Specification
Title of invention: Hot stamped product, steel sheet for hot stamping, and methods for producing the same
Technical field
[0001]
　TECHNICAL FIELD The present invention relates to a hot stamp molded product, a hot stamping steel plate, and a method for manufacturing the same.
　The present application claims priority based on Japanese Patent Application No. 2017-193095 filed in Japan on October 02, 2017, the contents of which are incorporated herein by reference.
Background technology
[0002]
　Nowadays when the industrial technology field is highly specialized, materials used in each technical field are required to have special and high performance. For example, steel sheets for automobiles are required to have high strength in consideration of the global environment in order to improve fuel efficiency by reducing the weight of a vehicle body. When the high-strength steel plate is applied to the vehicle body of an automobile, it is possible to impart desired strength to the vehicle body while reducing the thickness of the steel plate to reduce the weight of the vehicle body.
[0003]
　However, in press forming, which is a step of forming a vehicle body member of an automobile, cracks and wrinkles are more likely to occur as the thickness of the steel sheet used is smaller. Therefore, the steel sheet for automobiles is also required to have excellent press formability.
[0004]
　Since ensuring the press formability and increasing the strength of the steel sheet are contradictory factors, it is difficult to satisfy these characteristics at the same time. Further, when the high strength steel plate is press-formed, the shape of the member changes greatly due to the springback when the member is taken out of the mold, so that it becomes difficult to secure the dimensional accuracy of the member. Thus, it is not easy to manufacture a high-strength vehicle body member by press molding.
[0005]
　As a method for manufacturing an ultra-high-strength vehicle body member, a technique of press-forming a heated steel plate using a low-temperature press die has been proposed, as disclosed in Patent Document 1, for example. .. This technique is called hot stamping or hot pressing, and press-molds a soft steel sheet that is heated to a high temperature, so that a member having a complicated shape can be manufactured with high dimensional accuracy. Further, since the steel plate is rapidly cooled by contact with the die, it becomes possible to significantly increase the strength at the same time as press forming by quenching. For example, Patent Document 1 describes that a member having a tensile strength of 1400 MPa or more can be obtained by hot stamping a steel sheet having a tensile strength of 500 to 600 MPa.
[0006]
　By the way, in the skeleton structural parts such as the center pillar and the side member among the vehicle body members, a hard part and a soft part may be provided in the member in order to control the deformed state of the member when the vehicle collides. Many.
[0007]
　As a method of manufacturing a member having a soft portion by hot stamping, Patent Document 2 discloses a method of partially changing the heating temperature of a steel sheet by induction heating or infrared heating to soften a portion heated to a low temperature. ing. Patent Document 3 discloses a method in which a heat insulating material is attached to a part of a steel sheet when the steel sheet is heated in a furnace, and the heating temperature is partially lowered to soften the steel sheet.
[0008]
　Patent Document 4 and Patent Document 5 disclose a method of partially changing the cooling rate of the steel sheet by changing the contact area between the steel sheet and the mold at the time of forming and softening the low cooling rate portion. There is. Patent Document 6 discloses a technique for hot stamping using a so-called tailored blank material in which two raw plates are welded and connected.
[0009]
　In hot stamping, usually, a steel sheet is heated to an austenite region and then cooled at a cooling rate equal to or higher than a critical cooling rate to form a single structure of martensite to enhance strength. On the other hand, in the methods described in Patent Documents 2 to 5, as described above, the heating temperature or the cooling rate of the steel sheet is partially lowered, and a structure other than martensite is generated to achieve softening. However, since the fraction of the structure other than martensite sensitively changes with the heating temperature and the cooling rate, the methods of Patent Documents 2 to 5 have a problem that the strength of the soft part is not stable.
[0010]
　Further, in the technique described in Patent Document 6, by using a steel plate having low hardenability as one of the base plates, the soft part can be formed under constant heating and cooling conditions. However, although the metallographic structure and strength characteristics of the soft part largely depend on the composition of the steel sheet, Patent Document 6 does not consider the composition of the steel sheet having low hardenability.
[0011]
　In order to solve such a problem, Patent Documents 7 and 8 disclose a method of stabilizing the strength of a soft portion in a hot stamp member including a hard portion and a soft portion, or a hot stamp member having a soft portion as a whole. Has been done.
　Specifically, Patent Document 7 discloses a 600-1200 MPa class high-strength automobile for which the C content is made low and a quenching element is contained in a certain amount or more to suppress the formation of ferrite, pearlite and martensite during cooling. A member and a method of manufacturing the member are disclosed. Further, Patent Document 8 discloses a hot stamp member having a tensile strength of 500 MPa or more and a method for manufacturing the hot stamp member, in which the content of C is controlled to be low and Ti is contained to control the production amount of martensite. ..
[0012]
　According to the techniques described in Patent Documents 7 and 8, it is possible to improve the uniformity of strength and elongation in the member. However, according to the study of the present inventors, since the metal structure contains a hard structure such as bainite and martensite, the thermal stability is low, and the strength may decrease when the member is subjected to coating baking treatment. I knew it was. Since automotive components are often subjected to paint baking, there is room for improvement in the techniques described in Patent Documents 7 and 8.
Prior art documents
Patent literature
[0013]
Patent Document 1: Japanese Unexamined
Patent Publication No. 2002-102980 Patent Document 2: Japanese Unexamined
Patent Publication No. 2005-193287 Patent Document 3: Japanese Unexamined
Patent Publication No. 2009-61473 Patent Document 4: Japanese Unexamined Patent Publication No. 2003-328031 JP
Patent Document 5: WO 2006/38868 pamphlet
Patent Document 6: Japanese Patent 2004-58082 JP
Patent Document 7: Japanese Patent 2005-248320 JP
Patent Document 8: WO 2008/132303 Issue pamphlet
Summary of the invention
Problems to be Solved by the Invention
[0014]
　As described above, it is not easy to manufacture a soft member or a member including a soft portion by hot stamping, and in particular, a low-strength hot stamp including a soft portion in a part or the whole and having excellent thermal stability. Manufacturing parts has been difficult in the prior art.
[0015]
　The present invention solves the above problems and is excellent in thermal stability. More specifically, the variation in strength (tensile strength) before and after the coating baking treatment accompanying the coating baking treatment is small, and the tensile strength is 700 MPa. It is an object of the present invention to provide a hot stamp molded product having a portion that is less than the above, a steel plate for hot stamp suitable as a material thereof, and a method for producing them.
