 ° C. or lower and holding the heating temperature at the heating temperature for 1 second or longer.
[0032]
　(6) In the manufacturing method for producing the alloyed hot-dip zinc-plated steel sheet of (4), the material steel sheet having the component composition of (1) or (2) is heated to a temperature range exceeding one Ac point. After the annealing step and the annealing step of annealing, the first cooling step of cooling the material steel sheet to 500 ° C. or lower with an average cooling rate of 2 ° C./sec or more and less than 100 ° C./sec in the temperature range of 650 to 500 ° C. 1 After the cooling process, the material steel sheet is subjected to hot-dip zinc plating, after the plating process, the material steel sheet is alloyed, and after the alloying process, the material steel sheet is subjected to the alloying treatment temperature. After the second cooling step and the second cooling step in which the average cooling rate in the temperature range of 300 ° C is set to 2 ° C / sec or more and the temperature is lowered to less than 300 ° C, the material steel sheet is annealed and rolled with an elongation rate of 0.10% or more. After the annealing rolling step and the annealing rolling step, the material steel sheet is heated to 300 ° C. with an average heating rate of less than 10 ° C./sec in the temperature range up to 300 ° C., and then exceeds 300 ° C. It is characterized by being provided with a two-stage heat treatment step in which the average heating rate in the temperature range is set to more than 10 ° C./sec, heating is performed in a temperature range of more than 300 ° C. and 600 ° C. or lower, and heat treatment is performed to maintain the heating temperature for 1 second or longer. A method for manufacturing an alloyed hot-dip zinc-plated steel sheet.
The invention's effect
[0033]
　According to the present invention, both uniform ductility and local ductility are good, press formability is excellent, yield strength and tensile strength are high, local ductility is good, shock absorption is excellent, and low-temperature impact is achieved. It is possible to provide a hot-dip galvanized steel sheet and an alloyed hot-dip galvanized steel sheet having excellent characteristics.
Forms for carrying out the invention
[0034]
　Hereinafter, the component composition, metallographic structure, and mechanical properties of the hot-dip galvanized steel sheet of the present invention and the alloyed hot-dip galvanized steel sheet (hereinafter collectively referred to as “the steel sheet of the present invention”) will be sequentially described.
[0035]
　First, the composition of the steel sheet of the present invention will be described. Hereinafter, "%" related to the component composition means "mass%".
[0036]
　(C: 0.03 to 0.70%)
　C is an element required to obtain retained austenite. Further, in the steel sheet of the present invention, it is an element that strengthens the grain boundaries by segregating at the former austenite grain boundaries. If C is less than 0.03%, it becomes difficult to obtain a metallic structure containing retained austenite and tempered martensite, so C is set to 0.03% or more. It is preferably 0.10% or more, more preferably 0.13% or more, still more preferably 0.16% or more.
[0037]
　On the other hand, if C exceeds 0.70%, the weldability of the steel sheet is significantly lowered, so C is set to 0.70% or less. It is preferably 0.30% or less, more preferably 0.26% or less, still more preferably 0.24% or less.
[0038]
　(Si: 0.25 to 2.50%)
　Si is an element that suppresses the precipitation of cementite and promotes the formation of retained austenite, and tempered martensite is excessively softened. It is an element that contributes to ensuring strength by suppressing.
[0039]
　If Si is less than 0.25%, the addition effect cannot be sufficiently obtained. Therefore, Si is 0.25% or more, preferably more than 0.60%, more preferably more than 1.00%, still more preferably. It is over 1.45%.
[0040]
　On the other hand, if Si exceeds 2.50%, the plating property of the steel sheet is remarkably lowered and the weldability of the steel sheet is lowered. Therefore, the Si is set to 2.50% or less. It is preferably 2.30% or less, more preferably 2.10% or less, still more preferably 1.90% or less.
[0041]
　(Mn: 1.00 to 5.00%)
　Mn is an element that contributes to the improvement of hardenability of steel and is an effective element for obtaining a metallographic structure containing retained austenite and tempered martensite. If Mn is less than 1.00%, the addition effect cannot be sufficiently obtained, so Mn is set to 1.00% or more. It is preferably more than 1.50%, more preferably more than 2.00%, and even more preferably more than 2.50%.
[0042]
　On the other hand, if Mn exceeds 5.00%, the weldability of the steel sheet deteriorates, so Mn is set to 5.00% or less. It is preferably 4.00% or less, more preferably 3.50% or less, still more preferably 3.00% or less.
[0043]
　(P: 0.0005 to 0.100%)
　P is a preferable element because it segregates at the old austenite grain boundaries and embrittles the steel sheet. However, the present invention is a technique for suppressing segregation of P to the former austenite grain boundaries and segregating C and B, and is premised on the fact that P remains in the steel to some extent. Therefore, it is not necessary to reduce P excessively. In particular, if P is reduced to less than 0.0005%, the manufacturing cost will increase significantly, so that P can be 0.0005% or more. It may be 0.0010% or more.
[0044]
　On the other hand, when P exceeds 0.100%, segregation becomes remarkable and the steel sheet becomes remarkably embrittled. Therefore, P is set to 0.100% or less. It is preferably less than 0.020%, more preferably less than 0.015%, and even more preferably less than 0.010%.
[0045]
　(S: 0.010% or less)
　S forms sulfide-based inclusions in the steel and inhibits the local ductility of the steel sheet. Therefore, the smaller the amount, the more preferable the element. If S exceeds 0.010%, the local ductility of the steel sheet is significantly reduced, so S is set to 0.010% or less. It is preferably 0.0050% or less, more preferably 0.0012% or less.
[0046]
　The lower limit includes 0%, but if S is reduced to less than 0.0001%, the manufacturing cost increases significantly. Therefore, 0.0001% is a substantial lower limit on the practical steel sheet.
[0047]
　(Sol.
　Al : 0.001 to 2.500%) Al, like Si, is an element that deoxidizes molten steel, promotes the formation of retained austenite, and retains austenite and tempered martensite. It is an element effective for the formation of a metallographic structure containing.
[0048]
　sol. If Al is less than 0.001%, the deoxidizing effect cannot be sufficiently obtained. Al is 0.001% or more. It is preferably 0.015% or more, more preferably 0.025% or more, still more preferably 0.045% or more. In terms of promoting retained austenite, it is preferably 0.055% or more, more preferably 0.065% or more, still more preferably 0.075% or more.
[0049]
　On the other hand, sol. If Al exceeds 2.500%, a large amount of alumina (Al 2 O 3 ), which causes surface defects, is generated , and the transformation point rises, making annealing difficult. Al is 2.500% or less. It is preferably less than 0.600%, more preferably less than 0.200%, and even more preferably less than 0.080%.
[0050]
　(N: 0.020% or less)
　N is a preferable element because it forms a nitride that causes cracking of the slab during continuous casting of steel. If N exceeds 0.020%, slab cracking frequently occurs, so N is set to 0.020% or less. It is preferably 0.010% or less, more preferably less than 0.008%, still more preferably 0.005% or less.
[0051]
　The lower limit includes 0%, but if N is reduced to less than 0.0005%, the manufacturing cost increases significantly. Therefore, 0.0005% is a substantial lower limit on the practical steel sheet.
