Steel plate, member and manufacturing method thereof

Abstract

The present invention provides a steel plate and a member having a tensile strength TS of 780 MPa or more and less than 1,180 MPa, high yield stress YS, and excellent total elongation, local elongation, bendability, and energy absorption characteristics, and a manufacturing method thereof. The base steel sheet has a specified chemical composition, the surface layer of the base steel sheet has a soft surface layer of 20 μm or more, and in the steel structure at the 1 / 4 position of the sheet thickness of the base steel sheet, ferrite, bainitic ferrite, tempered martensite, retained austenite, and fresh martensite are within specified ranges, and the average grain size of the hard second phase, including retained austenite and fresh martensite present in the bainitic ferrite, is 3.0 μm or less.

Description

[Technical field]

[0001] The present invention relates to a steel plate having high strength and excellent crashworthiness, a member, and a method for producing the same. The steel plate of the present invention is preferably usable mainly as a steel plate for automobiles. [Background technology]

[0002] From the viewpoint of protecting the global environment, it is always an important issue in the automobile industry to reduce the weight of automobile bodies while maintaining their strength, and to improve the fuel efficiency of automobiles in order to reduce CO2 emissions. In order to reduce the weight of automobile bodies while maintaining their strength, it is effective to make the steel plates used as materials for automobile parts thinner by increasing their strength. On the other hand, automobile parts made of steel plates are premised on ensuring the safety of people inside the vehicle in the event of a collision. Therefore, high-strength steel plates used as materials for automobile parts are required to have excellent collision properties in addition to the desired strength.

[0003] In recent years, the use of high-strength steel plates with a tensile strength TS of 780 MPa or more in automobile bodies is expanding. From the perspective of collision characteristics, automobile parts are broadly divided into non-deformable members such as pillars and bumpers and energy-absorbing members such as side members, and each is required to have the necessary collision characteristics to ensure the safety of passengers in the event of a collision while the automobile is traveling. In non-deformable members, high-strength steel plates with a tensile strength (hereinafter also simply referred to as TS) of 780 MPa or more have already been put into practical use. However, when applied to energy-absorbing members, high-strength steel plates of 780 MPa or more are prone to component breakage at the point where they have been subjected to primary processing by forming during a collision, and there is a problem that they cannot stably exhibit their collision energy absorption ability, so materials of 590 MPa or less are mainly used. Therefore, there is room to contribute to environmental conservation by reducing the weight while ensuring safety during a collision by suppressing component breakage during a collision and stably exhibiting high absorption energy. For the above reasons, it is required to apply high-strength steel plates with a TS of 780 MPa or more, which have excellent collision characteristics, to energy-absorbing members.

[0004] As an example of a steel sheet that can be used as a material for such automobile parts, Patent Document 1 discloses a high-strength hot-dip galvanized steel sheet having a component composition containing, by mass%, 0.05 to 0.3% C, 0.01 to 2.5% Si, 0.5 to 3.5% Mn, 0.003 to 0.100%, 0.02% or less S, 0.010 to 1.5% Al, 0.007% or less N, with the balance being Fe and unavoidable impurities, and having a microstructure containing, by area ratio, 20 to 87% ferrite, 3 to 10% martensite and retained austenite in total, and 10 to 60% tempered martensite, and having excellent workability with a high TS-El balance, excellent stretch flangeability, and low YR.

[0005] Patent Document 2 also describes a high-strength hot-dip galvanized steel sheet having a thickness of 0.6 to 5.0 mm and a plating layer on the surface of the steel sheet, the steel sheet structure containing a ferrite phase with a volume fraction of 40 to 90% and a retained austenite phase with a volume fraction of 3 to 25%, the retained austenite phase having a solute carbon content of 0.70 to 1.00%, an average particle size of 2.0 μm or less, an average distance between particles of 0.1 to 5.0 μm, a thickness of a decarburized layer in the surface layer of the steel sheet of 0.01 to 10.0 μm, an average particle size of oxides contained in the surface layer of the steel sheet of 30 to 120 nm, and an average density of 1.0 × 10 12 pieces / m 2 The above also discloses a high-strength hot-dip galvanized steel sheet that has an average work hardening coefficient (n value) of 0.080 or more at 3 to 7% plastic deformation, has high ductility while maintaining a high strength of 900 MPa or more in maximum tensile strength, and has excellent mechanical cutting properties.

[0006] Patent Document 3 discloses a high-strength hot-dip galvanized steel sheet having excellent delayed fracture resistance, characterized in that it has, by volume fraction, 40 to 90% of a ferrite phase and 5% or less of a retained austenite phase, the proportion of unrecrystallized ferrite in the entire ferrite phase being 50% or less, the grain size ratio, which is the value obtained by dividing the average grain size in the rolling direction of the ferrite phase by the average grain size in the sheet width direction, is 0.75 to 1.33, the length ratio, which is the value obtained by dividing the average length in the rolling direction of hard structures dispersed in an island shape by the average length in the sheet width direction, is 0.75 to 1.33, and the average aspect ratio of inclusions is 5.0 or less. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Patent No. 5256689 [Patent Document 2] Patent No. 5354135 [Patent Document 3] Patent No. 5352793 Summary of the Invention [Problem to be solved by the invention]

[0008] However, although Patent Document 1 discloses a high-strength hot-dip galvanized steel sheet having improved ductility, which is the press formability inside the steel sheet, and stretch flangeability, which is the press formability at the ends of the steel sheet, it cannot be said that the bendability and energy absorption characteristics are sufficient.

[0009] Patent Document 2 discloses a high-strength hot-dip galvanized steel sheet in which the ductility is improved by the formation of retained austenite inside the steel sheet and the mechanical cutting characteristics are improved by the formation of a decarburized layer on the surface of the steel sheet. However, it cannot be said that the bendability and energy absorption characteristics are sufficient.

[0010] Patent Document 3 discloses a high-strength hot-dip galvanized steel sheet in which the main structure inside the steel sheet is soft ferrite and the amount of unrecrystallized ferrite is limited to a small amount to improve ductility, and delayed fracture resistance and its anisotropy are improved by forming a decarburized layer on the surface of the steel sheet. However, it cannot be said that the bendability and energy absorption characteristics are sufficient.

[0011] From the above, it cannot be said that the steel sheets disclosed in Patent Documents 1 to 3 have a TS of 780 MPa or more and less than 1180 MPa, a high YS, excellent total elongation and local elongation, excellent bendability, and excellent energy absorption properties.

[0012] The present invention has been made in view of the above circumstances, and aims to provide a steel plate, a member and a manufacturing method thereof, which have a tensile strength TS of 780 MPa or more and less than 1,180 MPa, high yield stress YS, excellent total elongation and local elongation, excellent bendability and excellent energy absorption characteristics.

[0013] The steel sheet referred to here also includes zinc-plated steel sheet, and zinc-plated steel sheet refers to hot-dip galvanized steel sheet (hereinafter also referred to as GI), alloyed hot-dip galvanized steel sheet (hereinafter also referred to as GA), or electro-galvanized steel sheet (hereinafter also referred to as EG).

[0014] Here, the tensile strength TS is measured by a tensile test in accordance with JIS Z 2241 (2011).

[0015] In addition, having a high yield stress YS, excellent total elongation (T-El) and local elongation (L-El), excellent bendability, and excellent energy absorption characteristics means that the following is satisfied.

[0016] A high yield stress YS means that the YS measured in a tensile test conforming to JIS Z 2241 (2011) satisfies the following formula (A) or (B) depending on the TS measured in the tensile test. (A) When 780MPa≦TS<980MPa, 400MPa≦YS≦650MPa (B) When 980MPa≦TS, 600MPa≦YS≦980MPa

[0017] Total elongation is correlated with stretch formability, and excellent total elongation means that the total elongation (T-El) measured in a tensile test conforming to JIS Z 2241 (2011) satisfies the following formula (A) or (B) depending on the TS measured in the tensile test. (A) When 780MPa≦TS<980MPa, 17.0%≦T-El (B) When 980MPa≦TS, 11.0%≦T-El

[0018] Local elongation is correlated with stretch flange formability, and excellent local elongation means that the local elongation (L-El) measured in a tensile test conforming to JIS Z 2241 (2011) satisfies the following formula (A) or (B) depending on the TS measured in the tensile test. (A) When 780MPa≦TS<980MPa, 7.0%≦L-El (B) When 980MPa≦TS, 5.0%≦L-El

[0019] "Excellent bendability" means that the outer bend angle (limit bend angle (α)) at the center of a plate-shaped test piece when the load F from the push-bending jig is at its maximum in a bending test based on the VDA standard (VDA238-100) defined by the German Association of the Automotive Industry is 80° or greater.

[0020] Excellent energy absorption properties are defined as the time when the load reaches its maximum value after deformation begins, in the load F (N)-stroke X (mm) curve obtained by performing a bending test on a steel plate in accordance with the VDA standard (VDA238-100) stipulated by the German Association of the Automotive Industry. max This means that the integral value AE (N·mm) of the load F (N) up to the load satisfies the following formula (1), and preferably satisfies the following formula (2).

[0021]

number

[0022]

number

[0023] In the above formula (1) and formula (2), F: Load (N) X: Stroke (mm) X max : Stroke (mm) at which the load reaches its maximum value AE: Stroke X at which the load reaches its maximum value after deformation begins max Integral value of load F up to (N mm) [Means for solving the problem]

[0024] As a result of extensive research into solving the above problems, the present inventors have discovered the following. The composition of the base steel sheet is appropriately adjusted, and the steel has a steel structure in which, in terms of area ratio, ferrite is 20.0% or more and 70.0% or less, the total of tempered martensite and bainitic ferrite is 20.0% or more and 70.0% or less, retained austenite is 3.0% or more and 20.0% or less, and fresh martensite is 20.0% or less (including 0.0%), the area ratio of bainitic ferrite among the total area ratio of bainitic ferrite and tempered martensite is 40% or more, and the average grain size of the hard second phase including retained austenite and fresh martensite present in the bainitic ferrite is 3.0 μm or less, and in the base steel sheet, the total area ratio of structures other than ferrite is S A / 2 or less (S A When the area where the soft surface layer is the area where the area is the total area ratio of structures other than ferrite in the cross section at 1 / 4 of the plate thickness) is considered as the soft surface layer, the base steel plate has a soft surface layer of 20 μm or more in the plate thickness direction from the surface, and has a tensile strength of 780 MPa or more and less than 1180 MPa. It was found that this makes it possible to obtain a steel plate with high YS, excellent press formability (bendability and ductility of the steel plate), and excellent energy absorption properties during bending deformation.