Means for solving the problems
[0016]
　The present invention has been made to solve the above problems, and has as its gist a hot stamped product, a steel plate for hot stamping, and a method for producing them as described below.
[0017]
　(1) A hot stamp molded product according to an aspect of the present invention is a hot stamp molded product, wherein all or part of the hot stamp molded product is C: 0.001% or more and 0.080% by mass. %, Si: 2.50% or less, Mn: 0.01% or more and less than 0.50%, P: 0.200% or less, S: 0.0200% or less, sol. Al: 0.001 to 2.500%, N: 0.0200% or less, Cr: 0.30% to less than 2.00%, Ti: 0 to 0.300%, Nb: 0 to 0.300%, V: 0 to 0.300%, Zr: 0 to 0.300%, Mo: 0 to 2.00%, Cu: 0 to 2.00%, Ni: 0 to 2.00%, B: 0 to 0 0.0200%, Ca: 0 to 0.0100%, Mg: 0 to 0.0100%, REM: 0 to 0.1000%, Bi: 0 to 0.0500%, balance: Fe and chemical composition of impurities Having a metal structure in volume %, including ferrite: more than 60.0%, martensite: 0% or more and less than 10.0%, bainite: 0% or more and less than 20.0%, and the tensile strength is It is less than 700 MPa, and ΔTS, which is the amount of decrease in the tensile strength after heat treatment at 170° C. for 20 minutes, is 100 MPa or less.
　(2) In the hot stamp molded product according to (1), the chemical composition is, in mass %, Ti: 0.001 to 0.300%, Nb: 0.001 to 0.300%, V:0. One or more selected from 0.001 to 0.300% and Zr: 0.001 to 0.300% may be contained.
　(3) In the hot stamp molded product according to (1) or (2), the chemical composition is, by mass%, Mo: 0.001 to 2.00%, Cu: 0.001 to 2.00%. , And Ni: 0.001 to 2.00%, may be contained.
　(4) In the hot stamp molded article according to any one of (1) to (3), the chemical composition may contain B: 0.0001 to 0.0200% by mass.
　(5) In the hot stamp molded article according to any one of (1) to (4), the chemical composition is, by mass, Ca: 0.0001 to 0.0100%, Mg: 0.0001 to 0. One or more selected from 0.0100% and REM: 0.0001 to 0.1000%.
　(6) The hot-stamp molded article according to any one of (1) to (5) above may contain the chemical composition in mass% of Bi: 0.0001 to 0.0500%.
　(7) The hot stamp molded article according to any one of (1) to (6) above may have a plating layer on the surface.
　(8) The steel sheet for hot stamping according to another aspect of the present invention has a chemical composition, in mass %, of C: 0.001% or more and less than 0.080%, Si: 2.50% or less, Mn: 0. 01% or more and less than 0.50%, P: 0.200% or less, S: 0.0200% or less, sol. Al: 0.001 to 2.500%, N: 0.0200% or less, Cr: 0.30% to less than 2.00%, Ti: 0 to 0.300%, Nb: 0 to 0.300%, V: 0 to 0.300%, Zr: 0 to 0.300%, Mo: 0 to 2.00%, Cu: 0 to 2.00%, Ni: 0 to 2.00%, B: 0 to 0 0.0200%, Ca:0 to 0.0100%, Mg:0 to 0.0100%, REM:0 to 0.1000%, Bi:0 to 0.0500%, balance: Fe and impurities, metallographic structure Contains iron carbide, and the Mn content and Cr content in the iron carbide satisfy the following formula (i).
　[Mn] θ + [Cr] θ >2.5 (i)
　However, the meaning of each symbol in the above formula is as follows.
　[Mn] θ : Mn content in iron carbide in atomic% when the total content of Fe, Mn and Cr contained in the iron carbide is 100 atomic %.
　[Cr] θ : included in the iron carbide Cr content in iron carbide in atomic% when the total content of Fe, Mn, and Cr is 100 atomic%
　(9) In the steel sheet for hot stamping according to (8), the chemical composition is: In mass %, Ti: 0.001 to 0.300%, Nb: 0.001 to 0.300%, V: 0.001 to 0.300%, and Zr: 0.001 to 0.300%, You may contain 1 or more types selected from.
　(10) In the steel sheet for hot stamping according to the above (8) or (9), the chemical composition is% by mass, Mo: 0.001 to 2.00%, Cu: 0.001 to 2.00%. , And Ni: 0.001 to 2.00%, may be contained.
　(11) In the steel sheet for hot stamping according to any one of (8) to (10), the chemical composition may contain B: 0.0001 to 0.0200% in mass%.
　(12) In the steel sheet for hot stamping according to any one of the above (8) to (11), the chemical composition is% by mass, Ca: 0.0001 to 0.0100%, Mg: 0.0001 to 0. One or more selected from 0.0100% and REM: 0.0001 to 0.1000%.
　(13) In the steel sheet for hot stamping according to any one of (8) to (12), the chemical composition may contain, by mass%, Bi: 0.0001 to 0.0500%.
　(14) The steel sheet for hot stamping according to any one of (8) to (13) above may have a plating layer on the surface.
　(15) A method for producing a hot stamp molded article according to another aspect of the present invention is the method for producing the hot stamp molded article according to any one of (1) to (6) above, including (8) to A heating step of heating the hot stamping steel sheet according to any one of (13) to a heating temperature T° C., and a hot stamping step of hot stamping the hot stamping steel sheet after the heating step.
　(16) A method for producing a hot stamp molded article according to another aspect of the present invention is the method for producing the hot stamp molded article according to any one of (1) to (6) above, including (8) to A joining step of joining the hot stamping steel sheet according to any one of (13) to a joining steel sheet to form a joining steel sheet; a heating step of heating the joining steel sheet after the joining step to a heating temperature T° C.; A hot stamping step of hot stamping the joined steel sheet after the heating step.
　(17) A method for manufacturing a hot stamp molded product according to another aspect of the present invention is a method for manufacturing the hot stamp molded product according to (7) above, wherein the hot stamp steel plate according to (14) is used. A heating step of heating to a heating temperature T° C. and a hot stamping step of hot stamping the hot stamping steel plate after the heating step are provided.