[0052]
　(B: 0 to 0.0200%)
　Like C, B is an element that segregates into the former austenite grain boundaries and strengthens the grain boundaries. The steel sheet of the present invention has good uniform ductility and local ductility, is excellent in press formability, has high yield strength and tensile strength, has good local ductility, is excellent in shock absorption, and has low temperature impact characteristics. The hot-dip zinc-plated steel sheet and the alloyed hot-dip zinc-plated steel sheet, which are also excellent in Can be added according to the above. In addition, B is an element that improves the hardenability of steel and is effective in forming a metallographic structure containing retained austenite and tempered martensite. In order to obtain a sufficient effect of the addition, B is preferably 0.0002% or more. It is more preferably 0.0005% or more, still more preferably 0.0010% or more.
[0053]
　On the other hand, if B exceeds 0.0200%, the addition effect is saturated and the economic efficiency is lowered, so B is set to 0.0200% or less. It is preferably 0.0100% or less, more preferably 0.0050% or less, still more preferably 0.0020% or less.
[0054]
　In addition to the above elements, the steel sheet of the present invention may contain one or more of Ti, Nb, V, Cr, Mo, Cu, Ni, Ca, Mg, REM, and Bi in order to improve the characteristics. ..
[0055]
　(Ti: 0 to 0.30%, Nb: 0 to 0.30%, V: 0 to 0.30%)
　Ti, Nb, and V refine the metallographic structure and improve the strength and ductility of the steel sheet. It is an element that contributes to. In order to sufficiently obtain the effect of adding Ti, Nb, and V, the Ti, Nb, and V are preferably 0.001% or more. More preferably, Ti and Nb are 0.005% or more, V is 0.010% or more, and even more preferably, Ti and Nb are 0.010% or more, and V is 0.020% or more.
[0056]
　On the other hand, when Ti, Nb, and V exceed 0.30%, the addition effect is saturated, the recrystallization temperature during annealing rises, the metal structure after annealing becomes non-uniform, and local ductility decreases. Therefore, Ti, Nb, and V are all preferably 0.30% or less. More preferably, Ti is less than 0.080%, Nb is less than 0.050%, V is 0.20% or less, and more preferably Ti is 0.035% or less, Nb is 0.030% or less, and V is. It is less than 0.10%.
[0057]
　(Cr: 0 to 2.00%, Mo: 0 to 2.00%)
　Cr and Mo are elements that improve the hardenability of steel and contribute to the formation of a metallic structure containing retained austenite and tempered martensite. be. In order to sufficiently obtain the effect of adding Cr and Mo, the amount of Cr and Mo is preferably 0.001% or more. More preferably, Cr is 0.100% or more, and Mo is 0.050% or more.
[0058]
　On the other hand, if Cr and Mo exceed 2.00%, the addition effect is saturated and the economic efficiency is lowered. Therefore, both Cr and Mo are preferably 2.00% or less. More preferably, Cr is 1.00% or less and Mo is 0.50% or less.
[0059]
　(Cu: 0 to 2.00%, Ni: 0 to 2.00%)
　Cu and Ni are elements that contribute to the improvement of yield strength and tensile strength. In order to sufficiently obtain the effect of adding Cu and Ni, both Cu and Ni are preferably 0.001% or more. More preferably, each element is 0.010% or more.
[0060]
　On the other hand, if Cu and Ni exceed 2.00%, the addition effect is saturated and the economic efficiency is lowered. Therefore, both Cu and Ni are preferably 2.00% or less. More preferably, each element is 0.80% or less.
[0061]
　(Ca: 0 to 0.010%, Mg: 0 to 0.010%, REM: 0 to 0.10%)
　Ca, Mg, and REM control the shape of inclusions to improve local ductility. It is an element that contributes to. In order to sufficiently obtain the effect of adding Ca, Mg, and REM, the amount of Ca, Mg, and REM is preferably 0.0001% or more. More preferably, each element is 0.0005% or more.
[0062]
　On the other hand, if Ca and Mg exceed 0.010%, the addition effect is saturated and the economic efficiency is lowered. Therefore, Ca and Mg are preferably 0.010% or less. More preferably, each element is 0.002% or less.
[0063]
　If the REM exceeds 0.10%, the addition effect is saturated and the economic efficiency is lowered. Therefore, the REM is preferably 0.10% or less. More preferably, it is 0.010% or less.
[0064]
　REM is a general term for a total of 17 elements of Sc, Y, and lanthanoids. Lanthanoids are industrially added in the form of misch metal. The amount of REM is the total amount of these elements.
[0065]
　(Bi: 0 to 0.050%)
　Bi is an element that refines the solidified structure and contributes to the improvement of local ductility. In order to obtain a sufficient effect of adding Bi, Bi is preferably 0.0001% or more. More preferably, it is 0.0003% or more.
[0066]
　On the other hand, if Bi exceeds 0.050%, the addition effect is saturated and the economic efficiency is lowered. Therefore, Bi is preferably 0.050% or less. It is more preferably 0.010% or less, still more preferably 0.005% or less.
[0067]
　The rest of the composition of the steel sheet of the present invention is Fe and unavoidable impurities. The unavoidable impurity is an element that is unavoidably mixed from the steel raw material (ore, scrap, etc.) and / or in the manufacturing process, and is an element that is allowed within a range that does not impair the characteristics of the steel sheet of the present invention.
[0068]
　Next, the metal structure of the steel sheet of the present invention will be described. Hereinafter, "%" related to the tissue fraction means "volume%".
[0069]
　(Residual austenite: more than 5.0%, tempered martensite: more than 5.0%)
　The metal structure of the steel sheet of the present invention is 5% by volume, more than 5.0% retained austenite, and more than 5.0% tempered martensite. It is a metallographic structure containing more than 0%. By forming this metal structure, uniform ductility and local ductility can be improved while maintaining yield strength and tensile strength.
[0070]
　If the retained austenite is 5.0% or less, the uniform ductility is not improved, so the retained austenite is set to more than 5.0%. It is preferably more than 6.0%, more preferably more than 8.0%, and even more preferably more than 10.0%.
[0071]
　Since the volume% of retained austenite is not uniquely determined in relation to the volume% of other tissues, the upper limit cannot be set, but if it is 30.0% or more, local ductility and low temperature impact characteristics deteriorate. The retained austenite is preferably less than 30.0%. More preferably, it is less than 20.0%.
[0072]
　If the tempered martensite is 5.0% or less, it is difficult to increase the local ductility while maintaining the yield strength and the tensile strength. Therefore, the tempered martensite is set to more than 5.0%. It is preferably more than 8.0%, more preferably more than 10.0%, and even more preferably more than 12.0%.
[0073]
　Since the volume% of tempered martensite is not uniquely determined in relation to the volume% of other tissues, the upper limit cannot be set, but if it exceeds 70.0%, the uniform ductility decreases, so tempered martensite is used. 70.0% or less is preferable. It is more preferably 50.0% or less, still more preferably 30.0% or less.
[0074]
　The rest of the metallographic structure includes polygonal ferrite, martensite (referred to as untempered martensite, also called fresh martensite), low-temperature transformation formation of acicular ferrite and bainite, pearlite, and cementite. It is a structure containing the precipitate of.
[0075]
　Polygonal ferrite is a structure effective for enhancing uniform ductility, and therefore is preferably contained in an amount of more than 2.0%. More preferably, it is 3.0% or more.
[0076]
　Since the volume% of polygonal ferrite is not uniquely determined in relation to the volume% of other structures, the upper limit cannot be set, but if the polygonal ferrite is 50.0% or more, the yield strength and tensile strength will increase. The polygonal ferrite is preferably less than 50.0% because it is lowered and the local ductility is also lowered. It is more preferably less than 20.0%, still more preferably less than 10.0%.