[0025] The present invention was made based on these findings, and the gist of the present invention is as follows. [1] A steel plate having a base steel plate, the base steel plate comprising: In mass%, C: 0.05-0.30%, Si: 0.10-2.00%, Mn: 1.00-3.50%, P: 0.001 to 0.100%, S: 0.0001~0.0200%, Sol.Al: 0.005~2.000% and N: 0.010% or less and the balance being Fe and unavoidable impurities; At a 1 / 4 position of the thickness of the base steel plate, In terms of area ratio, Ferrite: 20.0% or more and 70.0% or less; The sum of bainitic ferrite and tempered martensite: 20.0% or more and 70.0% or less; Retained austenite: 3.0% or more and 20.0% or less; Fresh martensite: 20.0% or less (including 0.0%), The ratio of the area ratio of bainitic ferrite to the total area ratio of the bainitic ferrite and tempered martensite is 40% or more, a steel structure in which a hard second phase, which is present in the bainitic ferrite and includes the retained austenite and the fresh martensite, has an average grain size of 3.0 μm or less; having Furthermore, the layer is formed to a thickness of 20 μm or more from the surface of the base steel sheet in the thickness direction. The steel structure has a surface soft layer in which the total area ratio of structures other than ferrite in the steel structure is 1 / 2 or less at a 1 / 4 position of the plate thickness, A steel plate having a tensile strength of 780 MPa or more but less than 1180 MPa. [2] The composition further includes, in mass%, Nb: 0.200% or less, Ti: 0.200% or less, V: 0.200% or less, B: 0.0100% or less, Cr: 1.000% or less, Ni: 1.000% or less, Mo: 1.000% or less Sb: 0.200% or less, Sn: 0.200% or less, Cu: 1.000% or less, Ta: 0.100% or less, W: 0.500% or less, Mg: 0.0200% or less, Zn: 0.0200% or less, Co:0.0200% or less, Zr: 0.1000% or less, Ca: 0.0200% or less, Se: 0.0200% or less, Te: 0.0200% or less, Ge: 0.0200% or less, As: 0.0500% or less, Sr: 0.0200% or less, Cs: 0.0200% or less, Hf: 0.0200% or less, Pb: 0.0200% or less, Bi: 0.0200% or less and REM: 0.0200% or less The steel sheet according to the above [1], comprising at least one selected from the following: [3] The steel sheet according to [1] or [2] above, having a zinc-plated layer on one or both sides of the base steel sheet. [4] A member made using the steel plate according to any one of [1] to [3] above. [5] A hot rolling process in which a steel slab having the composition described in [1] or [2] is hot rolled to obtain a hot rolled steel sheet; A pickling process for pickling the hot-rolled steel sheet; An annealing process in which the steel sheet after the pickling process is heated and annealed under the conditions of an annealing temperature of 720°C to 860°C, a holding time of 20 seconds or more, and a dew point of -10°C or more; A rapid heating process of rapidly heating the annealing temperature to the annealing temperature + 10 ° C. or higher at a temperature increase rate of 10 ° C. / sec or higher; A first cooling step of cooling the steel plate after the rapid heating step to a first cooling stop temperature of 400°C or more and 600°C or less; A holding step of holding the temperature in the range of 400° C. to 600° C. for a holding time of less than 300 seconds; A second cooling step of cooling the steel plate after the holding step to a second cooling stop temperature of 100°C or more and 300°C or less; A reheating and holding process of heating the steel sheet after the second cooling process to a tempering temperature of 460° C. or less and holding the steel sheet in the temperature range for a tempering time of 10 seconds or more and 2000 seconds or less. Or even The method for producing a steel sheet includes a cold rolling step of cold rolling the steel sheet after the pickling step and before the annealing step at a rolling reduction of 20% or more and 80% or less to obtain a cold-rolled steel sheet. [6] The method for producing a steel sheet according to [5] above, wherein a threading speed LS (m / min) of the steel sheet in the annealing step satisfies the following formula (1): 3.0 / (t×W×4.716×10 ―4 ) <LS<390.0 / (t×W×4.716×10 ―4 )...Equation (1) Where: t: Plate thickness (mm) after pickling process and before annealing process, W: Sheet width after pickling and before annealing (mm) It is. [7] A hot-dip galvanizing process is included which performs a galvanizing process on a surface of the steel sheet to form a galvanized layer on the surface of the steel sheet after the holding process and before the second cooling process; Alternatively, the method for producing a steel sheet according to [5] or [6] above further comprises an alloying treatment step of subjecting the steel sheet on which the zinc plating layer has been formed to an alloying treatment. [8] The method for producing a steel sheet according to [5] or [6], further comprising, after the reheating and holding step, an electrogalvanizing step of immersing the steel sheet in an electrogalvanizing bath to form a zinc plating layer on the steel sheet. [9] A method for producing a component, comprising the step of subjecting the steel plate according to any one of [1] to [3] above to at least one of forming and joining to produce a component. Effect of the Invention

[0026] According to the present invention, it is possible to obtain a steel sheet having a tensile strength TS of 780 MPa or more and less than 1180 MPa, a high yield stress YS, and excellent total elongation, local elongation, bendability, and energy absorption properties. A member obtained by subjecting the steel sheet of the present invention to forming processing, welding, etc. can be suitably used as an energy absorption member in the automotive field. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0027] The present invention will be described in detail below.

[0028] [1. Steel plate] The steel sheet of the present invention is a steel sheet having a base steel sheet, the base steel sheet having a composition containing, by mass%, C: 0.05-0.30%, Si: 0.10-2.00%, Mn: 1.00-3.50%, P: 0.001-0.100%, S: 0.0001-0.0200%, sol. Al: 0.005-2.000%, and N: 0.010% or less, with the balance being Fe and unavoidable impurities, and having, at a 1 / 4 position of the sheet thickness of the base steel sheet, an area ratio of ferrite: 20.0% or more and 70.0% or less, a total of bainitic ferrite and tempered martensite: 20.0% or more and 70.0% or less, and retained austenite: 3.0% or more and 20.0% or less. and a steel structure in which fresh martensite is 20.0% or less (including 0.0%), the area ratio of bainitic ferrite accounts for 40% or more of the total area ratio of bainitic ferrite and tempered martensite, and the average grain size of a hard second phase including retained austenite and fresh martensite is 3.0 μm or less, the steel structure further has a soft surface layer formed with a thickness of 20 μm or more from the surface of the base steel sheet in the sheet thickness direction, and the total area ratio of structures other than ferrite in the steel structure is 1 / 2 or less at the 1 / 4 position of the sheet thickness, and the tensile strength is 780 MPa or more and less than 1,180 MPa. The steel sheet of the present invention has a tensile strength TS (hereinafter, tensile strength TS may be simply referred to as TS) of 780 MPa or more and less than 1180 MPa, and also has a high yield stress YS (hereinafter, yield stress YS may be simply referred to as YS), and is excellent in total elongation, local elongation, bendability and energy absorption characteristics. The steel sheet may have a zinc plating layer as the outermost layer on one or both sides of the steel sheet. The steel sheet having a zinc plating layer may be a zinc-plated steel sheet. More specifically, a steel sheet having a hot-dip galvanized layer among the zinc-coated layers may be referred to as a hot-dip galvanized steel sheet. Furthermore, a steel sheet having a galvannealed layer among the galvanized layers may be referred to as a galvannealed steel sheet. Furthermore, among the zinc-plated layers, the steel sheet having an electrolytic zinc-plated layer may be referred to as an electrolytic zinc-plated steel sheet.

[0029] Ingredient composition First, the composition of the base steel sheet of the steel sheet according to the embodiment of the present invention will be described. Note that the unit of the composition is always "mass%", but hereinafter, unless otherwise specified, it will be simply shown as "%".

[0030] C: 0.05 to 0.30% C is an effective element for ensuring a TS of 780 MPa or more and less than 1180 MPa and a high YS by generating appropriate amounts of fresh martensite, tempered martensite, bainitic ferrite, and retained austenite. If the C content is less than 0.05%, the area ratio of ferrite increases, making it difficult to achieve a TS of 780 MPa or more. On the other hand, if the C content exceeds 0.30%, the area ratio of fresh martensite increases excessively, making it difficult to make the TS less than 1180 MPa. In addition, the fresh martensite becomes the starting point for void generation during the VDA bending test, so that the desired bendability of the steel sheet may not be achieved. Therefore, the C content is set to 0.05% or more and 0.30% or less. The C content is preferably 0.07% or more, and more preferably 0.09% or more. The C content is preferably 0.25% or less, and more preferably 0.20% or less.

[0031] Silicon: 0.10 to 2.00% Silicon inhibits the formation of carbides during cooling after annealing and promotes the formation of retained austenite. That is, silicon is an element that affects the volume fraction of retained austenite. If the silicon content is less than 0.10%, the volume fraction of retained austenite decreases, and ductility decreases. On the other hand, if the Si content exceeds 2.00%, the area ratio of ferrite increases, and the C concentration in austenite during annealing increases excessively, making it impossible to achieve the desired local elongation. Therefore, the Si content is set to 0.10% or more and 2.00% or less. The Si content is preferably 1.50% or less. Also, the Si content is preferably 0.50% or more.

[0032] Mn: 1.00-3.50% Mn is an element that adjusts the area ratio of ferrite, bainitic ferrite, tempered martensite, etc. Here, if the Mn content is less than 1.00%, the area ratio of ferrite increases excessively, making it difficult to achieve a TS of 780 MPa or more. On the other hand, if the Mn content exceeds 3.50%, the area ratio of ferrite decreases, and the desired total elongation cannot be obtained. Therefore, the Mn content is set to 1.00% or more and 3.50% or less. The Mn content is preferably 2.00% or more. Also, the Mn content is preferably 3.00% or less.

[0033] P: 0.001 to 0.100% P is an element that has a solid solution strengthening effect and increases the TS and YS of a steel sheet. To obtain this effect, the P content is set to 0.001% or more. On the other hand, if the P content exceeds 0.100%, P segregates at the prior austenite grain boundaries and embrittles the grain boundaries, which increases the amount of voids generated during the VDA bending test, making it impossible to achieve the desired bendability. Therefore, the P content is set to 0.001% or more and 0.100% or less, and preferably 0.030% or less.

[0034] S: 0.0001~0.0200% S exists as sulfide in steel. In particular, if the S content exceeds 0.0200%, the amount of voids generated increases after the steel sheet is subjected to shearing, and the desired bendability of the sheared edge cannot be achieved. Therefore, the S content is set to 0.0200% or less. The S content is preferably set to 0.0080% or less. Furthermore, due to restrictions on production technology, the S content is set to 0.0001% or more.

[0035] Sol.Al: 0.005~2.000% Al suppresses the formation of carbides during cooling after annealing and promotes the formation of retained austenite. In other words, Al is an element that affects the volume fraction of retained austenite. To obtain these effects, the sol. Al content is set to 0.005% or more. On the other hand, if the sol.Al content exceeds 2.000%, the area ratio of ferrite increases excessively, making it difficult to achieve a TS of 780 MPa or more. It also leads to a decrease in YS. In addition, the C concentration in austenite during annealing increases excessively, making it impossible to achieve the desired bendability. Therefore, the Al content is set to 0.005% or more and 2.000% or less. The Al content is preferably 0.010% or more. The Al content is more preferably 0.015% or more. Moreover, the Al content is preferably 1.000% or less.

[0036] N: 0.010% or less N exists as nitrides in steel. In particular, if the N content exceeds 0.010%, the amount of voids generated increases after a VDA bending test is performed on a steel sheet, and the desired bendability cannot be achieved. Therefore, the N content is set to 0.010% or less. The N content is preferably set to 0.0050% or less. Although there is no particular lower limit for the N content, due to restrictions on production technology, the N content is preferably 0.0005% or more.

[0037] The basic composition of the base steel sheet of the steel sheet according to one embodiment of the present invention has been described above, but the base steel sheet of the steel sheet according to one embodiment of the present invention has a composition that contains the basic components, with the balance other than the basic components including Fe (iron) and unavoidable impurities. Here, it is preferable that the base steel sheet of the steel sheet according to one embodiment of the present invention has a composition that contains the basic components, with the balance consisting of Fe and unavoidable impurities.

[0038] In addition to the basic components, the base steel sheet of the steel sheet according to one embodiment of the present invention may contain at least one selected from the optional components shown below. Note that the effects of the present invention can be obtained so long as the optional components shown below are contained in amounts not exceeding the upper limit amounts shown below, so no lower limit is particularly set. Note that when the optional elements shown below are contained in amounts less than the preferred lower limit values ​​described below, the elements are considered to be contained as unavoidable impurities.

[0039] Nb: 0.200% or less, Ti: 0.200% or less, V: 0.200% or less, B: 0.0100% or less, Cr: 1.000% or less, Ni: 1.000% or less, Mo: 1.000% or less, Sb: 0. 200% or less, Sn: 0.200% or less, Cu: 1.000% or less, Ta: 0.100% or less, W: 0.500% or less, Mg: 0.0200% or less, Zn: 0.0200% or less, Co: 0.0200 %, Zr: 0.1000% or less, Ca: 0.0200% or less, Se: 0.0200% or less, Te: 0.0200% or less, Ge: 0.0200% or less, As: 0.0500% or less, Sr: 0.0200% or less, Cs: 0.0200% or less, Hf: 0.0200% or less, Pb: 0.0200% or less, Bi: 0.0200% or less, and at least one selected from REM: 0.0200% or less.

[0040] Nb: 0.200% or less Nb forms fine carbides, nitrides, or carbonitrides during hot rolling or annealing, thereby increasing TS and YS. To obtain such an effect, the Nb content is preferably 0.001% or more. The Nb content is more preferably 0.005% or more. On the other hand, if the Nb content exceeds 0.200%, a large amount of coarse precipitates and inclusions may be generated. In such a case, the coarse precipitates and inclusions may become the starting point of cracks during the VDA bending test, and the desired bendability of the steel sheet may not be achieved. Therefore, when Nb is contained, the Nb content is preferably 0.200% or less. The Nb content is more preferably 0.060% or less.

[0041] Ti: 0.200% or less Ti, like Nb, forms fine carbides, nitrides, or carbonitrides during hot rolling or annealing, thereby increasing TS and YS. To obtain such an effect, the Ti content is preferably 0.001% or more. The Ti content is more preferably 0.005% or more. On the other hand, if the Ti content exceeds 0.200%, a large amount of coarse precipitates and inclusions may be generated. In such a case, the coarse precipitates and inclusions may become the starting point of cracks during the VDA bending test, and the desired bendability of the steel sheet may not be achieved. Therefore, when Ti is contained, the Ti content is preferably 0.200% or less. The Ti content is more preferably 0.060% or less.