　(18) A method for manufacturing a hot stamp molded product according to another aspect of the present invention is a method for manufacturing the hot stamp molded product according to (7) above, which is the hot stamping steel plate according to (14). A joining step of joining the joined steel sheet to a joined steel sheet, a heating step of heating the joined steel sheet after the joining step to a heating temperature T° C., and a hot stamping on the joined steel sheet after the heating step A hot stamping step.
　(19) In the method for producing a hot stamp molded product according to any one of (15) to (18), in the heating step, the heating temperature T° C. is a temperature higher than Ac 1 point of the hot stamping steel sheet. In the hot stamping step, the hot stamping starting temperature may be a temperature of (T-300)° C. or higher.
　(20) A method for producing a steel sheet for hot stamping according to another aspect of the present invention is the method for producing the steel sheet for hot stamping according to any of (8) to (14), wherein the chemical composition is mass. %, C: 0.001% or more and less than 0.080%, Si: 2.50% or less, Mn: 0.01% or more and less than 0.50%, P: 0.200% or less, S: 0.0200. % Or less, sol. Al: 0.001 to 2.500%, N: 0.0200% or less, Cr: 0.30% to less than 2.00%, Ti: 0 to 0.300%, Nb: 0 to 0.300%, V: 0 to 0.300%, Zr: 0 to 0.300%, Mo: 0 to 2.00%, Cu: 0 to 2.00%, Ni: 0 to 2.00%, B: 0 to 0 0.0200%, Ca: 0 to 0.0100%, Mg: 0 to 0.0100%, REM: 0 to 0.1000%, Bi: 0 to 0.0500%, balance: Fe and impurities as slabs Then, after hot rolling, a hot rolling step of winding in a temperature range of 800° C. or less to form a hot rolled steel sheet, and a hot rolled sheet annealing of heating the hot rolled steel sheet to a temperature range of more than 650° C. And a hot-rolled sheet annealing step of forming a hot-rolled annealed steel sheet.
　(21) The method for producing a steel sheet for hot stamping according to the above (20), further, one or both of cold rolling and annealing are optionally performed on the hot rolled annealed steel sheet after the hot rolled sheet annealing step. After performing the above, a plating step of performing plating may be provided.
Effect of the invention
[0018]
　According to the present invention, it is possible to obtain a hot-stamp molded article having a portion having a small strength variation (excellent thermal stability) due to coating baking processing and a tensile strength of less than 700 MPa.
Brief description of the drawings
[0019]
FIG. 1 is a schematic view showing the shape of a hot stamp molded product manufactured in Example 1.
FIG. 2 is a schematic diagram showing the shape of a hot stamp molded product manufactured in Example 2.
MODE FOR CARRYING OUT THE INVENTION
[0020]
　The inventors of the present invention have earnestly studied a method for suppressing a decrease in strength during hot baking of a hot stamp molded product having a tensile strength of less than 700 MPa. As a result, the following findings were obtained.
[0021]
　(A) When the metal structure of the hot stamped molded product contains a large amount of hard structure such as martensite or bainite, the tensile strength of the molded product is greatly reduced by the coating baking treatment. It is considered that this is because the hard structure is tempered and softened.
[0022]
　(B) On the other hand, even in the case of a hot stamp molded product having a low fraction of hard structure and a metal structure mainly composed of a soft structure containing ferrite, the tensile strength is greatly reduced by the coating baking treatment depending on the component composition. There are cases.
[0023]
　(C) In a hot stamp molded product having a metal structure mainly composed of a soft structure containing ferrite, the Mn content is limited to a low level and a predetermined amount of Cr is contained, and in a steel sheet before hot stamping, iron carbide By controlling the Mn content and Cr content in the medium to a certain level or more, the decrease in tensile strength due to the coating baking treatment is suppressed.
　The reason for this is not clear, but if the content of (a) Mn is excessive, the transformation temperature from austenite to ferrite decreases, and in the cooling process after hot stamping, fine iron carbide or fine iron is contained in the ferrite. When carbon clusters are generated and ferrite is hardened, (b) Cr is contained, and the Mn content in the iron carbide and the Cr content are set to a certain level or more, the iron carbide is stabilized, and fine particles in the ferrite are stabilized. The formation of iron carbides or fine iron-carbon clusters is suppressed, and (c) the fine iron carbides or fine iron-carbon clusters present in the ferrite are transformed into coarse iron carbides by the heat treatment during coating baking. It is presumed that this is due to the decrease in the strength of ferrite.
[0024]
　From the results of (A) to (C) above, for hot stamping in which the Mn content is limited to a low level and a certain amount or more of Cr is contained, and the Mn content and the Cr content in the iron carbide are controlled to be above a certain value. It was found that by hot stamping using a steel sheet, it is possible to manufacture a hot stamped product having a metal structure mainly composed of ferrite, having excellent thermal stability and having a small decrease in strength due to coating baking treatment.
　Hereinafter, a hot stamp molded product according to an embodiment of the present invention (hot stamp molded product according to the present embodiment), a steel plate for hot stamping suitable as a material thereof (a steel plate for hot stamping according to the present embodiment), and them Each requirement of the manufacturing method will be described in detail.
　
　All or part of the hot stamp molded product according to the present embodiment has the chemical composition shown below. The reasons for limiting each element are as follows. In the following description, “%” regarding the content means “mass %”. When the hot stamped product has a portion having a tensile strength of less than 700 MPa and a portion having a tensile strength of 700 MPa or more, at least the portion having a tensile strength of less than 700 MPa has the following chemical composition. All you have to do is do it.
[0026]
　C: 0.001% or more and less than 0.080%
　C is an element having the effect of increasing the tensile strength of the steel sheet after hot stamping (steel sheet included in hot stamped products). If the C content is less than 0.001%, the increase in tensile strength due to hot stamping cannot be expected. The preferred C content is 0.010% or more, 0.020% or more, or 0.030% or more.
　On the other hand, when the C content is 0.080% or more, the volume ratio of martensite and/or bainite increases in the metal structure after hot stamping, and the tensile strength of the hot stamped product becomes 700 MPa or more. In this case, even if the Mn and Cr contents are adjusted as described later, it becomes impossible to secure the thermal stability of the hot stamped product. Therefore, the C content is less than 0.080%. A preferred C content is less than 0.075%, less than 0.070%, less than 0.060%, or less than 0.050%.