[0077]
　Martensite is a tissue that inhibits maintaining yield strength and increasing local ductility, so a small amount is preferable, and less than 5.0% is preferable. It is more preferably less than 2.0%, still more preferably less than 1.0%.
[0078]
　Precipitates such as low-temperature transformation-forming structures of acicular ferrite and bainite, pearlite, and cementite inhibit yield strength and tensile strength, and therefore, a total of 40.0% or less is preferable. It is more preferably 20.0% or less, still more preferably 10.0% or less.
[0079]
　Pearlite is preferably less than 10.0% because it inhibits uniform ductility as well as yield strength and tensile strength. It is more preferably less than 5.0%, still more preferably less than 3.0%.
[0080]
　Precipitates such as martensite, acicular ferrite and bainite low temperature transformation formation structure, pearlite, and cementite may be inevitably formed. Therefore, the lower limit is not set, but the rest of the metal structure is these. The lower limit is 0% as it does not need to contain tissue.
[0081]
　The volume% of the metal structure of the steel sheet of the present invention is measured as follows.
[0082]
　A test piece is taken from a steel plate, a vertical cross section parallel to the rolling direction is polished, and a metal structure at a depth of 1/4 of the thickness of the base steel plate is scanned from the boundary between the base steel plate and the plating layer by a scanning electron microscope. Observe and image with (SEM). The image is image-processed, the area ratio of each tissue is calculated, and the calculated area ratio is defined as the volume ratio.
[0083]
　Tempering martensite can be distinguished from bainite in this respect because the iron carbides present inside extend in multiple directions. Polygonal ferrite can be distinguished from acicular ferrite in that it is lumpy in morphology and has a low dislocation density.
[0084]
　(C amount of retained austenite: 0.85% by mass or more) In
　the steel sheet of the present invention, in order to stabilize retained austenite and improve uniform ductility and local ductility, the C amount of retained austenite is 0.85% by mass or more. And. It is preferably 0.87% by mass or more, more preferably 0.89% by mass or more. The C amount of retained austenite means the C concentration in the austenite phase.
[0085]
　Since the C amount of retained austenite varies depending on the C amount of the steel sheet and the manufacturing conditions, the upper limit cannot be set, but if the C amount is 1.50% by mass or more, the TRIP effect cannot be obtained and the uniform ductility is lowered. Therefore, the amount of C of retained austenite is preferably less than 1.50% by mass. It is more preferably less than 1.20% by mass, still more preferably less than 1.10% by mass.
[0086]
　The volume% of retained austenite and the amount of C of retained austenite are the rolled surface of the test piece collected from the steel sheet from the boundary between the base steel plate and the plating layer to a depth of 1/4 of the plate thickness of the base steel plate. Is chemically polished, and the X-ray diffraction intensity and the diffraction peak position of the polished surface are measured and calculated by an X-ray diffractometer (XRD).
[0087]
　([C] γgb / [P] γgb : 4.0 or more)
　C segregation amount at the former austenite grain boundary (atomic number / nm 2 ): [C] γgb and P segregation amount at the former austenite grain boundary (atomic number / nm 2 ): [P] γgb ratio: [C] γgb / [P] By setting γgb to 4.0 or more, the low temperature impact characteristics are remarkably improved.
[0088]
　If [C] γgb / [P] γgb is less than 4.0, the low temperature impact characteristics are not improved, so [C] γgb / [P] γgb is set to 4.0 or more. It is preferably 5.0 or more, more preferably 6.0 or more. The upper limit is not particularly limited, but is preferably 30.0 or less from the viewpoint of productivity.
[0089]
　[C] γgb and [P] γgb at the prior austenite grain boundaries are measured as follows to calculate [C] γgb / [P] γgb .
[0090]
　In the test piece collected from the steel plate, the old austenite grain boundaries are confirmed by observing the metal structure at a depth of 1/4 of the plate thickness of the base steel plate from the boundary between the base steel plate and the plating layer. A block containing the old austenite grain boundaries is cut out by the Lift-out method, and a needle sample for a three-dimensional atom probe (3DAP) is prepared using a focused ion beam device (FIB).
[0091]
　With 3DAP, the distribution of C and P atoms in the region including the former austenite grain boundaries was measured, and the concentration distribution was analyzed by a ladder diagram to obtain the segregated atomic density (Interfacial excess) per unit grain boundary area. Let it be [C] γgb and [P] γgb . [C] γgb and [P] γgb were measured at five or more different former austenite grain boundaries, and the average value of [C] γgb / [P] γgb values obtained at each former austenite grain boundary was obtained. Ask for.
[0092]
　([B] γgb / [P] γgb : 4.0 or more) When
　the steel plate of the present invention contains B, the amount of B segregation at the former austenite grain boundaries (atomic number / nm 2 ): [B] γgb And, by setting the ratio of P segregation amount (atomic number / nm 2 ): [P] γgb : [B] γgb / [P] γgb at the former austenite grain boundary to 4.0 or more, the low temperature impact characteristics become remarkable. improves.
[0093]
　If [B] γgb / [P] γgb is less than 4.0, the low temperature impact characteristics are not improved, so [B] γgb / [P] γgb is set to 4.0 or more. It is preferably 5.0 or more, more preferably 6.0 or more. The upper limit is not particularly limited, but is preferably 30.0 or less from the viewpoint of productivity.
[0094]
　[B] γgb and [P] γgb at the prior austenite grain boundaries are measured as follows to calculate [B] γgb / [P] γgb .
[0095]
　In the test piece collected from the steel plate, the old austenite grain boundaries are confirmed by observing the metal structure at a depth of 1/4 of the plate thickness of the base steel plate from the boundary between the base steel plate and the plating layer. A block containing the old austenite grain boundaries is cut out by the Lift-out method, and a needle sample for a three-dimensional atom probe (3DAP) is prepared using a focused ion beam device (FIB).
[0096]
　With 3DAP, the distribution of B and P atoms in the region including the former austenite grain boundaries was measured, and the concentration distribution was analyzed by a ladder diagram to obtain the segregated atomic density (Interfacial excess) per unit grain boundary area. Let it be [B] γgb and [P] γgb . [B] γgb and [P] γgb were measured at five or more different former austenite grain boundaries, and the average value of [B] γgb / [P] γgb values obtained at each former austenite grain boundary was obtained. Ask for.
[0097]
　Next, the hot-dip galvanized layer of the steel sheet of the present invention and the alloyed hot-dip galvanized layer will be described.
[0098]
　The hot-dip galvanized layer and the alloyed hot-dip galvanized layer may be formed under normal plating conditions and alloying conditions. However, if the Fe amount of the alloyed hot-dip galvanized layer is less than 7% by mass, weldability and slidability cannot be ensured. Therefore, the Fe amount of the alloyed hot-dip galvanized layer is preferably 7% by mass or more. The upper limit of the amount of Fe is preferably 20% by mass or less, more preferably 15% by mass or less, from the viewpoint of suppressing powdering resistance. The amount of Fe in the alloyed hot-dip galvanized layer is adjusted by appropriately adjusting the alloying treatment conditions.
[0099]
　Next, the mechanical properties of the steel sheet of the present invention will be described.
[0100]
　Regarding the elongation characteristics of the steel plate, the uniform elongation in the direction orthogonal to the rolling direction is defined as UEl (Uniform Elongation), and the total elongation (TER 0 ) in the direction orthogonal to the rolling direction is the plate thickness based on the following formula (1). The value converted to the total elongation equivalent to 1.2 mm is defined as TEl (Total Elongation), and based on the following formula (2), the local elongation in the direction orthogonal to the rolling direction corresponding to the plate thickness 1.2 mm is LEl. Defined as (Local Elongation).