[0042] V:0.200% or less Like Nb and Ti, V forms fine carbides, nitrides, or carbonitrides during hot rolling or annealing, thereby increasing TS and YS. To obtain such an effect, the V content is preferably 0.001% or more. The V content is more preferably 0.005% or more. The V content is further preferably 0.010% or more, and even more preferably 0.030% or more. On the other hand, if the V content exceeds 0.200%, a large amount of coarse precipitates and inclusions may be generated. In such a case, the coarse precipitates and inclusions may become the starting point of cracks during a VDA bending test, and the desired bendability of the steel sheet may not be achieved. Therefore, when V is contained, the V content is preferably 0.200% or less. The V content is more preferably 0.060% or less.

[0043] B: 0.0100% or less B is an element that enhances hardenability by segregating at austenite grain boundaries. Also, B is an element that inhibits the formation and grain growth of ferrite during cooling after annealing. In order to obtain such effects, the B content is preferably 0.0001% or more. The B content is more preferably 0.0002% or more. The B content is further preferably 0.0005% or more, and even more preferably 0.0007% or more. On the other hand, if the B content exceeds 0.0100%, cracks may occur inside the steel sheet during hot rolling, and the amount of voids generated may increase when the steel sheet is subjected to a VDA bending test, making it difficult to achieve the desired bendability. Therefore, when B is contained, the B content is preferably 0.0100% or less, and more preferably 0.0050% or less.

[0044] Cr:1.000% or less Since Cr is an element that enhances hardenability, the addition of Cr produces a large amount of tempered martensite, ensuring a TS of 780 MPa or more and a high YS. To obtain such effects, the Cr content is preferably 0.0005% or more. The Cr content is more preferably 0.010% or more. The Cr content is further preferably 0.030% or more, and even more preferably 0.050% or more. On the other hand, if the Cr content exceeds 1.000%, the area ratio of hard fresh martensite increases excessively, and the fresh martensite becomes the starting point for void generation in the VDA bending test, and the desired bendability of the steel sheet may not be achieved. Therefore, when Cr is contained, the Cr content is preferably 1.000% or less. Moreover, the Cr content is more preferably 0.800% or less, and further preferably 0.700% or less.

[0045] Ni: 1.000% or less Since Ni is an element that enhances hardenability, the addition of Ni produces a large amount of tempered martensite, ensuring a TS of 780 MPa or more and a high YS. To obtain such effects, the Ni content is preferably 0.005% or more. The Ni content is more preferably 0.020% or more. The Ni content is further preferably 0.040% or more, and even more preferably 0.060% or more. On the other hand, if the Ni content exceeds 1.000%, the area ratio of fresh martensite increases excessively, and the fresh martensite becomes the starting point of void generation in the VDA bending test, and there is a risk that the desired bendability of the steel sheet cannot be achieved. Therefore, when Ni is contained, the Ni content is preferably 1.000% or less. The Ni content is more preferably 0.800% or less. The Ni content is further preferably 0.600% or less, and even more preferably 0.400% or less.

[0046] Mo: 1.000% or less Mo is an element that enhances hardenability, so the addition of Mo produces a large amount of tempered martensite, ensuring a TS of 780 MPa or more and a high YS. To achieve this effect, the Mo content is preferably 0.010% or more. More preferably, the Mo content is 0.030% or more. On the other hand, if the Mo content exceeds 1.000%, the area ratio of fresh martensite increases excessively, and the fresh martensite becomes the starting point of void generation in the VDA bending test, and there is a risk that the desired bendability of the steel sheet cannot be achieved. Therefore, when Mo is contained, the Mo content is preferably 1.000% or less. The Mo content is more preferably 0.500% or less, further preferably 0.450% or less, and further preferably 0.400% or less. The Mo content is more preferably 0.350% or less, and further more preferably 0.300% or less.

[0047] Sb: 0.200% or less Sb is an element that is effective in suppressing the diffusion of C near the steel sheet surface during annealing and controlling the formation of a soft layer near the steel sheet surface. If the soft layer increases excessively near the steel sheet surface, it becomes difficult to achieve a TS of 780 MPa or more. It also leads to a decrease in YS. Therefore, the Sb content is preferably 0.002% or more. The Sb content is more preferably 0.005% or more. On the other hand, if the Sb content exceeds 0.200%, a soft layer is not formed near the surface of the steel sheet, which may lead to a decrease in the bendability of the steel sheet. Therefore, when Sb is contained, the Sb content is preferably 0.200% or less. The Sb content is more preferably 0.020% or less.

[0048] Sn: 0.200% or less Like Sb, Sn is an element that is effective in suppressing the diffusion of C near the steel sheet surface during annealing and controlling the formation of a soft layer near the steel sheet surface. If the soft layer increases excessively near the steel sheet surface, it becomes difficult to achieve a TS of 780 MPa or more. It also leads to a decrease in YS. Therefore, the Sn content is preferably 0.002% or more. The Sn content is more preferably 0.005% or more. On the other hand, if the Sn content exceeds 0.200%, a soft layer is not formed near the surface of the steel sheet, which may lead to a decrease in the bendability of the steel sheet. Therefore, when Sn is contained, the Sn content is preferably 0.200% or less. The Sn content is more preferably 0.020% or less.

[0049] Cu:1.000% or less Since Cu is an element that enhances hardenability, the addition of Cu produces a large amount of tempered martensite, ensuring a TS of 780 MPa or more and a high YS. To obtain such effects, the Cu content is preferably 0.005% or more. The Cu content is more preferably 0.008% or more, and even more preferably 0.010% or more. The Cu content is more preferably 0.020% or more. On the other hand, if the Cu content exceeds 1.000%, the area ratio of fresh martensite increases excessively, and a large amount of coarse precipitates and inclusions may be generated. In such a case, the fresh martensite and the coarse precipitates and inclusions may become the starting point of void generation during the VDA bending test, and the desired bendability of the steel sheet may not be achieved. Therefore, when Cu is contained, the Cu content is preferably 1.000% or less. The Cu content is more preferably 0.200% or less.

[0050] Ta:0.100% or less Ta, like Ti, Nb and V, increases TS and YS by forming fine carbides, nitrides or carbonitrides during hot rolling or annealing. In addition, Ta partially dissolves in Nb carbides or Nb carbonitrides to generate composite precipitates such as (Nb,Ta)(C,N). This inhibits the coarsening of precipitates and stabilizes precipitation strengthening. This further improves TS and YS. To obtain such effects, the Ta content is preferably 0.001% or more. The Ta content is more preferably 0.002% or more, and even more preferably 0.004% or more. On the other hand, if the Ta content exceeds 0.100%, a large amount of coarse precipitates and inclusions may be generated. In such a case, the coarse precipitates and inclusions may become the starting point for void generation during the VDA bending test, and the desired bendability of the steel sheet may not be achieved. Therefore, when Ta is contained, the Ta content is preferably 0.100% or less. The Ta content is more preferably 0.090% or less, and even more preferably 0.080% or less.

[0051] W: 0.500% or less W is an element that enhances hardenability, so the addition of W produces a large amount of tempered martensite, ensuring a TS of 780 MPa or more and a high YS. To achieve this effect, the W content is preferably 0.001% or more. The W content is more preferably 0.030% or more. On the other hand, if the W content exceeds 0.500%, the area ratio of hard fresh martensite increases excessively, and the fresh martensite becomes the starting point of void generation in the VDA bending test, and there is a risk that the desired bendability of the steel sheet cannot be achieved. Therefore, when W is contained, the W content is preferably 0.500% or less. The W content is more preferably 0.450% or less, and even more preferably 0.400% or less. The W content is even more preferably 0.300% or less.

[0052] Mg: 0.0200% or less Mg is an element that is effective in making the shape of inclusions such as sulfides and oxides spherical and improving the bendability of the sheared end surface. To obtain such an effect, the Mg content is preferably 0.0001% or more. The Mg content is more preferably 0.0005% or more, and further preferably 0.0010% or more. On the other hand, if the Mg content exceeds 0.0200%, a large amount of coarse precipitates and inclusions may be generated. In such a case, the coarse precipitates and inclusions may become the starting point for void generation during the VDA bending test, and the desired bendability of the steel sheet may not be achieved. Therefore, when Mg is contained, the Mg content is preferably 0.0200% or less. The Mg content is more preferably 0.0180% or less, and further preferably 0.0150% or less.

[0053] Zn: 0.0200% or less Zn is an element effective in making the shape of inclusions spherical and improving the bendability of the sheared end surface. To obtain such effects, the Zn content is preferably 0.0010% or more. The Zn content is more preferably 0.0020% or more, and even more preferably 0.0030% or more. On the other hand, if the Zn content exceeds 0.0200%, a large amount of coarse precipitates and inclusions may be generated. In such a case, the coarse precipitates and inclusions may become the starting point for void generation during the VDA bending test, and the desired bendability of the steel sheet may not be achieved. Therefore, when Zn is contained, the Zn content is preferably 0.0200% or less. The Zn content is more preferably 0.0180% or less, and further preferably 0.0150% or less.

[0054] Co:0.0200% or less Co, like Zn, is an element that is effective in making the shape of inclusions spherical and improving the bendability of the sheared end surface. To obtain such an effect, the Co content is preferably 0.0010% or more. The Co content is more preferably 0.0020% or more, and even more preferably 0.0030% or more. On the other hand, if the Co content exceeds 0.0200%, a large amount of coarse precipitates and inclusions may be generated. In such a case, the coarse precipitates and inclusions may become the starting point for void generation during the VDA bending test, and the desired bendability of the steel sheet may not be achieved. Therefore, when Co is contained, the Co content is preferably 0.0200% or less. The Co content is more preferably 0.0180% or less, and further preferably 0.0150% or less.

[0055] Zr: 0.1000% or less Zr, like Zn and Co, is an element that is effective in making the shape of inclusions spheroidal and improving the bendability of the sheared end surface. In order to obtain such effects, the Zr content is preferably 0.0010% or more. On the other hand, if the Zr content exceeds 0.1000%, a large amount of coarse precipitates and inclusions may be generated. In such a case, the coarse precipitates and inclusions may become the starting point for void generation during the VDA bending test, and the desired bendability of the steel sheet may not be achieved. Therefore, when Zr is contained, the Zr content is preferably 0.1000% or less. The Zr content is more preferably 0.0300% or less, and further preferably 0.0100% or less.

[0056] Ca:0.0200% or less Ca exists as inclusions in steel. If the Ca content exceeds 0.0200%, a large amount of coarse inclusions may be generated. In such a case, the coarse precipitates and inclusions may become the starting point for void generation during the VDA bending test, and the desired bendability of the steel sheet may not be achieved. Therefore, when Ca is contained, the Ca content is preferably 0.0200% or less. The Ca content is preferably 0.0020% or less. Although there is no particular lower limit for the Ca content, the Ca content is preferably 0.0005% or more, and more preferably 0.0010% or more due to restrictions on production technology.