[0027]
　Si: 2.50% or less
　Si is an element contained as an impurity in steel. If the Si content exceeds 2.50%, the weldability deteriorates and the transformation point becomes too high, making it difficult to heat the steel sheet to a temperature equal to or higher than the transformation point during the heating process of the hot stamp. Therefore, the Si content is set to 2.50% or less. A preferable Si content is 2.00% or less, 1.50% or less, or 1.00% or less. When a plated steel sheet is used as the hot stamping steel sheet, the Si content is preferably less than 0.50%, and more preferably less than 0.40% in order to secure the plating property.
　The lower limit of the Si content is not particularly limited, but excessively lowering the Si content causes an increase in steelmaking cost, so it is preferable to contain Si in an amount of 0.001% or more. Further, Si has the effect of increasing the tensile strength of the steel sheet after hot stamping, so Si may be positively contained. From the viewpoint of high strength, the preferable Si content is 0.10% or more, 0.20% or more, or 0.30% or more.
[0028]
　Mn: 0.01% or more and less than 0.50%
　Mn is an element that deteriorates the thermal stability of the hot stamped product. In particular, when the Mn content is 0.50% or more, the thermal stability of the molded product after hot stamping remarkably deteriorates. Therefore, the Mn content is less than 0.50%. The Mn content is preferably less than 0.40%, less than 0.35%, less than 0.30%, or less than 0.25%.
　On the other hand, Mn is an element that combines with S, which is an impurity, to form MnS, and has an action of suppressing the adverse effect of S. In order to obtain this effect, the Mn content is 0.01% or more. The Mn content is preferably 0.05% or more, 0.10% or more, or 0.15% or more.
[0029]
　P: 0.200% or less
　P is an element contained as an impurity in steel. When the P content exceeds 0.200%, the weldability and the toughness after hot stamping are significantly deteriorated, so the P content is set to 0.200% or less. A preferable P content is 0.100% or less, 0.050% or less, or 0.020% or less.
　Although the lower limit of the P content is not particularly limited, excessive reduction of the P content causes an increase in steelmaking cost, so it is preferable to contain 0.001% or more. Further, P has a function of increasing the tensile strength of the molded product after hot stamping, and thus may be positively contained. From the viewpoint of increasing the strength, the preferable P content is 0.010% or more, 0.020% or more, or 0.030% or more. When a plated steel sheet is used as the hot stamping steel sheet, the P content is preferably 0.05% or less, and more preferably 0.040% or less, in order to secure the plating property.
[0030]
　S: 0.0200% or less
　S is an element contained as an impurity in the steel and embrittles the steel. Therefore, the smaller the S content, the more preferable, but if the S content exceeds 0.0200%, the adverse effect thereof becomes particularly large, so the S content is set to 0.0200% or less. The preferred S content is 0.0100% or less, 0.0050% or less, or 0.0030% or less.
　Although the lower limit of the S content is not particularly limited, excessive reduction of the S content causes an increase in steelmaking cost, so 0.0001% or more is preferably contained.
[0031]
　sol. Al: 0.001 to 2.500%
　Al is an element having an action of deoxidizing molten steel. sol. If the Al content is less than 0.001%, deoxidation becomes insufficient. Therefore, sol. The Al content is 0.001% or more. sol. The Al content is preferably 0.010% or more, 0.020% or more, or 0.040% or more.
　On the other hand, sol. If the Al content is too high, the transformation point rises, and it becomes difficult to heat the steel sheet to a temperature equal to or higher than the transformation point during the heating process of the hot stamp. Therefore, sol. The Al content is 2.500% or less. sol. The Al content is preferably 1.000% or less, 0.500% or less, 0.100% or less, or 0.060% or less.
[0032]
　N: 0.0200% or less
　N is an element contained in steel as an impurity and forming a nitride during continuous casting of steel. Since this nitride deteriorates the toughness after hot stamping, a lower N content is preferable. If the N content is more than 0.0200%, the adverse effect thereof becomes particularly large, so the N content is set to 0.0200% or less. The N content is preferably less than 0.0100%, less than 0.0080%, or less than 0.0050%.
　Although the lower limit of the N content is not particularly limited, excessive reduction of the N content causes an increase in steelmaking cost, and therefore N is preferably contained in 0.001% or more.
[0033]
　Cr: 0.30% or more and less than 2.00%
　Cr is an element having an action of improving the thermal stability of a hot stamp molded product (steel plate after hot stamping) having a metal structure mainly composed of ferrite. .. When the Cr content is less than 0.30%, the effect due to the above-mentioned action cannot be sufficiently obtained. Therefore, the Cr content is 0.30% or more. The Cr content is preferably 0.50% or more, 0.70% or more, or 0.90% or more.
　On the other hand, when the Cr content is 2.00% or more, the volume ratio of martensite and/or bainite contained in the metal structure of the hot stamp molded product becomes excessive, and the thermal stability of the hot stamp molded product deteriorates. .. Therefore, the Cr content is less than 2.00%. The Cr content is preferably 1.50% or less, 1.20% or less, or 1.00% or less.
[0034]
　Furthermore, the thermal stability of the hot stamped article improves as the Mn content decreases and the Cr content increases. Therefore, the ratio ([Cr]/[Mn]) of the Cr content ([Cr]) and the Mn content ([Mn]) is preferably 1.00 or more. More preferably, it is 1.05 or more, 1.50 or more, 2.50 or more, or 3.00 or more.
[0035]
　Ti: 0 to 0.300%
　Nb: 0 to 0.300%
　V: 0 to 0.300%
　Zr: 0 to 0.300%
　Ti, Nb, V and Zr are hot stamped through the refinement of the metal structure. It is an element that has the effect of increasing the tensile strength of a product. In order to obtain this effect, one or more selected from Ti, Nb, V and Zr may be contained if necessary.
[0036]
　When it is desired to obtain the above effects, it is preferable to contain one or more selected from Ti, Nb, V and Zr in an amount of 0.001% or more. It is more preferable to contain at least one of 0.005% or more Ti, 0.005% or more Nb, 0.010% or more V, and 0.005% or more Zr.