　　TEl = Tel 0 × (1.2 / t 0 ) 0.2　　　　・ ・ ・ (1)
　　LEl = Tel-UEl ・ ・ ・ (2)
[0101]
　UEl is the measured value of uniform elongation measured using the JIS No. 5 tensile test piece, TEL 0 is the measured value of the total elongation measured using the JIS No. 5 tensile test piece, and t 0 is used for the measurement. The thickness of the JIS No. 5 tensile test piece. TEl and LEl are the total elongation and the local elongation converted when the plate thickness is 1.2 mm, respectively.
[0102]
　TS × UEl becomes a large value when both tensile strength (TS) and uniform elongation (UEl) are excellent, and is therefore used as an index for evaluating uniform ductility.
[0103]
　Since TS × LEl becomes a large value when both tensile strength (TS) and local elongation (LEl) are excellent, it is used as an index for evaluating local ductility.
[0104]
　In the steel sheet of the present invention, TS × UEl is preferably 10,000 MPa ·% or more, and TS × LEl is preferably 5000 MPa ·% or more from the viewpoint of press moldability. More preferably, TS × UEl is 11000 MPa ·% or more, and TS × LEl is 6000 MPa ·% or more. More preferably, TS × UEl is 12000 MPa ·% or more, and TS × LEl is 7,000 MPa ·% or more.
[0105]
　In order to improve the impact absorption of the steel sheet of the present invention, the tensile strength (TS) is preferably 780 MPa or more, more preferably 980 MPa or more, still more preferably 1180 MPa or more. The yield ratio (YR) is preferably 0.59 or more, more preferably 0.66 or more, and even more preferably 0.72 or more.
[0106]
　The more excellent the local ductility, the more the cracking at the time of impact load is suppressed and the absorbed energy is increased. Therefore, from the viewpoint of cracking suppression, TS × LEl is preferably 5500 MPa ·% or more, more preferably 6500 MPa ·% or more.
[0107]
　Regarding the low temperature impact characteristics of steel sheets, a plurality of subsize Charpy impact test pieces having a length of 55 mm, a thickness of 10 mm, and a width of the steel sheet thickness are provided in the width direction, with the direction orthogonal to the rolling direction as the length direction. Perform a Charpy impact test in a stacked state. The notch shape of the test piece is defined as the V notch defined by JIS Z 2242, and the Charpy impact values ​​when the Charpy impact test is performed with the test temperatures set to -60 ° C and 40 ° C are defined as IV LT and IV HT , respectively. ..
[0108]
　IV LT / IV HT can be used as an index for evaluating low temperature impact characteristics, and in the steel sheet of the present invention, IV LT / IV HT is preferably more than 0.50, more preferably more than 0.60, and 0. More than 70 is more preferred.
[0109]
　Next, the method for manufacturing the steel sheet of the present invention will be described.
[0110]
　(Material Steel Sheet)
　The steel sheet before plating of the present invention (hereinafter referred to as “material steel sheet”) may be a steel sheet having the component composition of the steel sheet of the present invention, and the method for producing the material steel sheet is not limited to a specific production method. .. As the material steel sheet, a hot-rolled steel sheet can be used. Further, as the hot-rolled steel sheet, a cold-rolled steel sheet that has been pickled and then cold-rolled can also be used. Hereinafter, an example of a method for manufacturing a material steel sheet will be described.
[0111]

　The casting method of the 　(casting) slab is not limited to a specific casting method, but a continuous casting method is preferable. Steel ingots cast by other casting methods may be made into steel pieces by ingot rolling or the like. In the continuous casting step, it is preferable to flow the molten steel by electromagnetic stirring or the like in the mold in order to suppress the occurrence of surface defects due to inclusions. The steel ingot in a high temperature state after continuous casting or the steel piece in a high temperature state after ingot rolling may be cooled once, then reheated, and subjected to hot rolling.
[0112]
　Further, the steel ingot in a high temperature state after continuous casting or the steel piece in a high temperature state after ingot rolling may be subjected to hot rolling as it is, or may be subjected to hot rolling after performing auxiliary heating. .. Steel ingots and steel pieces used for hot rolling are collectively referred to as "slabs".
[0113]
　In order to prevent coarsening of austenite, the temperature of the slab subjected to hot rolling is preferably less than 1250 ° C. More preferably, it is 1200 ° C. or lower. The lower limit of the temperature of the slab to be subjected to hot rolling is not particularly limited, but it is preferable that the temperature is such that hot rolling can be completed at 3 points or more.
[0114]
　(Hot rolling)
　The conditions of hot rolling are not limited to specific conditions, but if the completion temperature of hot rolling is too low, coarse low-temperature transformation generated in the rolling direction in the metal structure of the hot-rolled steel sheet. Tissue may occur.
[0115]
　Since this coarse low-temperature transformation-forming structure inhibits uniform ductility and local ductility, the completion temperature of hot rolling is preferably Ar 3 points or more and more than 850 ° C. More preferably, it is Ar 3 points or more and more than 880 ° C., and even more preferably Ar 3 points or more and more than 900 ° C. The upper limit of the completion temperature of hot rolling is not particularly limited, but is preferably 1000 ° C. or lower in terms of fine-graining the metal structure of the hot-rolled steel sheet.
[0116]
　When the hot rolling consists of rough rolling and finish rolling, the rough rolled material may be heated between the rough rolling and the finish rolling in order to maintain the completion temperature of the hot rolling in the above temperature range.
[0117]
　At this time, the rough-rolled material is heated so that the rear end of the rough-rolled material has a higher temperature than the tip of the rough-rolled material, and the temperature variation over the entire length of the rough-rolled material at the start of finish rolling is 140 ° C. or less. It is preferable to suppress the temperature. This temperature suppression improves the uniformity of characteristics in the coil around which the hot-rolled steel sheet is wound.
[0118]
　The rough-rolled material may be heated by using a known means. For example, a solenoid type induction heating device is provided between the rough rolling mill and the finish rolling mill, and heating by the solenoid type induction heating device is performed based on the temperature distribution in the longitudinal direction of the rough rolled material on the upstream side of the induction heating device. The amount of temperature rise may be controlled.
[0119]
　The conditions from the end of hot rolling to the start of winding may be normal conditions, but the winding temperature should be 600 ° C or higher in order to improve the cold rollability of the hot-rolled steel sheet by softening the hot-rolled steel sheet. preferable. The winding temperature is more preferably 640 ° C. or higher, and even more preferably 680 ° C. or higher. If the take-up temperature is too high, the pickling property of the hot-rolled steel sheet is lowered. Therefore, the take-up temperature is preferably 750 ° C. or lower, more preferably less than 720 ° C.
[0120]
　(Cold rolling)
　The conditions for cold rolling are not limited to specific conditions. Prior to cold rolling, the hot-rolled steel sheet may be descaled by pickling or the like. In order to homogenize the metallographic structure after annealing and further improve local ductility, the reduction ratio of cold rolling is preferably 40% or more. If the rolling ratio is too high, the rolling load increases and rolling becomes difficult. Therefore, the rolling ratio is preferably less than 70%, more preferably less than 60%.
[0121]
　(Annealing) The
　material steel sheet is annealed by heating it to a temperature exceeding one Ac point. Ac 1 point is the temperature at which austenite begins to form in the metallographic structure when the material steel sheet is heated.