[0057] Se: 0.0200% or less, Te: 0.0200% or less, Ge: 0.0200% or less, As: 0.0500% or less, Sr: 0.0200% or less, Cs: 0.0200% or less, Hf: 0.0200% or less, Pb: 0.0200% or less, Bi: 0.0200% or less, REM: 0.0200% or less Se, Te, Ge, As, Sr, Cs, Hf, Pb, Bi and REM are all elements effective for improving bendability during VDA testing. In order to obtain such effects, it is preferable that the contents of Se, Te, Ge, As, Sr, Cs, Hf, Pb, Bi and REM are each 0.0001% or more. On the other hand, if the contents of Se, Te, Ge, Sr, Cs, Hf, Pb, Bi and REM exceed 0.0200%, or if the content of As exceeds 0.0500%, a large amount of coarse precipitates and inclusions may be generated. In such cases, the coarse precipitates and inclusions may become the starting point for void generation during VDA bending testing, and the desired bendability of the steel sheet may not be achieved. Therefore, when at least one of Se, Te, Ge, As, Sr, Cs, Hf, Pb, Bi, and REM is contained, it is preferable that the contents of Se, Te, Ge, As, Sr, Cs, Hf, Pb, Bi, and REM are each 0.0200% or less, and the As content is 0.0500% or less. The Se content is more preferably 0.0005% or more, and further preferably 0.0008% or more. The Se content is more preferably 0.0180% or less, and further preferably 0.0150% or less. The Te content is more preferably 0.0005% or more, and further preferably 0.0008% or more. The Te content is more preferably 0.0180% or less, and further preferably 0.0150% or less. The Ge content is more preferably 0.0005% or more, and further preferably 0.0008% or more. The Ge content is more preferably 0.0180% or less, and further preferably 0.0150% or less. The As content is more preferably 0.0010% or more, and further preferably 0.0015% or more. The As content is more preferably 0.0400% or less, and further preferably 0.0300% or less. The Sr content is more preferably 0.0005% or more, and further preferably 0.0008% or more. The Sr content is more preferably 0.0180% or less, and further preferably 0.0150% or less. The Cs content is more preferably 0.0005% or more, and further preferably 0.0008% or more. The Cs content is more preferably 0.0180% or less, and further preferably 0.0150% or less. The Hf content is more preferably 0.0005% or more, and further preferably 0.0008% or more. The Hf content is more preferably 0.0180% or less, and further preferably 0.0150% or less. The Pb content is more preferably 0.0005% or more, and further preferably 0.0008% or more. The Pb content is more preferably 0.0180% or less, and further preferably 0.0150% or less. The Bi content is more preferably 0.0005% or more, and further preferably 0.0008% or more. The Bi content is more preferably 0.0180% or less, and further preferably 0.0150% or less. The REM content is more preferably 0.0005% or more, and further preferably 0.0008% or more. The REM content is more preferably 0.0180% or less, and further preferably 0.0150% or less.

[0058] In the present invention, REM refers to scandium (Sc) with atomic number 21, yttrium (Y) with atomic number 39, and lanthanoids from lanthanum (La) with atomic number 57 to lutetium (Lu) with atomic number 71. The REM content in the present invention is the total content of one or more elements selected from the above-mentioned REM. The REM is not particularly limited, but is preferably La and / or Ce.

[0059] That is, the base steel sheet of the steel sheet of the present invention contains, in mass%, C: 0.05 to 0.30%, Si: 0.10 to 2.00%, Mn: 1.00 to 3.50%, P: 0.001 to 0.100%, S: 0.0001 to 0.0200%, sol. Al: 0.005 to 2.000%, and N: 0.010% or less, and optionally Nb: 0.200% or less, Ti: 0.200% or less, V: 0.200% or less, B: 0.0100% or less, Cr: 1.000% or less, Ni: 1.000% or less, Mo: 1.000% or less, Sb: 0.200% or less, Sn: 0.200% or less, Cu: 1.000% or less, Ta: 0.10 and at least one selected from among 0% or less, W: 0.500% or less, Mg: 0.0200% or less, Zn: 0.0200% or less, Co: 0.0200% or less, Zr: 0.1000% or less, Ca: 0.0200% or less, Se: 0.0200% or less, Te: 0.0200% or less, Ge: 0.0200% or less, As: 0.0500% or less, Sr: 0.0200% or less, Cs: 0.0200% or less, Hf: 0.0200% or less, Pb: 0.0200% or less, Bi: 0.0200% or less and REM: 0.0200% or less, with the balance being Fe and unavoidable impurities.

[0060] steel structure Next, the steel structure of a steel sheet (base steel sheet) according to one embodiment of the present invention will be described. The steel plate of the present invention has a steel structure in which, at a 1 / 4 position in the thickness direction of the base steel plate, the area ratios are ferrite: 20.0% or more and 70.0% or less, the total of bainitic ferrite and tempered martensite: 20.0% or more and 70.0% or less, retained austenite: 3.0% or more and 20.0% or less, and fresh martensite: 20.0% or less (including 0.0%), the area ratio of bainitic ferrite out of the total area ratio of bainitic ferrite and tempered martensite is 40% or more, and the average grain size of the hard second phase present in the bainitic ferrite and including retained austenite and fresh martensite is 3.0 μm or less.

[0061] Ferrite area ratio: 20.0% to 70.0% The soft ferrite is a phase that improves ductility. From the viewpoint of ensuring high ductility, the area ratio of ferrite is set to 20.0% or more. The area ratio of ferrite is preferably 30.0% or more, and more preferably 40.0% or more. On the other hand, if the area ratio of ferrite is excessively increased, it becomes difficult to achieve a TS of 780 MPa or more. Therefore, the area ratio of ferrite is set to 70.0% or less. The area ratio of ferrite is preferably 60.0% or less, and more preferably 50.0% or less.

[0062] Total area ratio of bainitic ferrite and tempered martensite (excluding retained austenite): 20.0% to 70.0% Bainitic ferrite and tempered martensite have intermediate hardness between soft ferrite and hard fresh martensite, and are important phases for ensuring good yield stress (YS) of steel sheets. They are also important for ensuring TS. Therefore, the total area ratio of bainitic ferrite and tempered martensite is set to 20.0% or more. It is preferably 30.0% or more, and more preferably 40% or more. On the other hand, if the total area ratio of bainitic ferrite and tempered martensite increases excessively, the total elongation and / or local elongation decreases. Therefore, the total area ratio of bainitic ferrite and tempered martensite is set to 70.0% or less. The total area ratio of bainitic ferrite and tempered martensite is preferably 60.0% or less, and more preferably 50.0% or less. Bainitic ferrite refers to upper bainitic ferrite that is generated in a relatively high temperature range and has few carbides.

[0063] Area ratio of retained austenite: 3.0% to 20.0% From the viewpoint of obtaining good ductility, the area ratio of the retained austenite is set to 3.0% or more, preferably 5.0% or more, and more preferably 7.0% or more. On the other hand, if the area ratio of the retained austenite increases excessively, the fresh martensite formed by the deformation-induced transformation during shear processing becomes the starting point for void generation, and the desired local elongation (L-El) cannot be obtained. Therefore, the area ratio of the retained austenite is set to 20.0% or less. The area ratio of the retained austenite is preferably 15.0% or less, and more preferably 10.0% or less.

[0064] Area ratio of fresh martensite: 20.0% or less (including 0.0%) If the area ratio of fresh martensite is excessively increased, the fresh martensite becomes the starting point of void generation in a bending test conforming to the VDA standard (VDA238-100) specified by the German Association of the Automotive Industry, and the desired bendability of the steel sheet (limit bending angle (α) in the VDA bending test) cannot be achieved. From the viewpoint of ensuring good bendability of the steel sheet, the area ratio of fresh martensite is set to 15.0% or less, preferably 10.0% or less. The lower limit of the area ratio of fresh martensite is not particularly limited, and may be 0.0%. It should be noted that fresh martensite is martensite that has been quenched (not tempered).

[0065] The ratio of the area ratio of bainitic ferrite to the total area ratio of bainitic ferrite and tempered martensite: 40% or more Bainitic ferrite contributes to improving ductility. If the area ratio of bainitic ferrite is less than 40% of the total area ratio of bainitic ferrite and tempered martensite, the effect of improving ductility cannot be sufficiently obtained. Therefore, the area ratio of bainitic ferrite is 40% or more of the total area ratio of bainitic ferrite and tempered martensite, preferably 50% or more, and more preferably 60% or more. There is no particular upper limit to the ratio of the area fraction of bainitic ferrite to the total area fraction of bainitic ferrite and tempered martensite. However, if it is increased excessively, the tensile strength (TS) may decrease, so the upper limit is preferably 90% or less, and more preferably 80% or less.

[0066] Average grain size of hard second phase that exists in bainitic ferrite and contains retained austenite and fresh martensite: 3.0 μm or less Among the hard second phases including retained austenite and fresh martensite, the hard second phase present in bainitic ferrite has a high concentration of solute C. Therefore, retained austenite is highly stable and improves total elongation due to stress-induced transformation, and fresh martensite is hard, but by contacting with bainitic ferrite, which is an intermediate hardness phase, it improves TS while ensuring local elongation. In this way, the above-mentioned hard second phase plays an extremely important role in ensuring total elongation and TS. However, when the average grain size of the retained austenite and fresh martensite present in the bainitic ferrite exceeds 3.0 μm, the interface between the hard fresh martensite and / or the retained austenite formed by processing-induced transformation and the bainitic ferrite during tensile testing becomes the origin of void generation, reducing local elongation. Therefore, the average grain size of the hard second phase including the retained austenite and fresh martensite present in the bainitic ferrite is 3.0 μm or less, preferably 2.0 μm or less, and more preferably 1.0 μm or less. Although there is no particular lower limit, the average grain size of the hard secondary phase present in the bainitic ferrite may be 0.1 μm or more, or may be 0.3 μm or more. In the present invention, the hard second phase contains retained austenite and fresh martensite, and may consist of retained austenite and fresh martensite.

[0067] The area ratio of the remaining structure other than the above is preferably 20.0% or less. The area ratio of the remaining structure is more preferably 10.0% or less. The area ratio of the remaining structure may be 0.0%. The remaining structure is not particularly limited, and examples thereof include carbides such as pearlite, cementite, etc. The type of the remaining structure can be confirmed, for example, by observation with a SEM (Scanning Electron Microscope).

[0068] Soft surface layer: The total area ratio of structures other than ferrite is 1 / 2 or less at the 1 / 4 position of the plate thickness. The soft surface layer contributes to suppressing the progression of bending cracks during press forming and during a vehicle collision, and improves the energy absorption characteristics during bending deformation. The soft surface layer means the decarburized layer, and the total area ratio of the structure other than ferrite in the cross section at 1 / 4 of the plate thickness is S A When the total area ratio of structures other than ferrite is S A This refers to the surface area where the surface area is less than / 2. In order to obtain excellent energy absorption properties during bending deformation of the steel sheet, the soft surface layer is formed in an area of ​​20 μm or more from the surface of the base steel sheet in the sheet thickness direction. The thickness of the soft surface layer is preferably 25 μm or more, and more preferably 30 μm or more. There is no particular upper limit to the thickness of the soft surface layer, but since an excessively formed soft surface layer causes a decrease in TS, the thickness is preferably 200 μm or less, more preferably 150 μm or less, and even more preferably 120 μm or less. In addition, the total area ratio S of the structures other than ferrite mentioned above A The 1 / 4 sheet thickness position of the steel sheet where the measurement is performed is a non-surface soft layer (a layer in which no change in structure has occurred due to decarburization, and which does not satisfy the conditions of a surface soft layer defined in the present invention).

[0069] Here, the area ratios of ferrite, bainitic ferrite, tempered martensite and hard phase (hard second phase (retained austenite + fresh martensite)) at the 1 / 4 sheet thickness position of the base steel sheet are measured as follows. That is, a sample is cut out from the base steel sheet so that the cross section of the sheet thickness parallel to the rolling direction of the base steel sheet becomes the observation surface. The observation surface of the sample is then mirror-polished using diamond paste. The observation surface of the sample is then finish-polished using colloidal silica, and etched with 3 vol.% nital to reveal the structure. Then, using a SEM (Scanning Electron Microscope), images are taken of three fields of view of 25.6 μm x 17.6 μm at the outermost layer position and 1 / 4 of the plate thickness position of the observation surface of the sample under conditions of an acceleration voltage of 15 kV and a magnification of 5000 times. From the obtained structural image, ferrite, bainitic ferrite, tempered martensite, and hard phases (hard second phases (retained austenite + fresh martensite)) are identified as follows.

[0070] Ferrite: A black region that is lumpy in shape. It contains almost no iron-based carbides. However, if it does contain iron-based carbides, the area of ​​the ferrite includes the area of ​​the iron-based carbides. The same applies to bainitic ferrite and tempered martensite, which will be described later. Bainitic ferrite: This is a region that is black to dark gray in color and has a massive or amorphous shape. It also contains no iron carbides or contains a relatively small amount of them. Tempered martensite: This is a gray area with an amorphous shape. It also contains a relatively large number of iron-based carbides. Hard phase (hard second phase (retained austenite + fresh martensite)): This is a region that is white to light gray in color and has an amorphous form. It does not contain iron-based carbides. If the size is relatively large, the color gradually darkens as it moves away from the interface with other structures, and the interior may be dark gray. Carbides: White areas that are dot-like or linear in shape. They are included in tempered martensite, bainitic ferrite, and ferrite. Remaining structure: Pearlite, internal oxides, etc., and the morphology thereof are as known.

[0071] Next, the area of ​​each phase identified in the structural image is calculated by the following method. An equally spaced 20 x 20 grid is placed on an area of ​​25.6 μm x 19.2 μm in actual length on the 5000x magnification SEM image, and the area ratios of ferrite, bainitic ferrite, tempered martensite, and hard phases (hard second phases) are investigated by the point counting method, which counts the number of points on each phase. The area ratio is the average value of three area ratios determined on separate 5000x magnification SEM images.