[0037]
　When Ti is contained, the Ti content is more preferably 0.010% or more, and particularly preferably 0.020% or more. When Nb is contained, the Nb content is more preferably 0.020% or more, and particularly preferably 0.030% or more. When V is contained, the V content is more preferably 0.020% or more. When Zr is contained, the Zr content is more preferably 0.010% or more.
[0038]
　On the other hand, when the contents of Ti, Nb, V and Zr exceed 0.300%, respectively, the effect is saturated and the manufacturing cost of the steel sheet increases. Therefore, even when it is contained, the content of Ti, Nb, V, and Zr is each 0.300% or less.
　Further, when the contents of Ti, Nb, V and Zr are high, there is a concern that carbides of these elements are deposited in a large amount and the toughness after hot stamping is impaired. Therefore, the Ti content is preferably less than 0.060%, more preferably less than 0.040%. The Nb content is preferably less than 0.060%, more preferably less than 0.040%. The V content is preferably less than 0.200%, more preferably less than 0.100%. The Zr content is preferably less than 0.200%, more preferably less than 0.100%.
[0039]
　Mo: 0 to 2.00%
　Cu: 0 to 2.00%
　Ni: 0 to 2.00%
　Mo, Cu and Ni have the effect of increasing the tensile strength of the hot stamped product (steel plate after hot stamping). Have. Therefore, one or more selected from Mo, Cu and Ni may be contained if necessary.
[0040]
　In order to obtain the above effect, it is preferable to contain one or more selected from Mo, Cu and Ni in an amount of 0.001% or more. The preferable Mo content is 0.05% or more, the preferable Cu content is 0.10% or more, and the preferable Ni content is 0.10% or more.
[0041]
　On the other hand, when the contents of Mo, Cu and Ni exceed 2.00%, the volume ratio of martensite and/or bainite contained in the metal structure of the molded product after hot stamping becomes excessive and the hot stamped product Thermal stability is degraded.
　Therefore, even if it is contained, the contents of Mo, Cu and Ni are each set to 2.00% or less. A preferable Mo content is 0.50% or less, a preferable Cu content is 1.00% or less, and a preferable Ni content is 1.00% or less.
[0042]
　B: 0 to 0.0200%
　B is an element having a function of segregating at grain boundaries and improving the toughness of the steel sheet after hot stamping. In order to obtain this effect, you may contain as needed.
[0043]
　In order to obtain the above effect, the B content is preferably 0.0001% or more. The B content is more preferably 0.0006% or more, still more preferably 0.0010% or more.
[0044]
　On the other hand, when the B content exceeds 0.0200%, the volume ratio of martensite and/or bainite contained in the metal structure of the hot stamp molded product becomes excessive, and the thermal stability of the hot stamp molded product deteriorates. Therefore, even if it is contained, the B content is 0.0200% or less. The B content is preferably 0.0050% or less, more preferably 0.0030% or less.
[0045]
　Ca: 0 to 0.0100%
　Mg: 0 to 0.0100%
　REM: 0 to 0.1000%
　Ca, Mg and REM have the function of improving the toughness after hot stamping by adjusting the shape of inclusions. It is an element that has. Therefore, you may contain as needed. In order to obtain the above effect, it is preferable to contain one or more selected from Ca, Mg and REM in an amount of 0.0001% or more.
　On the other hand, when the content of Ca or Mg is more than 0.0100%, or when the content of REM is more than 0.1000%, the effect is saturated and excessive cost is generated. Therefore, even when it is contained, the Ca and Mg contents are each 0.0100% or less, and the REM content is 0.1000% or less.
[0046]
　In the present embodiment, REM refers to a total of 17 elements of Sc, Y and lanthanoid, and the REM content means the total content of these elements. Lanthanoids are industrially added in the form of misch metal.
[0047]
　Bi: 0 to 0.0500%
　Bi is an element having an action of improving the toughness after hot stamping by refining the solidified structure. Therefore, you may contain as needed. In order to obtain the above effect, the Bi content is preferably 0.0001% or more. The Bi content is more preferably 0.0003% or more, still more preferably 0.0005% or more.
　On the other hand, when the Bi content exceeds 0.0500%, the above effect is saturated and an excessive cost occurs. Therefore, even if it is contained, the Bi content is 0.0500% or less. The Bi content is preferably 0.0100% or less, more preferably 0.0050% or less.
[0048]
　In the above chemical composition, the balance is Fe and impurities. Here, the "impurities" are components that are mixed by ores, raw materials such as scrap, and various factors of the manufacturing process when industrially manufacturing a steel sheet, and are allowed within a range that does not adversely affect the present invention. Means something.
[0049]
　
　The metal structure of the hot stamp molded product according to the present embodiment will be described. All or part of the hot stamp molded product according to the present embodiment has a metal structure containing the amounts of ferrite, martensite, and bainite shown below. In the following description regarding the metal structure, “%” means “volume ratio %”.
[0050]
　Ferrite: 60.0% If
　the volume ratio of super ferrite is 60.0% or less, the tensile strength of the molded product after hot stamping becomes 700 MPa or more, and thermal stability cannot be secured. Therefore, the volume ratio of ferrite is set to more than 60.0%. The volume ratio of ferrite is preferably more than 70.0%, more preferably more than 80.0%.
　The upper limit of the volume ratio of ferrite is not particularly limited, but in order to increase the strength of the hot stamped product, it is preferably less than 98.0%, more preferably less than 96.0%, and 94 More preferably, it is less than 0.0%.
　The above-mentioned ferrite includes, in addition to polygonal ferrite, pseudo-polygonal ferrite having a higher dislocation density than polygonal ferrite, granular bainitic ferrite, and acicular ferrite having sawtooth grain boundaries. From the viewpoint of thermal stability, the ratio of polygonal ferrite to the whole ferrite is preferably 10.0% or more by volume.
[0051]
　Martensite: 0% or more and less than 10.0%
　Bainite: 0% or more and less than 20.0% When the
　metal structure contains martensite and bainite, the thermal stability of the hot stamped product deteriorates. Therefore, the volume ratio of martensite is less than 10.0% and the volume ratio of bainite is less than 20.0%. The volume ratio of martensite is preferably less than 5.0%, more preferably less than 2.0%, and even more preferably less than 1.0%. The volume ratio of bainite is preferably less than 10.0%, more preferably less than 5.0%, and even more preferably less than 2.0%.