[0122]
　When the heating temperature is Ac 1 point or less, austenite is not generated, retained austenite is not obtained in the metal structure of the steel sheet of the present invention, and the uniform ductility is lowered. Therefore, the heating temperature is preferably more than Ac 1 point. More preferably, it is above (Ac 1 + 30) ° C.
[0123]
　In order to make the metal structure of the steel sheet uniform and further improve the local ductility, the heating temperature is preferably (Ac 3 points-40) ° C. or higher. More preferably, it is more than 3 points of Ac . Ac 3 points are the temperatures at which ferrite disappears in the metallographic structure when the material steel sheet is heated.
[0124]
　When the heating temperature is too high, austenite becomes coarse, so local ductility is impaired, the heating temperature (Ac 3 point +100) ° C. or less are preferred, (Ac 3 point +50) ° C. or less is more preferable.
[0125]
　The holding time at the heating temperature is not particularly limited, but is preferably 10 seconds or more in order to make the metal structure of the material steel sheet uniform. 240 seconds or less is preferable from the viewpoint of suppressing the coarsening of austenite.
[0126]
　After annealing, the material steel sheet is cooled to a temperature range of 500 ° C. or lower without maintaining an isothermal temperature in the middle, with an average cooling rate of 2 ° C./sec or more and less than 100 ° C./sec in the temperature range of 650 to 500 ° C.
[0127]
　The cooling temperature range that defines the average cooling rate is the temperature range of 650 to 500 ° C. Since ferrite and pearlite precipitate in this temperature range, it is necessary to control the cooling rate in order to adjust the amount of precipitation and secure the required mechanical properties.
[0128]
　If the average cooling rate in the temperature range of 650 to 500 ° C. is less than 2 ° C./sec, polygonal ferrite and pearlite are excessively generated, and the yield strength and tensile strength are lowered. Therefore, the average cooling rate in the above temperature range is 2 ° C./sec or higher is preferable. It is more preferably 4 ° C./sec or higher, and even more preferably 10 ° C./sec or higher.
[0129]
　On the other hand, if the average cooling rate in the temperature range of 650 to 500 ° C. is 100 ° C./sec or more, the accuracy of the shape and dimensions of the steel sheet is lowered. Therefore, the average cooling rate in the above temperature range is preferably less than 100 ° C./sec. .. More preferably, it is 30 ° C./sec or less.
[0130]
　The material steel sheet is cooled to 500 ° C. or lower at an average cooling rate of 2 ° C./sec or more and less than 100 ° C./sec in a temperature range of 650 to 500 ° C. The cooling conditions after cooling to 500 ° C. or lower are not particularly limited, but in the metal structure after plating, the volume% of retained austenite and the amount of C of retained austenite are adjusted to improve uniform ductility and local ductility, and yield strength. It is preferable to hold the material steel sheet in a temperature range of 500 ° C. or lower and 460 ° C. or higher for 4 to 45 seconds in order to increase the temperature.
[0131]
　(Hot-dip galvanizing) The
　material steel sheet is hot-dip galvanized according to a conventional method to form a hot-dip galvanizing layer on one side or both sides of the material steel sheet. The material steel sheet may be appropriately cooled and / or heated before hot-dip galvanizing the material steel sheet.
[0132]
　The bath temperature and bath composition of the hot-dip galvanized bath may be a normal bath temperature and bath composition. The plating adhesion amount may be a normal adhesion amount. For example, the range of 20 to 80 g / m 2 per side of the material steel sheet is preferable.
[0133]
　The material steel sheet having the hot-dip galvanized layer may be heated to a required temperature to alloy the hot-dip galvanized layer. The alloying treatment may be carried out under normal conditions. For example, the alloying treatment may be performed at 470 to 560 ° C. for 5 to 60 seconds. However, the condition that the amount of Fe in the plating layer is 7% by mass or more is preferable.
[0134]
　(Cooling after
　plating or alloying ) The average cooling rate of the steel plate after plating or alloying in the temperature range from the plating temperature to 300 ° C or from the alloying temperature to 300 ° C. Cool to less than 300 ° C. at 2 ° C./sec or higher.
[0135]
　If the average cooling rate is less than 2 ° C./sec, pearlite is excessively generated, the yield strength and the tensile strength are lowered, and the amount of retained austenite is reduced, so that the uniform ductility is lowered. The cooling rate is preferably 2 ° C./sec or higher. More preferably, it is over 10 ° C./sec.
[0136]
　The upper limit of the average cooling rate is not particularly limited, but is preferably 500 ° C./sec or less from the viewpoint of economy. Although the cooling shutdown temperature is less than 300 ° C., the cooling shutdown temperature is preferably room temperature from the viewpoint of effectively performing the subsequent temper rolling.
[0137]
　(Temperature rolling)
　Before the steel sheet having the hot-dip galvanizing layer or the alloyed plating layer is subjected to the two-stage heat treatment, tempering rolling with an elongation rate of 0.10% or more is performed. By this temper rolling, the concentration of C in austenite is promoted in the subsequent two-stage heat treatment, the uniform ductility and the local ductility are improved, and the yield strength is improved.
[0138]
　If the elongation rate is less than 0.10%, the concentration of C in austenite is not promoted in the subsequent two-stage heat treatment, the uniform ductility and local ductility are not improved, and the yield strength is not improved. The elongation rate is preferably 0.10% or more. It is more preferably 0.30% or more, still more preferably 0.50% or more.
[0139]
　The upper limit of the elongation rate is not particularly specified, but if it is too high, the rolling load increases, so the elongation rate is preferably 2.00% or less. It is more preferably less than 1.50%, still more preferably less than 1.00%.
[0140]
　The tempering rolling temperature is not particularly specified, but a low temperature is preferable, and room temperature is particularly preferable, in terms of effectively imparting processing strain to austenite.
[0141]
　(Two-stage heat treatment)
　A steel sheet having a hot-dip galvanized layer or an alloyed hot-dip galvanized layer is subjected to temper rolling with an elongation rate of 0.10% or more, and then the steel sheet is heated to 300 ° C. at an average heating rate of 10. Heat at less than ° C./sec, then in a temperature range above 300 ° C. and below 600 ° C., with an average heating rate of 10 ° C./sec or above, and to a heating temperature in the temperature range above 300 ° C. and below 600 ° C. Hold for 1 second or longer.
[0142]
　By this two-stage heat treatment, the ratio of C segregation amount (atomic number / nm 2 ): [C] γgb and P segregation amount (atomic number / nm 2 ): [P] γgb at the old austenite grain boundaries : [C] γgb / [P] γgb satisfies [C] γgb / [P] γgb ≧ 4.0, C is concentrated to retained austenite, reaches 0.85% by mass or more, and martensite is tempered . As a result, uniform ductility and local ductility are improved, yield strength is improved, and low temperature impact characteristics are improved.
[0143]
　Further, when the steel plate contains B, the ratio of B segregation amount (atomic number / nm 2 ): [B] γgb and P segregation amount (atomic number / nm 2 ): [P] γgb : [B] γgb / [ P] γgb satisfies [B] γgb / [P] γgb ≧ 4.0, uniform ductility and local ductility are improved, yield strength is improved, and low temperature impact characteristics are improved.
[0144]
　Hereinafter, each process condition of the two-stage heat treatment will be described.
[0145]
　(Average heating rate up to 300 ° C: less than 10 ° C / sec) In
　the metal structure of the steel sheet after temper rolling, C is concentrated to austenite and martensite is tempered, so the metal structure exceeds 300 ° C. Heat to a temperature range of 600 ° C or lower. At this time, heating is performed at an average heating rate of less than 10 ° C./sec up to 300 ° C. This heating promotes segregation of C and B into the old austenite grain boundaries.