[0072] The average grain size of the hard secondary phase in the bainitic ferrite is measured as follows. In the structural image, bainitic ferrite is identified as described above, and the hard second phase contained in the bainitic ferrite region is identified. The long and short diameters of the hard second phase contained in the bainitic ferrite region are measured, and the sum of the long and short diameters is divided by 2 to obtain the grain size. In the 5000x SEM image, 10 hard second phases with the largest grain size are selected from among the hard second phases contained in the bainitic ferrite region with the largest continuous area, and their average grain size is measured. At this time, if the 10 hard second phases are not included in the bainitic ferrite region, the average grain size of all the hard second phases is measured. The three average grain sizes obtained by the same procedure for three separate SEM images are then averaged to obtain the average grain size of the hard second phase in the bainitic ferrite.

[0073] The area ratio of retained austenite is measured as follows. The base steel plate is mechanically ground in the plate thickness direction (depth direction) to a position that is 1 / 4 of the plate thickness, and then chemically polished with oxalic acid to obtain the observation surface. The observation surface is then observed by X-ray diffraction. MoKα rays are used as the incident X-rays, and the ratio of the diffraction intensity of the (200), (211), and (220) faces of the fcc iron (austenite) to the diffraction intensity of the (200), (220), and (311) faces of the bcc iron is calculated, and the volume fraction of the retained austenite is calculated from the ratio of the diffraction intensity of each face. The retained austenite is then considered to be three-dimensionally homogeneous, and the volume fraction of the retained austenite is taken as the area fraction of the retained austenite.

[0074] The area ratio of fresh martensite is determined by subtracting the area ratio of retained austenite from the area ratio of the hard phase (hard second phase) determined as described above. [Area ratio of fresh martensite (%)] = [Area ratio of hard second phase (%)] - [Area ratio of retained austenite (%)]

[0075] The area ratio of the remaining structure is determined by subtracting the area ratio of ferrite, the area ratio of bainitic ferrite, the area ratio of tempered martensite, and the area ratio of the hard phase (hard second phase) determined as described above from 100.0%. [Area ratio of remaining structure (%)] = 100.0 - [Area ratio of ferrite (%)] - [Area ratio of bainitic ferrite (%)] - [Area ratio of tempered martensite (%)] - [Area ratio of hard second phase (%)]

[0076] For the measurement of the structure of the soft surface layer, if a zinc-coated layer is formed on the steel sheet, first peel off the zinc-coated layer, and measure the structure in the same way as at the 1 / 4 position in the sheet thickness direction at 1 μm intervals from the surface of the base steel sheet in the sheet thickness direction to a position 100 μm in the sheet thickness direction. After that, measurements are made at 20 μm intervals up to the center of the sheet thickness.

[0077] Next, the mechanical properties of the steel plate according to one embodiment of the present invention will be described.

[0078] Tensile strength (TS): 780MPa or more, less than 1180MPa The tensile strength TS of the steel plate according to one embodiment of the present invention is 780 MPa or more and less than 1180 MPa. The yield stress (YS), total elongation (T-El), local elongation (L-El), bendability of the steel plate according to one embodiment of the present invention, and energy absorption characteristics during bending deformation are as described above.

[0079] The tensile strength (TS), yield stress (YS), total elongation (T-El), and local elongation (L-El) are measured by a tensile test in accordance with JIS Z 2241 (2011) described later in the examples. The bendability of the steel sheet and the energy absorption characteristics during bending deformation are measured by a VDA bending test described later in the examples.

[0080] Zinc plating layer A steel sheet according to one embodiment of the present invention may have a zinc-plated layer formed on a base steel sheet, and this zinc-plated layer may be provided on only one surface of the base steel sheet, or on both surfaces.

[0081] The galvanized layer referred to here refers to a plated layer containing Zn as a main component (Zn content: 50.0 mass % or more), and examples thereof include a hot-dip galvanized layer, a galvannealed hot-dip galvanized layer, and an electrolytic galvanized layer.

[0082] Here, the hot-dip galvanized layer is preferably composed of, for example, Zn, 20.0 mass% or less of Fe, and 0.001 mass% to 1.0 mass% or less of Al. The hot-dip galvanized layer may optionally contain one or more elements selected from the group consisting of Pb, Sb, Si, Sn, Mg, Mn, Ni, Cr, Co, Ca, Cu, Li, Ti, Be, Bi, and REM in a total amount of 0.0 mass% to 3.5 mass%. The Fe content of the hot-dip galvanized layer is more preferably less than 7.0 mass%. The remainder other than the above elements is unavoidable impurities.

[0083] In addition, the galvannealed layer is preferably composed of, for example, Zn, 20% by mass or less of Fe, and 0.001% by mass or more and 1.0% by mass or less of Al. In addition, the galvannealed layer may optionally contain one or more elements selected from the group consisting of Pb, Sb, Si, Sn, Mg, Mn, Ni, Cr, Co, Ca, Cu, Li, Ti, Be, Bi, and REM in a total amount of 0.0% by mass or more and 3.5% by mass or less. The Fe content of the galvannealed layer is more preferably 7.0% by mass or more, and more preferably 8.0% by mass or more. The Fe content of the galvannealed layer is more preferably 15.0% by mass or less, and more preferably 12.0% by mass or less. The remainder other than the above elements is unavoidable impurities.

[0084] The electrolytic zinc plating layer is preferably composed of, for example, Zn and 9.0 mass % to 25.0 mass % Ni, the remainder being unavoidable impurities other than the above elements.

[0085] In addition, the coating weight of the hot-dip galvanized layer and the galvannealed hot-dip galvanized layer per side is not particularly limited, but is preferably 20 g / m 2 The coating weight of the hot-dip galvanized layer and the galvannealed hot-dip galvanized layer on one side is preferably 80 g / m or more. 2 In addition, the plating weight of the electrolytic zinc plating layer per side is not particularly limited, but is preferably 10 g / m 2 The plating weight of the electrolytic zinc plating layer on one side is preferably 70 g / m or more. 2 It is preferable that:

[0086] The coating weight of the zinc-plated layer (hot-dip galvanized layer, alloyed hot-dip galvanized layer, electrolytic zinc-plated layer) is measured as follows. That is, a treatment solution is prepared by adding 0.6 g of a corrosion inhibitor for Fe (Ivit 700BK (registered trademark) manufactured by Asahi Chemical Industry Co., Ltd.) to 1 L of a 10 mass % aqueous hydrochloric acid solution. Next, a steel sheet to be used as a test material is immersed in the treatment solution to dissolve the zinc plating layer. The amount of mass loss of the test material before and after dissolution is then measured, and this value is divided by the surface area of ​​the base steel sheet (the surface area of ​​the part that was covered with plating) to obtain the plating coverage (g / m 2 ) is calculated.

[0087] The thickness of the steel plate according to one embodiment of the present invention is not particularly limited, but is preferably 0.6 mm or more, more preferably 0.8 mm or more, further preferably 1.0 mm or more, and most preferably 1.2 mm or more. The thickness of the steel plate is preferably 3.5 mm or less, and more preferably 2.3 mm or less. The width of the steel plate of the present invention is not particularly limited, but is preferably 500 mm or more, more preferably 750 mm or more. The width of the steel plate is preferably 1600 mm or less, more preferably 1450 mm or less. [2. Steel plate manufacturing method] Next, a method for manufacturing a steel sheet according to one embodiment of the present invention will be described.

[0088] A method for producing a steel sheet according to an embodiment of the present invention includes a hot rolling process in which a steel slab having the above-mentioned component composition is hot-rolled to produce a hot-rolled steel sheet; a pickling process in which the hot-rolled steel sheet is pickled; an annealing process in which the steel sheet after the pickling process is annealed under conditions of an annealing temperature of 720°C to 860°C, a holding time of 20 seconds or more, and a dew point of -10°C or more in an atmosphere; a rapid heating process in which the steel sheet after the rapid heating process is rapidly heated from the annealing temperature to an annealing temperature +10°C or higher under conditions of a heating rate of 10°C / second or more; and a first cooling process in which the steel sheet after the rapid heating process is cooled to a first cooling stop temperature of 400°C to 600°C. a holding step of holding the steel sheet in a temperature range of 400°C or more and 600°C or less for a holding time of less than 300 seconds; a second cooling step of cooling the steel sheet after the holding step to a second cooling stop temperature of 100°C or more and 300°C or less; and a reheating and holding step of heating the steel sheet after the second cooling step to a temperature range of a tempering temperature of 460°C or less and holding it in the above temperature range for a tempering time of 10 seconds or more and 2000 seconds or less, or further comprising a cold rolling step of cold rolling the steel sheet after the pickling step and before the annealing step with a rolling reduction of 20% or more and 80% or less to obtain a cold-rolled steel sheet. It is preferable that the steel sheet passing speed LS (m / min) in the annealing process satisfies the following formula (1). 3.0 / (t×W×4.716×10 ―4 ) <LS<390.0 / (t×W×4.716×10 ―4 )...Equation (1) In formula (1), t is the sheet thickness (mm) after the pickling process and before the annealing process, and W is the sheet thickness (mm) after the pickling process and before the annealing process. Unless otherwise specified, the above temperatures refer to the surface temperatures of the steel slab and the steel plate.

[0089] First, a steel slab having the above-mentioned composition is prepared. For example, a steel material is melted to obtain molten steel having the above-mentioned composition. The melting method is not particularly limited, and known melting methods such as converter melting and electric furnace melting can be used. The obtained molten steel is then solidified to obtain a steel slab. The method for obtaining a steel slab from molten steel is not particularly limited, and for example, a continuous casting method, an ingot casting method, or a thin slab casting method can be used. From the viewpoint of preventing macrosegregation, it is preferable to adopt a continuous casting method as a method for obtaining a steel slab from molten steel.

[0090] (Hot rolling process) Next, in the hot rolling process, the steel slab is hot rolled to produce a hot rolled steel sheet. The hot rolling may be performed by applying an energy-saving process, such as direct rolling (a method in which the steel slab is not cooled to room temperature, but is charged as a hot piece into a heating furnace and hot rolled) or direct rolling (a method in which the steel slab is briefly kept at a constant temperature and then immediately rolled).

[0091] The hot rolling conditions are not particularly limited, and the hot rolling can be performed, for example, under the following conditions. That is, the steel slab is once cooled to room temperature, and then reheated and rolled. The slab heating temperature (reheating temperature) is preferably 1100°C or higher from the viewpoints of dissolving carbides and reducing the rolling load. In addition, in order to prevent an increase in scale loss, the slab heating temperature is preferably 1300°C or lower. The slab heating temperature is based on the temperature of the steel slab surface.

[0092] Next, the steel slab is subjected to rough rolling according to a conventional method to obtain a rough rolled plate (hereinafter also referred to as a sheet bar). Next, the sheet bar is subjected to finish rolling to obtain a hot rolled steel plate. In addition, when the slab heating temperature is set low, it is preferable to heat the sheet bar using a bar heater or the like before the finish rolling in order to prevent problems during the finish rolling. In order to reduce the rolling load, the finish rolling temperature is preferably set to 800°C or higher. In addition, if the rolling reduction rate in the non-recrystallized state of austenite becomes high, an abnormal structure elongated in the rolling direction may develop, which may reduce the workability of the annealed sheet. Furthermore, by setting the finish rolling temperature to 800°C or higher, the steel structure at the hot rolled steel plate stage and, in turn, the steel structure of the final product are likely to become uniform. In addition, if the steel structure becomes non-uniform, the bendability tends to decrease. On the other hand, if the finish rolling temperature exceeds 950°C, the amount of oxide (scale) generated increases. As a result, the interface between the base steel and the oxide becomes rough, which may deteriorate the surface quality of the steel sheet after pickling and cold rolling. In addition, the crystal grains may become coarse, which may cause a decrease in the strength and bendability of the steel sheet. Therefore, it is preferable that the finish rolling temperature is in the range of 950°C or less. For the above reasons, it is preferable that the finish rolling temperature is in the range of 800°C or more and 950°C or less.

[0093] After the finish rolling, the hot-rolled steel sheet is coiled at a coiling temperature of preferably 450° C. or higher and preferably 750° C. or lower.