[0052]
　Since martensite and bainite are not necessarily contained, the lower limit of the volume ratio of martensite and bainite is 0%.
　However, since martensite and bainite have the effect of increasing the strength of the hot stamped product, they may be contained in the metal structure within the above range. If the volume ratio of martensite and bainite is less than 0.1%, the effect due to the above action cannot be sufficiently obtained. Therefore, when increasing the strength, the lower limit values ​​of the volume ratios of martensite and bainite are both preferably 0.1% or more, and more preferably 0.5% or more.
[0053]
　The balance of the metal structure may contain pearlite or retained austenite, and may further contain precipitates such as cementite. Since it is not necessary to contain pearlite, residual austenite and precipitates, the lower limit of the volume ratio of pearlite, residual austenite and precipitates is 0%.
[0054]
　Since pearlite has an effect of increasing the strength of the hot stamped product, when increasing the strength, the volume ratio of pearlite is preferably 1.0% or more, and more preferably 2.0% or more. It is more preferably 5.0% or more.
　On the other hand, when pearlite is excessively contained, the toughness after hot stamping deteriorates. Therefore, the volume ratio of pearlite is preferably 20.0% or less, more preferably 10.0% or less.
[0055]
　The retained austenite has a function of improving the impact absorption of the hot stamped product. Therefore, when obtaining this effect, the volume ratio of retained austenite is preferably 0.5% or more, and more preferably 1.0% or more.
　On the other hand, if the residual austenite is excessively contained, the toughness after hot stamping is lowered. Therefore, the volume ratio of retained austenite is preferably 5.0% or less, more preferably 3.0% or less.
[0056]
　In this embodiment, the volume ratio of each metal structure is calculated as follows.
　First, after taking a test piece from the hot stamped product and polishing the longitudinal section parallel to the rolling direction of the steel plate, in the case of a non-plated steel plate, a 1/4 depth position of the steel plate from the steel plate surface, plating In the case of a steel plate, the structure is observed from the boundary between the steel plate of the base material and the plating layer at a depth position of ¼ of the plate thickness of the steel plate that is the base material. When the hot stamped article has a portion having a tensile strength of less than 700 MPa and a portion having a tensile strength of 700 MPa or more, a specimen is taken from the portion having a tensile strength of less than 700 MPa and observed. I do.
　Specifically, after the polished surface is corroded by Nital, the structure is observed using an optical microscope and a scanning electron microscope (SEM), and the obtained structure photograph is subjected to image analysis to obtain ferrite and pearlite, respectively. And the total area ratio of bainite, martensite, and retained austenite. After that, after repeller corrosion was performed on the same observation position, the structure was observed using an optical microscope and a scanning electron microscope (SEM), and image analysis was performed on the obtained structure photograph to obtain residual austenite. Calculate the total area ratio of martensite.
　Further, at the same observation position, the longitudinal cross section is electrolytically polished, and then the area ratio of the retained austenite is measured by using an SEM equipped with an electron beam backscattering pattern analyzer (EBSP).
　Based on these results, the area ratios of ferrite and pearlite, bainite, martensite, and retained austenite are obtained. Then, assuming that the area ratio is equal to the volume ratio, the measured area ratio is set as the volume ratio of each tissue.
[0057]
　
　All or part of the hot stamp molded product according to the present embodiment has a tensile strength of the base steel sheet of less than 700 MPa. This is because if the tensile strength is 700 MPa or more, the thermal stability of the hot stamp molded product cannot be secured. Preferably, the tensile strength of the hot stamped article is less than 600 MPa or less than 560 MPa in all or part thereof. On the other hand, in order to improve the impact absorption of the hot stamped product, the tensile strength of the hot stamped product is preferably 440 MPa or more, more preferably 490 MPa or more.
[0058]
　In the hot stamped molded product according to the present embodiment, a soft part having a tensile strength of less than 700 MPa and a hard part having a tensile strength of 700 MPa or more may be mixed in the molded product. By providing the parts having different strengths, it becomes possible to control the deformation state of the hot stamp molded product at the time of collision, and it is possible to improve the impact absorbability of the molded product. The hot stamped product having the portions having different strengths can be manufactured by hot stamping after joining two or more types of steel plates having different component compositions, as described later.
[0059]
In the hot stamp molded product according to the
　present embodiment, the amount of decrease in the tensile strength with respect to the tensile strength before hot stamping when subjected to heat treatment at 170° C. for 20 minutes ( ΔTS) is 100 MPa or less. ΔTS is preferably 60 MPa or less, and more preferably 30 MPa or less.
　In hot stamped products with a structure mainly composed of ferrite, the reason for the decrease in strength during paint baking is that the fine iron carbide or fine iron-carbon clusters present in ferrite are coarse iron due to heat treatment during paint baking. It is considered that this is because it changes to carbide. Although it is not easy to directly and quantitatively evaluate the state of existence of these fine iron carbides or fine iron-carbon clusters, the amount of decrease in tensile strength after heat treatment at 170° C. for 20 minutes ( It can be evaluated indirectly by ΔTS). When ΔTS is 100 MPa or less, generation of fine iron carbide or fine iron-carbon clusters in ferrite is suppressed, and it is judged that the thermal stability is excellent.
[0060]
　
　The hot stamp molded article according to the present embodiment may have a plating layer on the surface. By providing a plating layer on the surface, it is possible to prevent scale formation during hot stamping and further improve the corrosion resistance of the hot stamped product. The type of plating is not particularly limited as long as it is suitable for the above purpose. The plating layer of the hot stamp molded product can be formed by hot stamping using a plated steel sheet, as described later. Examples of the type of the plated layer include a zinc-based plated layer and an aluminum-based plated layer that are hot stamped using a zinc-based plated steel sheet and an aluminum-based plated steel sheet.
[0061]
　A steel plate for hot stamping suitable for manufacturing the above hot stamped product will be described.
Since the chemical composition
　does not substantially change by hot stamping, the chemical composition of the steel sheet for hot stamping has the same chemical composition as the above-mentioned hot stamped product.
[0062]
　<
　Metal Structure of Steel Sheet for Hot Stamping > The metal structure of the steel sheet for hot stamping according to the present embodiment contains iron carbide, and the chemical composition of the iron carbide (Mn content and Cr content in the iron carbide) is as follows (i. ) Satisfies the formula.