[0146]
　If the average heating rate up to 300 ° C. is 10 ° C./sec or higher, segregation of C and B into the old austenite grain boundaries is not promoted, so the average heating rate up to 300 ° C. is set to less than 10 ° C./sec. It is preferably 7 ° C./sec or less, more preferably 3 ° C./sec or less.
[0147]
　(Average heating rate in the temperature range of more than
　300 ° C and 600 ° C or less : 10 ° C / sec or more) By setting the average heating rate to the heating temperature in the temperature range of more than 300 ° C and 600 ° C or less to 10 ° C / sec or more. , Segregation of P to the old austenite grain boundaries can be suppressed.
[0148]
　That is, the following equation (3) can be realized by changing the average heating rate of less than 10 ° C./sec to 10 ° C./sec or more at 300 ° C. Equation (4) can be realized. The average heating rate in the temperature range of more than 300 ° C. and 600 ° C. or lower is preferably more than 20 ° C./sec.
[0149]
　　[C] γgb / [P] γgb ≧ 4.0 ・ ・ ・ (3)
　　[B] γgb / [P] γgb ≧ 4.0 ・ ・ ・ (4)
[0150]
　If the above formula is realized at the former austenite grain boundaries, the strengthening action of C and B is increased and the embrittlement action of P is suppressed at the former austenite grain boundaries, and the low temperature impact characteristics can be improved.
[0151]
　(Retention time in a temperature range of more than 300 ° C. and 600 ° C. or less: 1 second or more) After the
　above two-stage heating, the steel sheet is held at a heating temperature in a temperature range of more than 300 ° C. and 600 ° C. or less for 1 second or more. When the heating temperature is 300 ° C. or lower, the concentration of C in austenite is insufficient, the uniform ductility is not improved, and hard martensite remains, the local ductility is impaired and the yield strength is lowered. Therefore, the heating temperature is set to more than 300 ° C. It is preferably over 350 ° C., more preferably over 400 ° C.
[0152]
　On the other hand, when the heating temperature exceeds 600 ° C., the amount of retained austenite is insufficient, the uniform ductility is lowered, and the tempered martensite is excessively softened, the yield strength and the tensile strength are lowered, and the hardness is also reduced. The heating temperature is set to 600 ° C. or lower because the fresh martensite of the above is generated and the local ductility is lowered and the yield strength is lowered. It is preferably 550 ° C. or lower, more preferably 500 ° C. or lower.
[0153]
　If the heating holding time is less than 1 second, the concentration of C in austenite becomes insufficient and the uniform ductility is not improved, and hard martensite remains, the local ductility is lowered and the yield strength is increased. Since it decreases, the heating holding time is set to 1 second or more. It is preferably 5 seconds or longer, more preferably 15 seconds or longer.
[0154]
　If the heat retention time is too long, the amount of retained austenite will decrease, the uniform ductility will decrease, and the tempered martensite will become excessively soft, resulting in a decrease in yield strength and tensile strength, and hard fresh martensite. The heating retention time is preferably 96 hours or less because sites are generated and the local ductility is lowered and the yield strength is lowered. It is more preferably 48 hours or less, still more preferably 24 hours or less.
[0155]
　The heating holding time is appropriately adjusted according to the heating temperature. For example, when the heating temperature is 400 to 600 ° C., the heating holding time is preferably 20 minutes or less. It is more preferably 6 minutes or less, still more preferably less than 3 minutes. From the viewpoint of productivity, a heating temperature of more than 400 ° C. and a heating holding time of 20 minutes or less are preferable.
[0156]
　After the steel sheet is heat-treated in two stages, the steel sheet may be temper-rolled in order to correct the flatness of the steel sheet, or the steel sheet may be coated with an oiling or lubricating film. good.
[0157]
　The thickness of the steel sheet of the present invention is not particularly limited to a specific range, but the effect of the two-stage heat treatment is remarkably exhibited in a versatile steel sheet having a thickness of 0.8 to 2.3 mm.
Example
[0158]
　Next, an example of the present invention will be described. The conditions in the examples are one condition example adopted for confirming the feasibility and effect of the present invention, and the present invention is described in this one condition example. It is not limited. The present invention can adopt various conditions as long as the gist of the present invention is not deviated and the object of the present invention is achieved.
[0159]
　(Example 1)
　Molten steel was cast using a vacuum melting furnace to produce steels A to U having the component compositions shown in Table 1. The points Ac 1 and Ac 3 in Table 1 were obtained from the changes in thermal expansion when the cold-rolled steel sheets of steels A to P were heated at 2 ° C./sec. Steels A to U were heated to 1200 ° C. and held for 60 minutes, and then hot rolled under the hot rolling conditions shown in Tables 2-1 and 2-2.
[0160]
[table 1]

[0161]
　Specifically , steels A to U were rolled for 10 passes in a temperature range of 3 points or more to obtain a hot-rolled steel sheet having a thickness of 2.5 to 3.0 mm. After hot rolling, the hot-rolled steel sheet is cooled to 500 to 680 ° C. with a water spray, the cooling end temperature is set as the winding temperature, and the hot-rolled steel sheet is charged into an electric heating furnace maintained at this winding temperature. After that, the hot-rolled steel sheet was cooled to room temperature at a cooling rate of 20 ° C./hour to simulate slow cooling after winding.
[0162]
　The hot-rolled steel sheet after slow cooling is pickled to use as a base material for cold rolling, and cold-rolled at a reduction ratio of 47 to 52% to obtain a cold-rolled steel sheet (material) with a thickness of 1.2 to 1.6 mm. Steel plate). Using a hot-dip galvanizing simulator, the material steel sheet was heated to 650 ° C. at a heating rate of 10 ° C./sec, and then heated to the temperatures shown in Tables 2-1 and 2-2 at a heating rate of 2 ° C./sec. The heat was equalized for 30 to 90 seconds.
[0163]
　Then, the material steel sheet is cooled to 460 ° C. under the cooling conditions shown in Tables 2-1 and 2-2, the material steel sheet is immersed in a hot-dip galvanizing bath maintained at 460 ° C., and the material steel sheet is hot-dip galvanized. bottom. Some of the material steel sheets were hot-dip galvanized and then heated to 520 ° C. to be alloyed.
[0164]
　From the plating temperature (meaning the plating bath temperature) or the alloying temperature, the material steel sheet was secondarily cooled under the cooling conditions shown in Tables 2-1 and 2-2. In Tables 2-1 and 2-2, "RT" indicates room temperature.
[0165]
[Table 2-1]

[0166]
[Table 2-2]

[0167]
[Table 3-1]

[0168]
[Table 3-2]

[0169]
　The material steel sheet that has undergone secondary cooling is subjected to temper rolling with an elongation rate of 0.50%, and then heat-treated under the heat treatment conditions shown in Tables 3-1 and 3-2 to obtain a hot-dip galvanized steel sheet or alloyed melt. A galvanized steel sheet (hereinafter, hot-dip galvanized steel sheet and alloyed hot-dip galvanized steel sheet are collectively referred to as "plated steel sheet") was obtained.