[0094] In addition, the sheet bars may be joined together during hot rolling and continuous finish rolling may be performed. Also, the sheet bars may be wound once before the finish rolling. In order to reduce the rolling load during hot rolling, a part or all of the finish rolling may be lubricated rolling. Performing lubricated rolling is also effective from the viewpoint of uniformity of the steel sheet shape and material quality. In addition, the friction coefficient during lubricated rolling is preferably in the range of 0.10 to 0.25. In the hot rolling process, which includes rough rolling and finish rolling, steel slabs are generally made into sheet bars by rough rolling and then into hot rolled steel sheets by finish rolling. However, depending on the mill capacity, etc., such divisions do not matter as long as the specified size is achieved.

[0095] (pickling process) The hot-rolled steel sheet after the hot rolling process is pickled. Pickling can remove oxides from the steel sheet surface, ensuring good chemical conversion treatment properties and plating quality. Pickling may be performed once or multiple times. There are no particular limitations on the pickling conditions, and the usual methods may be followed.

[0096] (Cold rolling process) Next, the hot-rolled steel sheet is cold-rolled as necessary to obtain a cold-rolled steel sheet. The cold rolling is performed by multi-pass rolling requiring two or more passes, such as tandem multi-stand rolling or reverse rolling. The cold rolling reduction (cumulative reduction) should be between 20% and 80%. If the cold rolling reduction is less than 20%, the steel structure is likely to become coarse and non-uniform during the annealing process, which may result in reduced TS and bendability in the final product. On the other hand, if the cold rolling reduction exceeds 80%, the steel sheet is likely to have poor shape and the amount of zinc coating may become non-uniform. Furthermore, the cold-rolled steel sheet obtained after cold rolling may be optionally subjected to pickling.

[0097] (Annealing process) Next, in one embodiment of the present invention, after the pickling process (or after the cold rolling process in the case of performing cold rolling), the steel sheet obtained as described above is heated and annealed (held) at an annealing temperature of 720°C or more and 860°C or less for a holding time (annealing time) of 20 seconds or more. Note that the number of annealing steps may be two or more, but one time is preferable from the viewpoint of energy efficiency. The annealing step is a step including this heating treatment and soaking treatment.

[0098] Annealing temperature: 720℃ to 860℃ If the annealing temperature is less than 720°C, the rate of oxidation reaction of carbon in the surface layer of the steel sheet decreases, and the desired thickness of the surface soft layer cannot be achieved. Also, the ratio of austenite generated during heating in the two-phase region of ferrite and austenite may be insufficient. Therefore, the area ratio of ferrite may increase excessively after heat treatment, and TS may decrease. On the other hand, if the annealing temperature exceeds 860°C, the fraction of austenite, which has a slow carbon diffusion rate, increases, and the diffusion rate of carbon to the surface layer where the decarburization reaction proceeds decreases and becomes diffusion-limited, making it impossible to achieve the desired thickness of the soft surface layer. Also, the proportion of austenite generated during annealing becomes excessive, and the area ratio of ferrite after heat treatment may become less than 20.0%. Therefore, the annealing temperature is set to 720° C. or higher and 860° C. or lower, and preferably 850° C. or lower. The annealing temperature is the maximum temperature reached in the annealing process.

[0099] Holding time (annealing time): 20 seconds or more If the holding time is less than 20 seconds, the time for the decarburization reaction to occur in the surface layer is insufficient, and the desired thickness of the soft surface layer cannot be obtained. Therefore, the holding time (annealing time) is set to 20 seconds or more. The holding time is preferably 30 seconds or more, and more preferably 50 seconds or more. The upper limit of the holding time is not particularly limited, but the holding time is preferably 900 seconds or less, and more preferably 800 seconds or less. The holding time is the holding time in the temperature range of (annealing temperature -40°C) or more and (annealing temperature) or less. In other words, the holding time (annealing time) includes not only the holding time at the annealing temperature, but also the residence time in the temperature range of (annealing temperature -40°C) or more and (annealing temperature) or less during heating before reaching the annealing temperature.

[0100] Dew point of the annealing process atmosphere (annealing atmosphere): -10℃ or higher In one embodiment of the present invention, the dew point of the atmosphere in the annealing step (annealing atmosphere) is set to -10°C or higher. By performing annealing in the annealing step with the dew point of the annealing atmosphere being -10°C or higher, the decarburization reaction is promoted and a surface soft layer having a desired thickness can be formed. The dew point of the annealing atmosphere in the annealing step is more preferably 0°C or higher, further preferably 5°C or higher, and further more preferably 10°C or higher. Although there is no particular upper limit for the dew point of the annealing atmosphere in the annealing step, in order to improve the plating adhesion when a zinc plating layer is formed, the dew point of the annealing atmosphere in the annealing step is preferably 30° C. or lower.

[0101] 3.0 / (t×W×4.716×10 ―4 ) <LS<390.0 / (t×W×4.716×10 ―4 )...Equation (1) In the present invention, from the viewpoint of production efficiency, it is preferable that the steel sheet threading speed LS (m / min) in the annealing process satisfies formula (1). In formula (1), t: plate thickness (mm), W: plate width (mm), It is. The sheet thickness t is the sheet thickness after the pickling process and before the annealing process, and the sheet width W is the sheet width after the pickling process and before the annealing process. The above production efficiency can be expressed as E by the following equation (2). E(ton / h)=LS×t×W×ρ×60×10 ―6 ...Equation (2) In formula (2), t: plate thickness (mm), W: Board width (mm), ρ: specific gravity of iron (ton / m 3 ), and ρ = 7.86 (ton / m 3 ),. In this case, E is preferably greater than 3.0 and less than 390.0. Based on this equation (2), the above equation (1) can be derived. If E is 3.0 ton / h or less, the soft surface layer may be excessively formed, resulting in a shortage of TS. On the other hand, if E is 390.0 ton / h or more, the soft surface layer may not be sufficiently formed, resulting in insufficient energy absorption characteristics during bending deformation. Therefore, E is preferably greater than 3.0 and less than 390.0. Moreover, E is more preferably 16.0 ton / h or more, and more preferably 190.0 ton / h or less. From the above, LS (m / min) is 3.0 / (t×W×4.716×10 ―4 ) or more. LS (m / min) is 16.0 / (t×W×4.716×10 ―4 ) or more is more preferable. In addition, LS (m / min) is 390.0 / (t×W×4.716×10 ―4 ) is preferable. LS (m / min) is 190.0 / (t×W×4.716×10 ―4 ) or less is more preferable.

[0102] The sheet passing speed LS (m / min) is calculated by dividing the distance (m) traveled by the steel sheet in the annealing process by the time (min) required for the steel sheet to be transported in the annealing process.

[0103] A radiant tube furnace is generally used as the heat treatment furnace for the annealing process. The annealing temperature is obtained by a thermometer that measures the surface temperature of the steel sheet. The method of temperature measurement is not particularly limited, but for example, a radiation thermometer that measures the temperature by sensing infrared rays emitted by the steel sheet is suitable. In the case of a radiation thermometer, since it may be affected by reflected light of infrared rays emitted by the surrounding furnace body, a cover may be provided between the measurement part of the radiation thermometer and the detection part of the steel sheet. In addition, since it may be affected by the emissivity of the steel sheet surface, a multiple reflection type measurement method using a wedge-shaped space between the transport roll in the furnace and the steel sheet may be adopted.

[0104] (Rapid heating process) In one embodiment of the present invention, after the annealing step, the steel sheet obtained as described above is heated (rapidly heated) from the annealing temperature to the annealing temperature +10°C or higher at a heating rate of 10°C / sec or more. When annealing is performed two or more times in the above-mentioned annealing step, the treatment in the rapid heating step performed after the annealing may also be performed two or more times, but the treatment in the rapid heating step may be performed only once.

[0105] Heating rate from annealing temperature: 10℃ / sec or more If the heating rate from the annealing temperature in the rapid heating step is less than 10°C / sec, Mn will diffuse from ferrite to the austenite formed in the rapid heating step, reducing the amount of bainitic ferrite formed in the subsequent cooling step and holding step, making it impossible to obtain a structure in which the area ratio of bainitic ferrite is 40% or more of the total area ratio of bainitic ferrite and tempered martensite. Therefore, the heating rate from the annealing temperature in the rapid heating step is set to 10°C / sec or more. Furthermore, the heating rate in the rapid heating step is preferably 30° C. / sec or more. By setting the heating rate at 30° C. / sec or more, the diffusion of Mn from ferrite to austenite can be suppressed, austenite can be generated only by the diffusion of C, and the line length can be further shortened. On the other hand, if the heating rate exceeds 300°C / sec, there is a concern that the steel sheet may buckle due to thermal stress, and local high temperature areas may occur in the sheet width direction, resulting in a loss of uniformity. Therefore, the heating rate is preferably 300°C / sec or less, and more preferably 150°C / sec or less.

[0106] In the rapid heating process, the material is rapidly heated to a temperature 10°C higher than the annealing temperature (the temperature reached in the rapid heating process). If the temperature reached in the rapid heating step (rapid heating step temperature) is less than annealing temperature + 10°C, austenite cannot be sufficiently formed by rapid heating, and a structure with the desired total area ratio of bainitic ferrite and tempered martensite, or a structure in which the ratio of the area ratio of bainitic ferrite to the total area ratio of bainitic ferrite and tempered martensite is 40% or more, cannot be formed. Therefore, the effect of improving at least one of YS, TS, ductility, and bendability by performing rapid heating cannot be obtained, and the desired mechanical properties may not be secured. The temperature reached in the rapid heating step (rapid heating step temperature) is preferably annealing temperature + 30°C or more, more preferably annealing temperature + 50°C or more. Although there is no particular upper limit to the temperature reached in the rapid heating step, from the viewpoint of production efficiency, it is preferably annealing temperature + 210°C or less, more preferably annealing temperature + 150°C or less, and even more preferably annealing temperature + 120°C or less.

[0107] The induction heating (IH) device in the rapid heating process adjusts the output to rapidly heat the steel sheet so that the temperature of the steel sheet is within the annealing temperature +10°C or more (preferably, 740°C or more and 940°C or less). When heating in such a temperature range, it is preferable that the induction heating (IH) device is of the transverse type. The appropriate rapid heating temperature varies depending on the chemical composition of the steel sheet. Therefore, it is preferable that the suitable temperature range is predicted in advance by measurement, calculation or simulation, and set taking into consideration the temperature of the steel sheet in the annealing process. The temperature reached in the rapid heating process can be measured by a thermometer that measures the surface temperature of the steel sheet. The method of temperature measurement is not particularly limited, but for example, a radiation thermometer that measures the temperature by sensing infrared rays emitted by the steel sheet is suitable. In the case of a radiation thermometer, since it may be affected by reflected light of infrared rays emitted by the surrounding furnace body, a cover may be provided between the measurement part of the radiation thermometer and the detection part of the steel sheet. In addition, since it may be affected by the emissivity of the steel sheet surface, a multiple reflection type measurement method using a wedge-shaped space between the transport roll in the furnace and the steel sheet may be adopted. In addition, it is preferable to use a scanning radiation thermometer to measure the temperature at each position in the width direction of the steel sheet.

[0108] (First cooling process) Next, the steel sheet after annealing as described above (after the rapid heating step) is cooled in the first cooling step to a temperature of 400° C. or more and 600° C. or less (first cooling stop temperature).

[0109] First cooling stop temperature: 400℃ or more and 600℃ or less If the first cooling stop temperature is less than 400°C, the formation of bainitic ferrite is promoted, and the area ratio of the retained austenite formed together with the bainitic ferrite exceeds a certain amount, making it impossible to achieve the desired local elongation. On the other hand, if the first cooling stop temperature exceeds 600°C, the total area ratio of bainitic ferrite and tempered martensite becomes less than 20.0% due to an excessive increase in the area ratio of ferrite and pearlite, resulting in a decrease in TS. Therefore, the first cooling stop temperature in the first cooling step is set to 400°C or more and 600°C or less. The first cooling stop temperature is preferably 460°C or more. In addition, the first cooling stop temperature is preferably 550°C or less.

[0110] (holding process) After the first cooling step, in the holding step, the steel sheet is retained in a temperature range of 400° C. or more and 600° C. or less (hereinafter also referred to as a holding temperature range) for less than 300 seconds.

[0111] Holding time in holding temperature range: Less than 300 seconds In the holding step, bainitic ferrite is generated, and C diffuses from the generated bainitic ferrite to the untransformed austenite adjacent to the bainitic ferrite, thereby ensuring a predetermined area ratio of retained austenite. Here, if the holding time in the holding temperature range is 300 seconds or more, the diffusion of C and Mn from bainitic ferrite to untransformed austenite occurs excessively, and a coarse hard second phase (residual austenite and fresh martensite) is formed in the bainitic ferrite, which may result in failure to achieve the desired bendability or local elongation. In addition, the total area ratio of bainitic ferrite and tempered martensite becomes excessive due to the excessive formation of bainitic ferrite, and local elongation may decrease. Therefore, the holding time in the holding temperature range is preferably less than 300 seconds. The holding time in the holding temperature range is more preferably less than 80 seconds, and even more preferably less than 60 seconds. The holding time in the holding temperature range does not include the residence time in the temperature range after the galvanizing process is performed in the hot dip galvanizing process.