　[Mn] θ +[Cr] θ >2.5 (i)
　However, the meaning of each symbol in the above formula is as follows.
　[Mn] θ : Mn content (atomic %) in the iron carbide when the total content of Fe, Mn and Cr contained in the iron carbide is 100 atomic %.
　[Cr] θ : Fe contained in the iron carbide Content (atomic %) in the iron carbide when the total content of Al, Mn and Cr is 100 atomic %.
[0063]
　When the chemical composition of the iron carbide contained in the metal structure of the steel sheet for hot stamping satisfies the above formula (i), it becomes possible to improve the thermal stability of the steel sheet after hot stamping. The value on the left side of the above formula (i) is preferably more than 3.0, and more preferably more than 4.0.
[0064]
　On the other hand, in order to increase the Mn content and the Cr content in the iron carbide, it is necessary to anneal the hot-rolled steel sheet at a high temperature in the hot-rolled sheet annealing step described later, which impairs the productivity of the steel sheet. Therefore, the value on the left side of the above formula (i) is preferably less than 30.0, and more preferably less than 20.0.
[0065]
　In the present embodiment, the chemical composition of iron carbide is measured by the following procedure.
　First, a test piece is sampled from an arbitrary position of a steel sheet, a longitudinal section parallel to the rolling direction of the steel sheet is polished, and then a precipitate is extracted from the surface of the steel sheet by a replica method at a depth position of ¼ of the sheet thickness. The precipitate is observed using a transmission electron microscope (TEM), and the precipitate is identified and the composition is analyzed by electron beam diffraction and energy dispersive X-ray analysis (EDS).
[0066]
　Quantitative analysis of iron carbide by EDS was carried out for three elements of Fe, Mn and Cr, and the Mn content (atomic%) and the Cr content (atomic %) when the total content thereof was 100 atomic %. , [Mn] θ and [Cr] θ , respectively . This quantitative analysis is performed on a plurality of iron carbides, and the average value thereof is taken as the Mn content and Cr content in the iron carbide in the steel sheet. The number of iron carbides to be measured is 10 or more, and the larger the number of measurements, the more preferable. The iron carbide includes, in addition to the cementite forming pearlite, the cementite existing independently in the metal structure.
[0067]
　In the present embodiment, in the case of a hot-rolled annealed steel sheet, a cold-rolled steel sheet or an annealed steel sheet, a 1/4 depth position of the sheet thickness from the steel sheet surface, and in the case of a galvanized steel sheet, it is determined from the boundary between the steel sheet as a base material and the plating layer. The above-described metallographic structure is defined at the position of 1/4 depth of the plate thickness of the steel plate as the material.
[0068]
　The volume fraction of iron carbide does not have to be specified in particular, but the volume fraction of iron carbide is preferably 1% or more and 3% or more in order to refine the metal structure after hot stamping to increase the tensile strength. Is more preferable.
　On the other hand, if the volume fraction of iron carbide is excessive, the tensile strength of the steel sheet after hot stamping becomes too high and the thermal stability is impaired. Therefore, the volume fraction of iron carbide is preferably 20% or less, and more preferably 15% or less.
[0069]
　The balance of the metal structure of the steel sheet for hot stamping according to the present embodiment is mainly ferrite, but may include martensite, tempered martensite, bainite and retained austenite, and further, a precipitate other than iron carbide. May be included. However, since martensite, tempered martensite, bainite and retained austenite deteriorate the toughness after hot stamping, the smaller the volume fraction of these structures, the more preferable. The volume ratios of martensite, tempered martensite, bainite and retained austenite are all preferably less than 1.0%, more preferably less than 0.5%.
The scope of the claims
[Claim 1]
　A hot stamped article,
　wherein all or part of the hot stamped article is, in
　mass %,
　C: 0.001% or more and less than 0.080%,
　Si: 2.50% or less,
　Mn: 0.01 % Or more and less than 0.50%,
　P: 0.200% or less,
　S: 0.0200% or less,
　sol. Al: 0.001 to 2.500%,
　N: 0.0200% or less,
　Cr: 0.30% or more and less than 2.00%,
　Ti: 0 to 0.300%,
　Nb: 0 to 0.300%,
　V: 0 to 0.300%,
　Zr: 0 to 0.300%,
　Mo: 0 to 2.00%,
　Cu: 0 to 2.00%,
　Ni: 0 to 2.00%,
　B: 0 to 0 0.0200%,
　Ca: 0-0.0100%,
　Mg: 0-0.0100%,
　REM:
　0-0.1000%, Bi:0-0.0500%,
　balance: Fe and impurities Have,
　The metallographic structure includes volume%,
　　ferrite: more than 60.0%,
　　martensite: 0% or more and less than 10.0%,
　　bainite: 0% or more and less than 20.0%, and
　tensile strength is less than 700 MPa.
　There, after the heat treatment of 20 minutes at 170 ° C., said a decrease in tensile strength ΔTS is less than 100 MPa,
　hot stamping molded article.
[Claim 2]
　The chemical composition is mass%
　: Ti: 0.001 to 0.300%,
　Nb: 0.001 to 0.300%,
　V: 0.001 to 0.300%, and
　Zr: 0.001 to 　The hot stamp molded article according to claim 1
　, containing one or more selected from 0.300%
.
[Claim 3]
　The chemical composition in mass% 　is one selected from
　Mo: 0.001 to 2.00%,
　Cu: 0.001 to 2.00%, and
　Ni: 0.001 to 2.00%. 　The hot stamp molded product according to claim 1 or 2,
containing the above
.
[Claim 4]
　The hot stamp molded article according to any one of claims 1 to 3 　, wherein the chemical composition 　contains
　B: 0.0001 to 0.0200% in mass% .

[Claim 5]
　The chemical composition, in mass%, 　is one selected from
　Ca: 0.0001 to 0.0100%,
　Mg: 0.0001 to 0.0100%, and
　REM: 0.0001 to 0.1000%. 　The hot stamp molded product according to any one of claims 1 to 4
, containing the above
.
[Claim 6]
　The hot stamp molding according to any one of claims 1 to 5 　, wherein the chemical composition 　contains , by mass%,
　Bi: 0.0001 to 0.0500% .