[0170]
　When the stop temperature of the secondary cooling is set to 100 ° C., heat treatment rolling is performed without cooling to room temperature after the stop of secondary cooling, and then, without cooling to room temperature, it is shown in Tables 3-1 and 3-2. The heat treatment was performed under the heat treatment conditions. For some material steel sheets, temper rolling or heat treatment was omitted. "-" In the column of "heat treatment conditions" in Tables 3-1 and 3-2 indicates that the heat treatment was not performed.
[0171]
　A test piece for XRD measurement was collected from the plated steel sheet, and the rolled surface of the test piece was chemically polished from the boundary between the base steel sheet and the plating layer to a depth of 1/4 of the plate thickness of the base steel sheet. An X-ray diffraction test was performed on this rolled surface to measure the volume fraction of retained austenite and the amount of C of retained austenite.
[0172]
　Specifically, Mo-Kα rays are incident on the test piece to obtain the integrated intensities of the α-phase (200) and (211) diffraction peaks and the γ-phase (200), (220) and (311) diffraction peaks. The integrated intensity was measured to determine the volume fraction of retained austenite.
[0173]
　Further, the lattice constant (aγ) of austenite is obtained from the positions of the diffraction peaks of the γ phase (200), (220), and (311) by injecting Fe-Kα rays, and the C amount (Cγ) of retained austenite is determined. It was calculated using the relational expression of aγ (Å) = 3.578 + 0.033 × Cγ (mass%).
[0174]
　Further, after collecting a test piece for EBSP measurement from the plated steel sheet and electrolytically polishing the vertical cross section parallel to the rolling direction, the test piece is located at a depth of 1/4 of the thickness of the base steel sheet from the boundary between the base steel plate and the plating layer. The metallographic structure was observed, and the former austenite grain boundary was confirmed. Subsequently, FIB was used to prepare a needle sample for 3DAP measurement containing the old austenite grain boundaries.
[0175]
　Using 3DAP, the concentration distribution of C, B and P atoms was measured, and the C segregation amount ([C] γgb ), B segregation amount ([B] γgb ) and P segregation amount ([P ] γgb ) at the former austenite grain boundaries were measured. ] Γgb ) was calculated , and [C] γgb / [P] γgb and [B] γgb / [P] γgb were calculated.
[0176]
　Further, a test piece for SEM observation is collected from the plated steel sheet, and after polishing the vertical cross section parallel to the rolling direction of the test piece, nital corrosion and repeller corrosion are performed on the vertical cross section to perform boundary between the base steel sheet and the plating layer. The metallographic structure at a depth of 1/4 of the thickness of the base steel sheet was observed. By image processing, the volume fractions of tempered martensite, polygonal ferrite, fresh martensite, and the residual structure were measured.
[0177]
　The volume fraction of fresh martensite was obtained by subtracting the volume fraction of retained austenite measured by the above XRD measurement from the total volume fraction of retained austenite and fresh martensite measured by reperer corrosion.
[0178]
　For the yield stress (YS), tensile strength (TS), and uniform elongation (UEl), a JIS No. 5 tensile test piece is taken from the plated steel sheet along the direction perpendicular to the rolling direction, and the test piece is subjected to a tensile test. I asked for it.
[0179]
　The tensile speed was 1 mm / min until the yield point was reached, and 10 mm / min thereafter. The yield ratio (YR) was determined by dividing YS by TS. For the total elongation (TEl) and the local elongation (LEl), a tensile test was performed on the JIS No. 5 tensile test piece collected along the direction perpendicular to the rolling direction, and the measured value of the total elongation (TEL 0 ) and the measured value of the uniform elongation were obtained. Using (UEl), a conversion value corresponding to the case of a plate thickness of 1.2 mm was obtained based on the above formulas (1) and (2).
[0180]
　In addition, V-notch subsize Charpy impact test pieces were collected from the plated steel sheet along the direction orthogonal to the rolling direction, and 8 pieces were collected when the plate thickness was 1.2 mm, and 6 pieces were collected when the plate thickness was 1.6 mm. Was laminated and fastened with screws, and a Charpy impact test was conducted using this test piece. The Charpy impact value when the test temperature was -60 ° C was defined as IV LT , and the Charpy impact value when the test temperature was 40 ° C was defined as IV HT, and the value of IV LT / IV HT was determined.
[0181]
　If YR is 0.59 or more, TS × UEl is 10000 MPa ·% or more, and TS × LEl is 5000 MPa ·% or more, it is judged that the mechanical properties are good. Further, if IV LT / IV HT was more than 0.50, it was judged that the low temperature impact characteristics were good.
[0182]
　Tables 4-1 and 4-2 show the results of observing the metallographic structure of the plated steel sheet, and Tables 5-1 and 5-2 show the results of evaluating the mechanical properties and low-temperature impact characteristics of the plated steel sheet.
[0183]
[Table 4-1]

[0184]
[Table 4-2]

[0185]
[Table 5-1]

[0186]
[Table 5-2]

[0187]
　In Tables 4-1 and 4-2, the column of "C amount (mass%) of retained austenite", the column of "[C] γgb / [P] γgb ", and the column of "[B] γgb / [P] γgb ". In the column, "-" indicates that the C amount of retained austenite, [C] γgb / [P] γgb , and [B] γgb / [P] γgb were not measured.
[0188]
　In Tables 1 to 5, the underlined numerical values ​​or symbols mean that they are outside the scope of the present invention.
[0189]
　In each of the invention examples, TS × UEl was 10,000 or more and TS × LEl was 5000 or more, showing good uniform ductility and local ductility. Moreover, YR showed a high value of 0.59 or more. In addition, IV LT / IV HT showed a high value of 0.51 or more.
[0190]
　The test results for Comparative Examples in which the component composition or process conditions were not appropriate were inferior in any or all of the yield ratio, uniform ductility, local ductility, and low temperature impact characteristics.
[0191]
　Specifically, steels C, E, and N having a composition within the range of the present invention were used, but in test numbers 9, 22, and 40 in which temper rolling was not performed, the amount of C of retained austenite was high. Low, TS × UEl and TS × LEl are low. In test numbers 10, 23, and 41, where the heat treatment temperature was too low, the tempered martensite volume fraction, the amount of retained austenite C, and [C] γgb / [P] γgb were low, and YR, TS × UEl, TS × LEl, And IV LT / IV HT is low.
[0192]
　In the test using steel C (test number 14), since no heat treatment was performed, the tempered martensite volume fraction, the amount of retained austenite C, and [C] γgb / [P] γgb were low, and YR, TS × UEl, TS × LEl and IV LT / IV HT are low.
[0193]
　In the tests using steel C and steel F (test numbers 11 and 28), the heat treatment temperature was too high, so that the volume fraction of retained austenite, the amount of C of retained austenite, and [C] γgb / [P] γgb were low, and YR, TS × UEl, TS × LEl, and IV LT / IV HT are low.
[0194]
　Steel C having a component composition within the range of the present invention was used, but in Test No. 19, where the soaking temperature was too low in the annealing step, the retained austenite volume fraction and the tempered martensite volume fraction were low, and TS × UEl was low.
[0195]
　In the tests using steels A and C (test numbers 4 and 18), the average cooling rate in the temperature range of 650 to 500 ° C. was too low in the first cooling step. Therefore, in test number 4, the retained austenite volume fraction and tempered martensite were used. The site volume fraction is low, and YR and TS × LEl are low. In test number 18, the retained austenite volume fraction is low, and YR, TS × UEl, and TS × LEl are low.
[0196]
　Steel C having a component composition within the range of the present invention was used, but in Test No. 12, the average cooling rate (secondary cooling rate) in the temperature range of alloying temperature to 300 ° C. was too low in the second cooling step. The volume fraction of retained austenite and the amount of C of retained austenite are low, and TS × UEl and TS × LEl are low.