[0112] (Zinc plating process (hot-dip galvanizing process, alloying process)) When obtaining a hot-dip galvanized steel sheet or a galvannealed hot-dip galvanized steel sheet, the steel sheet is subjected to a galvanizing treatment (hot-dip galvanizing treatment, or further, a galvannealed hot-dip galvanizing treatment) after the above-mentioned holding step and before the second cooling step. By performing the galvanizing treatment, a galvanized steel sheet can be obtained. The galvanizing treatment here includes a hot-dip galvanizing treatment and a galvannealed treatment (hot-dip galvanizing treatment and an alloying treatment). A hot-dip galvanized steel sheet can be obtained by subjecting a steel sheet to a hot-dip galvanizing treatment, and a galvannealed steel sheet can be obtained by further subjecting the steel sheet to an alloying treatment. Hereinafter, the hot-dip galvanizing treatment and the alloying treatment will be collectively referred to as a galvannealed treatment.

[0113] In the case of hot-dip galvanizing, it is preferable to immerse the steel sheet in a hot-dip galvanizing bath at 440°C to 500°C, and then adjust the coating weight by gas wiping or the like. The hot-dip galvanizing bath is not particularly limited as long as it has the composition of the zinc-coated layer described above, but it is preferable to use a plating bath with an Al content of 0.10 mass% or more, with the balance being Zn and unavoidable impurities. The Al content is preferably 0.23 mass% or less.

[0114] In the case of galvannealing treatment, it is preferable to carry out alloying treatment by heating the galvanized steel sheet to an alloying temperature of 450° C. or more after carrying out the galvannealing treatment as described above. The alloying temperature is preferably 600° C. or less. If the alloying temperature is less than 450°C, the Zn-Fe alloying rate is slow, and alloying may be difficult. On the other hand, if the alloying temperature exceeds 600°C, untransformed austenite is transformed into pearlite, making it difficult to achieve a TS of 780 MPa or more, and ductility is reduced. The alloying temperature is more preferably 470°C or more. The alloying temperature is more preferably 570°C or less.

[0115] In addition, the coating weight of both hot-dip galvanized steel sheet (GI) and galvannealed steel sheet (GA) is 20 g / m per side. 2 The coating weight of the zinc-plated layer on one side is preferably 80 g / m or more. 2 It is preferable that the plating thickness is set to be equal to or less than 100 nm. The plating thickness can be adjusted by gas wiping or the like.

[0116] (Second cooling process) Next, the steel sheet after the galvanizing step in case of obtaining a hot-dip galvanized steel sheet or a galvannealed hot-dip galvanized steel sheet, or after the holding step in case of obtaining a steel sheet or an electrolytic galvanized steel sheet, is cooled to a second cooling stop temperature of 100°C or higher and 300°C or lower.

[0117] Second cooling stop temperature: 100℃ or more and 300℃ or less The second cooling step is a step necessary for controlling the area ratio of tempered martensite and the area ratio of retained austenite generated in the subsequent reheating and holding step within a predetermined range. If the second cooling stop temperature is less than 100°C, the untransformed austenite present in the steel in the second cooling step is almost entirely transformed into martensite. Ultimately, the area ratio of tempered martensite increases excessively, and the area ratio of bainitic ferrite cannot be made 40% or more of the total area ratio of bainitic ferrite and tempered martensite, resulting in a decrease in El. On the other hand, if the second cooling stop temperature exceeds 300°C, the area ratio of tempered martensite decreases and the area ratio of fresh martensite increases. As a result, the desired bendability cannot be achieved. Therefore, the second cooling stop temperature is set to 100°C or more and 300°C or less. The second cooling stop temperature is preferably 120°C or more. In addition, the second cooling stop temperature is preferably 280°C or less.

[0118] (Reheating and holding process) Next, the steel sheet is reheated to a tempering temperature range of 460°C or lower (hereinafter also referred to as reheating temperature range), and the steel sheet is held in the tempering temperature range of 460°C or lower for a tempering time of 10 seconds to 2000 seconds. This tempers the martensite present in the steel at the end of the second cooling step, and also causes the carbon that is supersaturated in the martensite to diffuse into the untransformed austenite, forming austenite that is stable at room temperature, i.e., retained austenite.

[0119] Tempering temperature (reheating temperature): 460℃ or less Although the lower limit of the tempering temperature (reheating temperature) is not particularly limited, if it is 300°C or lower, the tempering of the martensite present in the steel at the end of the second cooling step does not proceed sufficiently, and in addition to the excessive increase in fresh martensite, the coarsening of the carbides in the tempered martensite does not proceed sufficiently, and the density of the carbides in the tempered martensite increases, which may result in a decrease in bendability or local elongation. Therefore, it is preferable that the tempering temperature is more than 300°C. The tempering temperature is preferably 340°C or higher, more preferably 360°C or higher. On the other hand, if the tempering temperature (reheating temperature) exceeds 460°C, the tempering of martensite present in the steel at the end of the second cooling step proceeds excessively, making it difficult to achieve a TS of 780MPa or more. In addition, the untransformed austenite present in the steel at the end of the second cooling step decomposes as carbide (pearlite), resulting in a decrease in ductility. Therefore, the tempering temperature (reheating temperature) is set to 460°C or less. The tempering temperature is the maximum temperature reached in the reheating and holding step. The tempering temperature is preferably 440°C or less, more preferably 420°C or less.

[0120] Tempering time in the reheating temperature range (holding time): 10 seconds or more and 2000 seconds or less If the tempering time (holding time) in the reheating temperature range is less than 10 seconds, the martensite present in the steel at the end of the second cooling step is not tempered sufficiently, and fresh martensite increases excessively. Also, the coarsening of carbides in the tempered martensite does not proceed sufficiently, and the density of carbides in the tempered martensite becomes excessive. As a result, the desired bendability and local elongation cannot be achieved. On the other hand, if the tempering time (holding time) in the reheating temperature range exceeds 2000 seconds, the martensite present in the steel at the end of the second cooling process will be excessively tempered, making it difficult to achieve a TS of 780 MPa or more. Also, the untransformed austenite present in the steel at the end of the second cooling process will decompose as carbide (pearlite), which may reduce ductility. Therefore, the tempering time (holding time) in the reheating temperature range is set to 10 seconds or more and 2000 seconds or less. The tempering time is preferably 20 seconds or more, and more preferably 30 seconds or more. The tempering time is preferably 1000 seconds or less, more preferably 500 seconds or less, and further preferably 100 seconds or less. The tempering time (holding time) in the reheating temperature range includes not only the holding time at the reheating temperature, but also the residence time in that temperature range during heating and cooling before and after reaching the reheating temperature.

[0121] The cooling conditions after holding in the reheating temperature range are not particularly limited and may be in accordance with a conventional method. For example, gas jet cooling, mist cooling, roll cooling, water cooling, and air cooling may be used as the cooling method. In addition, from the viewpoint of preventing oxidation of the surface, it is preferable to cool to 50°C or less after holding in the reheating temperature range, and more preferably to about room temperature. The average cooling rate in the cooling after holding in the reheating temperature range is preferably, for example, 1°C / sec or more and 50°C / sec or less.

[0122] (Zinc plating process (electrolytic zinc plating process)) After cooling to room temperature, an electrogalvanizing treatment may be performed. By performing an electrogalvanizing treatment on the steel sheet, an electrogalvanized steel sheet can be obtained. When performing an electrogalvanizing treatment on the steel sheet surface, the treatment conditions for the electrogalvanizing treatment are not particularly limited, and may be performed according to a conventional method.

[0123] The steel sheet obtained as described above may be further subjected to temper rolling. If the reduction rate of temper rolling exceeds 2.00%, the yield stress increases, and the dimensional accuracy may decrease when the steel sheet is formed into a member. Therefore, the reduction rate of temper rolling is preferably 2.00% or less. The lower limit of the reduction rate of temper rolling is not particularly limited, but is preferably 0.05% or more from the viewpoint of productivity. The temper rolling may be performed on an apparatus continuous with the annealing apparatus for performing each of the above-mentioned steps (online), or may be performed on an apparatus discontinuous with the annealing apparatus for performing each of the steps (offline). The number of rolling times of temper rolling may be one or two or more. As long as the same elongation rate as that of temper rolling can be imparted, rolling using a leveler or the like may be used.

[0124] The other conditions than those mentioned above are not particularly limited, and may be those according to conventional methods.

[0125] [3. Materials] Next, a member according to one embodiment of the present invention will be described. A member according to an embodiment of the present invention is a member made using the above-mentioned steel plate (as a raw material). For example, the raw material steel plate is subjected to at least one of forming and joining to form a member. Here, the above steel sheet has a TS of 780 MPa or more, and has a high YS, excellent total elongation, local elongation, bendability, and energy absorption properties. Therefore, the member according to one embodiment of the present invention has high strength and excellent impact resistance properties. Therefore, the member according to one embodiment of the present invention is particularly suitable for application to impact energy absorbing members used in the automotive field.

[0126] [4. Manufacturing methods for components] Next, a method for manufacturing a member according to one embodiment of the present invention will be described. A method for manufacturing a component according to one embodiment of the present invention includes a step of subjecting the above-mentioned steel plate (e.g., a steel plate manufactured by the above-mentioned steel plate manufacturing method) to at least one of forming and joining to form a component. Here, the molding method is not particularly limited, and for example, a general processing method such as press processing can be used. The joining method is also not particularly limited, and for example, general welding such as spot welding, laser welding, and arc welding, rivet joining, crimp joining, etc. The molding conditions and joining conditions are not particularly limited, and may be in accordance with ordinary methods. EXAMPLES

[0127] Steel materials having the composition shown in Table 1 (the balance being Fe and unavoidable impurities) were melted in a converter and made into steel slabs by a continuous casting method. In Table 1, "-" indicates the content of unavoidable impurities.

[0128] The obtained steel slab was heated to 1200°C, and after heating, the steel slab was subjected to hot rolling consisting of rough rolling and finish rolling at a finish rolling temperature of 900°C to obtain hot rolled steel sheets. Next, No. 1, No. 4 to 24, No. 27 to 49, No. 51 to 58, No. 61 to 75, No. 77 to 84, and No. 87 to 98 of the obtained hot rolled steel sheets were pickled and cold rolled (reduction rate: 50%) to obtain cold rolled steel sheets with the thicknesses shown in Table 2. In addition, No. 2 to 3, No. 25 to 26, No. 50, No. 59 to 60, No. 76, and No. 85 to 86 of the obtained hot rolled steel sheets were pickled to obtain hot rolled steel sheets (white skin) with the thicknesses shown in Table 2. Next, the obtained cold-rolled steel sheet or hot-rolled steel sheet (white skin) was subjected to the annealing step, rapid heating step, first cooling step, holding step, galvanizing step, second cooling step and reheating and holding step under the conditions shown in Table 2 to obtain a steel sheet. Also, the cold-rolled steel sheet was obtained by carrying out the treatments excluding the galvanizing step. The electrogalvanized steel sheet was obtained by carrying out the treatments excluding the holding step and galvanizing step, more specifically, after the reheating and holding step, by carrying out the treatment in the electrogalvanizing step.

[0129] Here, in the galvanizing process, hot-dip galvanizing treatment, alloyed galvanizing treatment, or electrolytic galvanizing treatment was performed to obtain hot-dip galvanized steel sheet (hereinafter also referred to as GI), alloyed hot-dip galvanized steel sheet (hereinafter also referred to as GA), or electrolytic galvanized steel sheet (hereinafter also referred to as EG). In Table 2, the type of plating process is also indicated as "GI", "GA", or "EG". In Table 2, in the case of GI and EG steel sheets, alloying treatment was not performed, so the alloying temperature is indicated as -. In Table 2, cold-rolled steel sheets obtained without performing zinc plating treatment in the galvanizing process are indicated as "CR".