[Claim 7]

　The hot stamp molded product according to any one of claims 1 to 6 　, which has a plating layer on its surface .
[Claim 8]
　The chemical composition is% by mass,
　C: 0.001% or more and less than 0.080%,
　Si: 2.50% or less,
　Mn: 0.01% or more and less than 0.50%,
　P: 0.200% or less,
　S: 0.0200% or less,
　sol. Al: 0.001 to 2.500%,
　N: 0.0200% or less,
　Cr: 0.30% or more and less than 2.00%,
　Ti: 0 to 0.300%,
　Nb: 0 to 0.300%,
　V: 0 to 0.300%,
　Zr: 0 to 0.300%,
　Mo: 0 to 2.00%,
　Cu: 0 to 2.00%,
　Ni: 0 to 2.00%,
　B: 0 to 0 0.0200%,
　Ca: 0 to 0.0100%,
　Mg: 0 to 0.0100%,
　REM: 0 to 0.1000%,
　Bi: 0 to 0.0500%,
　balance: Fe and impurities,
　metallic structure Is a
　steel sheet for hot stamping , containing iron carbide, and the Mn content and Cr content in the iron carbide satisfy the following formula (i) .
　[Mn] θ +[Cr] θ >2.5 (i)
　However, the meaning of each symbol in the above formula is as follows.
　[Mn] θ : Mn content in the iron carbide in atomic% when the total content of Fe, Mn and Cr contained in the iron carbide is 100 atomic%
　[Cr] θ : included in the iron carbide Cr content in iron carbide in atomic% when the total content of Fe, Mn and Cr is 100 atomic%
[Claim 9]
　The chemical composition is mass %,
　Ti: 0.001 to 0.300%,
　Nb: 0.001 to 0.300%,
　V: 0.001 to 0.300%, and
　Zr: 0.001 to The 　steel sheet for hot stamping according to claim 8,
　containing at least one selected from 0.300%
.
[Claim 10]
　The chemical composition, in mass%, 　is one selected from
　Mo: 0.001 to 2.00%,
　Cu: 0.001 to 2.00%, and
　Ni: 0.001 to 2.00%.
The
　steel sheet for hot stamping according to claim 8 or 9, containing the above .
[Claim 11]
　The 　steel sheet for hot stamping according to any one of claims 8 to 10 , wherein the chemical composition 　contains
　B: 0.0001 to 0.0200% in mass% .

[Claim 12]
　The chemical composition in mass% 　is one selected from
　Ca: 0.0001 to 0.0100%,
　Mg: 0.0001 to 0.0100%, and
　REM: 0.0001 to 0.1000%.
The
　steel sheet for hot stamping according to any one of claims 8 to 11 , containing the above .
[Claim 13]
　The 　steel sheet for hot stamping according to any one of claims 8 to 12 , wherein the chemical composition 　contains , by mass%,
　Bi: 0.0001 to 0.0500% .

[Claim 14]
　The
　steel sheet for hot stamping according to any one of claims 8 to 13 , which has a plating layer on its surface .
[Claim 15]
　A method for producing the hot stamped product according to any one of claims 1 to 6, wherein
　the hot stamping steel plate according to any one of claims 8 to 13 is heated. A 　method for producing
　a
hot stamp molded product , comprising: a heating step of heating to a temperature T° C .; and a hot stamp step of hot stamping the hot stamping steel plate after the heating step .
[Claim 16]
　A method for producing the hot stamped product according to any one of claims 1 to 6, wherein
　the hot stamping steel plate according to any one of claims 8 to 13 is used. A joining
　step of joining the joined steel sheet to a joined steel sheet, a heating step of heating the joined steel sheet after the joining step to a heating temperature T° C., and a
　hot stamping of the joined steel sheet after the heating step. A stamping step, and a
　method for manufacturing a hot stamped article.
[Claim 17]
　A method for producing the hot stamped product according to
　claim 7,
　wherein the heating step of heating the hot stamping steel sheet according to claim 14 to a heating temperature T° C. and the steel sheet after the heating step are performed. A hot stamping step of performing hot stamping, and a
method for manufacturing a hot stamped article.
[Claim 18]
　A method for producing the hot stamped product according to
　claim 7,
　comprising a joining step of joining the hot stamping steel sheet according to claim 14 to a joining steel sheet to form a joined steel sheet, and a method after the joining step. A 　method for producing
　a
hot stamped product , comprising: a heating step of heating the bonded steel sheet to a heating temperature T° C .; and a hot stamping step of hot stamping the bonded steel sheet after the heating step .
[Claim 19]
　In the heating step, the heating temperature T° C. is a temperature higher than Ac 1 point of the hot stamping steel sheet , and in the
　hot stamping step, the hot stamping starting temperature is (T-300)° C. or higher.
　A method for manufacturing a hot stamp molded product according to any one of claims 15 to 18.
[Claim 20]
　A method for producing the steel sheet for hot stamping according to any one of claims 8 to 14, wherein the
　chemical composition is C: 0.001% or more and less than 0.080%, Si in mass%. : 2.50% or less, Mn: 0.01% or more and less than 0.50%, P: 0.200% or less, S: 0.0200% or less, sol. Al: 0.001 to 2.500%, N: 0.0200% or less, Cr: 0.30% to less than 2.00%, Ti: 0 to 0.300%, Nb: 0 to 0.300%, V:0 to 0.300%, Zr:0 to 0.300%, Mo:0 to 2.00%, Cu:0 to 2.00%, Ni:0 to 2.00%, B:0 to 0 0.0200%, Ca: 0 to 0.0100%, Mg: 0 to 0.0100%, REM: 0 to 0.1000%, Bi: 0 to 0.0500%, balance: Fe and impurities as slabs Then, after hot rolling, a hot rolling step of winding in a temperature range of 800° C. or lower to form a hot rolled steel sheet, and a hot rolled
　sheet annealing of heating the hot rolled steel sheet to a temperature range of higher than 650° C. And a hot-rolled sheet annealing step to obtain a hot-rolled annealed steel sheet
　.
[Claim 21]
　Furthermore, the hot rolled annealed steel sheet after the hot rolled sheet annealing step is
　optionally provided with a plating step of performing plating after performing one or both of cold rolling and annealing
　. Method for manufacturing steel sheet for hot stamping.