[0197]
　In the tests using steels C, E, and N (test numbers 17, 26, and 39), the average heating rate in the temperature range up to 300 ° C. in the two-stage heat treatment step was too high, so [C] γgb / [P] γgb is low and IV LT / IV HT is low.
[0198]
　In the tests using steels A, C, E, and G (test numbers 2, 16, 24, and 31), the average heating rate in the temperature range exceeding 300 ° C. in the two-stage heat treatment step was too low. [C] γgb / [P] γgb is low, and IV LT / IV HT is low.
[0199]
　In Test No. 5 using steel B, since the amount of Si in the steel is small, the volume fraction of retained austenite and the amount of C of retained austenite are low, and YR, TS × UEl, and TS × LEl are low. In test number 20 using steel D, since the amount of Mn in the steel is small, the retained austenite volume fraction is low, and YR and TS × LEl are low.
Industrial applicability
[0200]
　As described above, according to the present invention, both uniform ductility and local ductility are good, press formability is excellent, yield strength and tensile strength are high, local ductility is good, and impact absorption is excellent. Moreover, it is possible to provide a hot-dip zinc-plated steel sheet and an alloyed hot-dip zinc-plated steel sheet which are also excellent in low-temperature impact characteristics.
[0201]
　The hot-dip galvanized steel sheet and the alloyed hot-dip galvanized steel sheet of the present invention are optimal steel sheets as material steel sheets for structural parts of automobile bodies such as members and pillars, and other mechanical structural parts. Therefore, the present invention is highly applicable in the automobile industry and the machine parts manufacturing industry.
The scope of the claims
[Claim 1]
　A hot-dip galvanized steel sheet having a hot-dip galvanized layer on the surface of the
　steel sheet, wherein the component composition of the steel sheet is, in terms of mass%,
　　C: 0.03 to 0.70%,
　　Si: 0.25 to 2.50%. ,
　　Mn: 1.00 to 5.00%,
　　P: 0.0005 to 0.100%,
　　S: 0.010% or less,
　　sol. Al: 0.001 to 2.500%,
　　N: 0.020% or less,
　　B: 0 to 0.0200%,
　　Ti: 0 to 0.30%,
　　Nb: 0 to 0.30%,
　　V: 0 to 0.30%,
　　Cr: 0 to 2.00%,
　　Mo: 0 to 2.00%,
　　Cu: 0 to 2.00%,
　　Ni: 0 to 2.00%,
　　Ca: 0 to 0.010%,
　　It contains Mg: 0 to 0.010%,
　　REM: 0 to 0.10%, and
　　Bi: 0 to 0.050%
, and the balance is Fe and unavoidable impurities.
　The metallographic structure of the steel sheet contains retained austenite: more than 5.0% and tempered martensite: more than 5.0% by volume, and the retained austenite contains C: 0.85% by mass or more. ,
　The ratio of C segregation amount (number of atoms / nm 2 ): [C] γgb and P segregation amount (number of atoms / nm 2 ): [P] γgb at the former austenite grain boundaries in the metal structure of the steel sheet : [C] ] Γgb / [P] A hot-dip zinc-plated steel sheet having a γgb of 4.0 or more
.
[Claim 2]
　The composition of the steel plate is, in
　　terms of mass%, B: 0.0002 to 0.0200%,
　　Ti: 0.001 to 0.30%,
　　Nb: 0.001 to 0.30%,
　　V: 0.001 to 0.001 to 0.30%,
　　Cr: 0.001-2.00%,
　　Mo: 0.001-2.00%,
　　Cu: 0.001-2.00%,
　　Ni: 0.001-2.00%,
　　Ca :
　　0.0001 ~
　　0.010%, Mg: 0.0001 ~ 0.010%, REM: 0.0001 ~ 0.10%, and
　　Bi: 0.0001 ~ 0.050%
containing one or more The hot-dip zinc-plated steel sheet according to claim 1, wherein the hot-dip zinc-plated steel sheet is characterized by the above.
[Claim 3]
　In the composition of the steel sheet, the content of B is 0.0002% or more, and the
　segregation amount of B at the former austenite grain boundary in the metal structure of the steel sheet (atomic number / nm 2 ): [B] γgb and P. The hot-dip galvanized steel sheet according to claim 1 or 2, wherein the segregation amount (number of atoms / nm 2 ): [P] γgb ratio: [B] γgb / [P] γgb is 4.0 or more.
..
[Claim 4]
　The hot-dip galvanized steel sheet according to any one of claims 1 to 3, wherein the hot-dip galvanized layer is an alloyed hot-dip galvanized steel sheet.
[Claim 5]
　The method for producing a hot-dip zinc-plated steel sheet according to any one of claims 1 to 3, wherein
　the material steel sheet having the component composition according to claim 1 or 2 is heated to a temperature range exceeding one Ac point.
　After the annealing step and the annealing step, the material steel sheet is cooled to 500 ° C. or lower with an average cooling rate of 2 ° C./sec or more and less than 100 ° C./sec in the temperature range of 650 to 500 ° C. ,
　after the first cooling step, a plating step of performing molten zinc plating steel plate,
　after the plating step, the steel sheet, the average cooling rate in the temperature range of the plating temperature of ~ 300 ° C. as above 2 ° C. / sec, 300 ° C. After the second cooling step and
　the second cooling step of cooling to less than , the material steel sheet is subjected to the annealing rolling step of annealing the material steel sheet with an elongation rate of 0.10% or more, and
　after the annealing rolling process, the material steel sheet is used. , The average heating rate in the temperature range up to 300 ° C is less than 10 ° C / sec and heated to 300 ° C, and then the average heating rate in the temperature range over 300 ° C is more than 10 ° C / sec and the average heating rate is over 300 ° C and 600 ° C. A
method for producing a hot-dip zinc-plated steel sheet, which comprises a two-stage heat treatment step of heating to the following temperature range and performing a heat treatment for holding the heated temperature for 1 second or longer .
[Claim 6]
　The method for producing an alloyed hot-dip zinc-plated steel sheet according to claim 4, wherein
　the material steel sheet having the component composition according to claim 1 or 2 is annealed by heating it to a temperature range exceeding one Ac point. After the step and
　annealing step, the first cooling step and
　the first cooling step of cooling the material steel sheet to 500 ° C. or lower with an average cooling rate of 2 ° C./sec or more and less than 100 ° C./sec in the temperature range of 650 to 500 ° C. After the plating process of hot-dip zinc plating on the material steel sheet, after the plating process, the
　alloying process of
　alloying the material steel sheet, after the alloying process, the material steel sheet is alloyed at a temperature of ~ 300 ° C. After the second cooling step and
　the second cooling step of cooling to less than 300 ° C with an average cooling rate of 2 ° C./sec or more in the temperature range , the material steel sheet is annealed and rolled with an elongation rate of 0.10% or more. After the quality rolling step and the
　annealing rolling step, the material steel sheet is heated to 300 ° C. with an average heating rate of less than 10 ° C./sec in the temperature range up to 300 ° C., and then in a temperature range exceeding 300 ° C.
Alloying characterized by comprising a two-stage heat treatment step in which an average heating rate is set to more than 10 ° C./sec, heating is performed in a temperature range of more than 300 ° C. and 600 ° C. or lower, and heat treatment is performed to maintain the heating temperature for 1 second or longer . A method for manufacturing a hot-dip zinc-plated steel sheet.