[0130] The galvanizing bath temperature was 470°C for both GI and GA production. The amount of zinc coating is 10 to 70 g / m when manufacturing EG. 2 It was decided. When manufacturing GI, the thickness is 45 to 72 g / m per side. 2 When manufacturing GA, the thickness is 45 g / m per side. 2 It was decided. The composition of the zinc plating layer of the finally obtained zinc-plated steel sheet was as follows: GI: 0.1-1.0 mass% Fe, 0.2-0.33 mass% Al, and the balance was Zn and unavoidable impurities; GA: 8.0-12.0 mass% Fe, 0.1-0.23 mass% Al, and the balance was Zn and unavoidable impurities; EG: 9.0-25.0 mass% Ni, and the balance was Zn and unavoidable impurities. In addition, the zinc plating layers were formed on both sides of the base steel sheet in each case.

[0131] The steel structure of the obtained steel sheet was identified in the manner described above. The measurement results are shown in Table 2. In Table 3, F is ferrite, BF is bainitic ferrite, TM is tempered martensite, RA is retained austenite, FM is fresh martensite, and P is pearlite. In addition, in the manner described above, the surface layer in which the total area ratio of structures other than ferrite is ½ or less at the ¼ position of the sheet thickness was defined as the soft surface layer.

[0132] In addition, the tensile strength (TS), yield stress (YS), total elongation (T-El), local elongation (L-El), limit bending angle (α) in the VDA bending test, bending deformation absorbed energy amount, and production efficiency (E) were evaluated according to the following criteria.

[0133] ·TS 〇 (Pass): 780MPa or more and less than 1180MPa × (Fail): Less than 780MPa or more than 1180MPa

[0134] YS 〇(Pass): (A) When 780MPa≦TS<980MPa, 400MPa≦YS≦650MPa (B) When 980MPa≦TS<1180MPa, 600MPa≦YS≦900MPa ×(Fail): (A) When 780MPa≦TS<980MPa, 400MPa>YS or 650MPa <YS (B) If 980MPa≦TS<1180MPa, then 600MPa>YS or 900MPa <YS

[0135] T-El 〇(Pass): (A) When 780MPa≦TS<980MPa, 17.0%≦T-El (B) When 980MPa≦TS<1180MPa, 11.0%≦T-El ×(Fail): (A) When 780MPa≦TS<980MPa, 17.0%>T-El (B) When 980MPa≦TS<1180MPa, 11.0%>T-El L-El 〇(Pass): (A) When 780MPa≦TS<980MPa, 7.0%≦L-El (B) When 980MPa≦TS<1180MPa, 5.0%≦L-El ×(Fail): (A) When 780MPa≦TS<980MPa, 7.0%>L-El (B) When 980MPa≦TS<1180MPa, 5.0%>L-Ll

[0136] · Limit bending angle (α) in VDA bending test 〇(Pass): 80° or more × (Fail): Less than 80°

[0137] ·Integral value AE of load F from the start of deformation to the maximum load in the load-stroke curve in a VDA bending test 〇(Pass): 55000N mm or more × (Fail): Less than 55000N·mm

[0138] Production efficiency E(ton / h)=LS×t×W×ρ×60×10 ―6 Pass(A):16.0 <E<190.0 Pass(B):3.0 <E≦16.0、または190.0≦E<390.0 Pass (C): 3.0≧E or E≧390.0 Where: LS: Strip threading speed in the annealing process (m / min) t: Sheet thickness after pickling and before annealing (mm) W: Sheet width after pickling and before annealing (mm) ρ: specific gravity of iron (ton / m 3 ) and ρ = 7.86 (ton / m 3 ).

[0139] (1) Tensile test The tensile test was performed in accordance with JIS Z 2241 (2011). That is, JIS No. 5 test pieces were taken from the obtained steel sheets so that the longitudinal direction was perpendicular to the rolling direction of the base steel sheet. Using the taken test pieces, a tensile test was performed at a crosshead speed of 10 mm / min, and TS, YS, T-El and L-El were measured. The results are shown in Table 3.

[0140] (2) VDA bending test The VDA bending test was performed in accordance with the VDA standard (VDA238-100) prescribed by the German Association of the Automotive Industry. Specifically, a test piece of 70 mm × 60 mm was taken from the obtained steel plate by shearing, where the 60 mm side was parallel to the rolling (L) direction. The test pieces were ground on one side to make the plate thickness uniform at 1.2 mm, with the inside of the bend being the ground surface. The test piece was subjected to a VDA bending test under the following conditions. Steel plates with a thickness of less than 1.2 mm were tested without undergoing grinding or other processing. Test method: Roll support, punch intrusion Roll diameter: φ30mm Punch tip R: 0.4 mm Distance between rolls: (sheet thickness x 2) + 0.5 mm Stroke speed: 20mm / min Bending direction: perpendicular to rolling (C) direction At this time, the angle of the outer bend at the center of the plate-shaped test piece when the load F from the pushing bending jig from above is maximum is measured as the bending angle (limit bending angle) (°). In addition, the integral value (absorbed energy) of the load-stroke curve up to the maximum load is calculated using the obtained load-stroke curve. The average values ​​of the limit bending angle and the integral value (absorbed energy) of the load-stroke curve at the maximum load when the above VDA bending test is performed three times are respectively designated as α (°) and AE. The results are shown in Table 3.

[0141] In the VDA bending tests of steel plates with a thickness of over 1.2 mm, the effect of plate thickness was taken into consideration and all tests were conducted with steel plates with a thickness of 1.2 mm. Steel plates with a thickness of over 1.2 mm were ground on one side to make the thickness 1.2 mm. Since the grinding process may affect the bendability of the steel plate surface, the ground surface was placed on the inside of the bend (the side that comes into contact with the punch) in the VDA bending tests. On the other hand, in the VDA bending test of steel plates with a thickness of less than 1.2 mm, the effect of the plate thickness was small, so the test was performed without grinding.

[0142] [Table 1]

[0143] [Table 2-1]

[0144] [Table 2-2]

[0145] [Table 3-1]

[0146] [Table 3-2]

[0147] In Tables 1 to 3, the underlined parts indicate values ​​outside the appropriate range of the present invention. As shown in Table 3, all of the inventive examples passed the test in terms of tensile strength (TS), yield stress (YS), total elongation (T-El), local elongation (L-El), limit bending angle (α) in the VDA bending test, and integral value (absorbed energy) (AE) of the load-stroke curve in the VDA bending test. In addition, the production efficiency (E) was also good. On the other hand, in the comparative examples, at least one of the tensile strength (TS), yield stress (YS), total elongation (T-El), local elongation (L-El), limit bending angle (α) in the VDA bending test, and integral value (absorbed energy) (AE) of the load-stroke curve in the VDA bending test was insufficient.

[0148] In addition, it was found that the members obtained by forming or joining using the steel sheets of the present invention had excellent properties characteristic of the present invention in terms of tensile strength (TS), yield stress (YS), total elongation (T-El), local elongation (L-El), limit bending angle (α) in the VDA bending test, and integral value (absorbed energy) (AE) of the load-stroke curve in the VDA bending test. It was also found that the production efficiency (E) was good. [Industrial Applicability]

[0149] According to the present invention, it is possible to obtain a steel sheet having a TS of 780 MPa or more and excellent energy absorption properties. If a member obtained from the steel sheet of the present invention is used as an automobile part, it can contribute to weight reduction of the automobile and greatly contribute to high performance of the automobile body.

Claims

1. A steel plate comprising a base steel plate, the base steel plate comprising: In mass percent, C: 0.05-0.30%, Si: 0.10-2.00%, Mn: 1.00-3.50%, P: 0.001-0.100%, S: 0.0001-0.0200%, sol. Al: 0.005 to 2.000% and N: 0.010% or less and the balance being Fe and unavoidable impurities; At a 1 / 4 position of the thickness of the base steel sheet, In terms of area ratio, Ferrite: 20.0% or more and 70.0% or less, Sum of bainitic ferrite and tempered martensite: 20.0% or more and 70.0% or less, Retained austenite: 3.0% or more and 20.0% or less, Fresh martensite: 20.0% or less (including 0.0%), The ratio of the area ratio of bainitic ferrite to the total area ratio of the bainitic ferrite and tempered martensite is 40% or more; a steel structure in which a hard second phase, which is present in the bainitic ferrite and includes the retained austenite and the fresh martensite, has an average grain size of 3.0 μm or less; having Furthermore, the layer is formed to a thickness of 20 μm or more in the thickness direction from the surface of the base steel sheet, The steel structure has a surface soft layer in which the total area ratio of structures other than ferrite in the steel structure is 1 / 2 or less at a 1 / 4 position of the plate thickness, A steel plate having a tensile strength of 780 MPa or more and less than 1180 MPa.

2. The composition further includes, in mass%, Nb: 0.200% or less, Ti: 0.200% or less, V: 0.200% or less, B: 0.0100% or less, Cr: 1.000% or less, Ni: 1.000% or less, Mo: 1.000% or less, Sb: 0.200% or less, Sn: 0.200% or less, Cu: 1.000% or less, Ta: 0.100% or less, W: 0.500% or less, Mg: 0.0200% or less, Zn: 0.0200% or less, Co: 0.0200% or less, Zr: 0.1000% or less, Ca: 0.0200% or less, Se: 0.0200% or less, Te: 0.0200% or less, Ge: 0.0200% or less, As: 0.0500% or less, Sr: 0.0200% or less, Cs: 0.0200% or less, Hf: 0.0200% or less, Pb: 0.0200% or less, Bi: 0.0200% or less; REM: 0.0200% or less The steel sheet according to claim 1, comprising at least one selected from the following:

3. The steel sheet according to claim 1 , further comprising a zinc-plated layer on one or both sides of the base steel sheet.

4. The steel plate as described in claim 2, having a zinc plating layer on one or both sides of the base steel plate.

5. A member made using the steel plate according to any one of claims 1 to 4.

6. A hot rolling process in which a steel slab having the composition according to claim 1 or 2 is hot rolled to obtain a hot rolled steel sheet; A pickling process for pickling the hot-rolled steel sheet; An annealing process in which the steel sheet after the pickling process is heated and annealed under the conditions of an annealing temperature of 720°C to 860°C, a holding time of 20 seconds or more, and a dew point of -10°C or more in an atmosphere; A rapid heating process of rapidly heating the annealing temperature to the annealing temperature + 10 ° C. or higher at a temperature increase rate of 10 ° C. / sec or higher from the annealing temperature; A first cooling step of cooling the steel plate after the rapid heating step to a first cooling stop temperature: 400°C or more and 600°C or less; A holding step of holding the material at a temperature in the range of 400° C. to 600° C. for a holding time of less than 300 seconds; A second cooling step of cooling the steel sheet after the holding step to a second cooling stop temperature of 100°C or more and 300°C or less; A reheating and holding process of heating the steel sheet after the second cooling process to a tempering temperature of 460° C. or less and holding the steel sheet in the temperature range for a tempering time of 10 seconds or more and 2000 seconds or less. Or even The steel sheet after the pickling step and before the annealing step is subjected to cold rolling at a rolling reduction of 20% or more and 80% or less to obtain a cold-rolled steel sheet.

7. The method for producing a steel sheet according to claim 6, wherein a passing speed LS (m / min) of the steel sheet in the annealing step satisfies the following formula (1). 3.0 / (t×W×4.716×10 ―４ )<LS<390.0 / (t×W×4.716×10 ―４ ) …………Formula (1) Where: t: sheet thickness after pickling process and before annealing process (mm), W: Sheet width after pickling process and before annealing process (mm) It is.

8. A method for manufacturing a steel plate as described in claim 6, which satisfies the following (A) or (B). (A) a hot-dip galvanizing step of performing a galvanizing treatment on a surface of the steel sheet after the holding step and before the second cooling step to form a galvanized layer on the surface of the steel sheet, Alternatively, the method may further include an alloying treatment step of performing an alloying treatment on the steel sheet on which the zinc plating layer is formed. (B) After the reheating and holding step, the steel sheet is immersed in an electrolytic zinc plating bath to form a zinc plating layer on the steel sheet.

9. A method for manufacturing a steel plate as described in claim 7, which satisfies the following (A) or (B). (A) a hot-dip galvanizing step of performing a galvanizing treatment on a surface of the steel sheet after the holding step and before the second cooling step to form a galvanized layer on the surface of the steel sheet, Alternatively, the method may further include an alloying treatment step of performing an alloying treatment on the steel sheet on which the zinc plating layer is formed. (B) After the reheating and holding step, the steel sheet is immersed in an electrolytic zinc plating bath to form a zinc plating layer on the steel sheet.

10. A method for manufacturing a component, comprising a step of subjecting the steel plate according to any one of claims 1 to 4 to at least one of forming and joining to form a component.

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