Specification
Title of the Invention: Hot Stamp Molding
Technical field
[0001]
　The present invention relates to hot stamped bodies.
　This application claims priority based on Japanese Patent Application No. 2020-002408 filed in Japan on January 9, 2020, the content of which is incorporated herein.
Background technology
[0002]
　In recent years, from the viewpoint of environmental protection and resource saving, there is a demand for weight reduction of automobile bodies, and high-strength steel sheets are applied to automobile members. Automobile parts are manufactured by press forming, but as the strength of steel sheets increases, not only does the forming load increase, but formability also decreases. Therefore, high-strength steel sheets have a problem of formability into members having complicated shapes. In order to solve such problems, the application of hot stamping technology, in which press forming is performed after heating the steel sheet to a high temperature in the austenite region at which the steel sheet is softened, has been promoted. Hot stamping is attracting attention as a technology that achieves both formability and strength of automobile parts by performing quenching treatment in a mold at the same time as press working.
[0003]
　Automobile parts made by hot stamping steel sheets must have high strength and excellent collision resistance in order to obtain a higher weight reduction effect.
[0004]
　Patent Document 1 discloses a hot-dip galvanized steel sheet with improved strength, uniform deformability, and local deformability by containing 10% by volume or more of retained austenite stabilized by enriching C and Mn. and alloyed hot-dip galvanized steel sheets and methods for their manufacture are disclosed.
[0005]
　In Patent Document 2, by including 10% by volume or more of retained austenite and including high-temperature tempered martensite and low-temperature tempered martensite at a predetermined volume ratio, strength, uniform deformability, and local deformability are improved. An improved galvannealed steel sheet is disclosed.
[0006]
　Patent Document 3 discloses a high-strength hot press-formed member with improved ductility and bendability by making the structure of steel a composite structure and controlling the ratio of each structure that constitutes the composite structure. there is
[0007]
　From a safety point of view, there is a desire for an automotive component that has superior strength and better crash performance than the prior art.
prior art documents
patent literature
[0008]
Patent Document 1: Japanese Patent Application Publication No. 2017-53001
Patent Document 2: International Publication No. 2016/199922
Patent Document 3: International Publication No. 2018/033960
SUMMARY OF THE INVENTION
Problems to be Solved by the Invention
[0009]
　本発明は、強度および衝突特性に優れたホットスタンプ成形体を提供することを目的とする。
課題を解決するための手段
[0010]
　The gist of the present invention is as follows.
[1] A hot-stamped article according to an aspect of the present invention has a chemical composition, in mass %, of
C: 0.15 to 1.00%,
Si: 0.50 to 3.00%,
Mn: 3.0%. More than 00%, 5.00% or less
Al: 0.100 to 3.000%
Co: 0.100 to 3.000%
P: 0.100% or less
S: 0.1000% or less
N: 0.0100% or less,
Nb: 0-0.15%,
Ti: 0-0.150%,
Mo: 0-1.00%,
Cr: 0-1.00%,
Cu: 0-1.00% ,
V: 0 to 1.00%,
W: 0 to 1.00%,
Ni: 0 to 3.00%,
Mg: 0 to 1.00%,
Zr: 0 to 1.00%,
Sb: 0 to 1.00%,
Ca: 0 to 0.10%,
REM: 0 to 0.30%, and
B: 0 to 0.0100%, the
balance being Fe and impurities,
　In terms of area ratio, it consists of retained austenite of 10% or more and less than 20%, fresh martensite of 5 to 15%, bainite and tempered martensite of 65 to 85% in total, and residual structure of less than 5%. ,
　the length of the grain boundary at which the rotation angle is 4° to 12° with the <011> direction as the rotation axis of the grain boundary of the bainite and the tempered martensite, and the rotation angle is 49° to 54°. The ratio of the grain boundary length at which the rotation angle is 55° to 75° to the total length of the grain boundary length at which the rotation angle is 55° to 75° It has a microstructure that is greater than or equal to 30%.
[2] The hot stamped body according to [1] above has the chemical composition, in mass%, of
Nb: 0.01 to 0.15%,
Ti: 0.010 to 0.150%,
Mo: 0 .005-1.00%
Cr: 0.005-1.00%
Cu: 0.001-1.00%
V: 0.0005-1.00%
W: 0.001-1.00 %,
Ni: 0.001 to 3.00%,
Mg: 0.001 to 1.00%,
Zr: 0.001 to 1.00%,
Sb: 0.001 to 1.00%,
Ca: 0.001 to 1.00%. 001-0.10%,
REM: 0.001-0.30%, and
B: 0.0005-0.0100%
You may contain 1 type(s) or 2 or more types out of the group which consists of.
Effect of the invention
[0011]
　According to the aspect of the present invention, it is possible to obtain a hot-stamped article having excellent strength and impact resistance.
MODE FOR CARRYING OUT THE INVENTION
[0012]
　The inventors have found that the microstructure of the hot-stamped compact contains a predetermined amount of retained austenite, fresh martensite, bainite and tempered martensite, and the grain boundaries of said bainite and said tempered martensite Among them, the length of the grain boundary with a rotation angle of 4 ° to 12 ° with the rotation axis in the <011> direction, the length of the grain boundary with a rotation angle of 49 ° to 54 °, and the rotation angle of 55 ° to 75 ° The length of the grain boundary (high-angle grain boundary) with a rotation angle of 55 ° to 75 ° with respect to the total length of the grain boundary (hereinafter sometimes referred to as a large-angle grain boundary) It has been found that by setting the thickness ratio to 30% or more, it is possible to improve the collision characteristics while ensuring high strength. In the present embodiment, "excellent collision characteristics" refers to excellent strain dispersion characteristics and bendability.
[0013]
　The high-angle grain boundary is the grain boundary with the highest angle among the grain boundaries included in the crystal grains of bainite and tempered martensite. When transforming from austenite to bainite or martensite, strain accompanying the transformation occurs. If the austenite before transformation has a high hardness, or if the prior austenite grains are in a state where they cannot be easily deformed, large-angle grain boundaries that are highly effective in relieving strain are likely to be formed. The inventors of the present invention have found that by holding in a low temperature range after hot stamping, the prior austenite grains can be transformed into bainite or martensite while increasing the hardness of the former austenite grains, and that many high-angle grain boundaries can be formed. .
[0014]
　The hot-stamped article according to this embodiment will be described in detail below. First, reasons for limiting the chemical composition of the hot stamped body according to the present embodiment will be described.
　In addition, the lower limit value and the upper limit value are included in the numerical limitation range described below between "-". Numerical values ​​indicated as "less than" and "greater than" do not include the value within the numerical range. All percentages in the chemical composition are percentages by weight.
[0015]
　The hot stamped body according to the present embodiment has a chemical composition in mass% of C: 0.15 to 1.00%, Si: 0.50 to 3.00%, Mn: more than 3.00%, 5 .00% or less, Al: 0.100 to 3.000%, Co: 0.100 to 3.000%, P: 0.100% or less, S: 0.1000% or less, N: 0.0100% or less , and the balance: Fe and impurities. Each element will be described in detail below.
[0016]
"C: 0.15 to 1.00%"
　C is an element that improves the strength of the hot stamped product. C is also an element that stabilizes retained austenite. If the C content is less than 0.15%, the desired strength cannot be obtained in the hot stamped product. Therefore, the C content is made 0.15% or more. The C content is preferably 0.30% or more, more preferably 0.45% or more. On the other hand, if the C content exceeds 1.00%, the steel becomes embrittled. Therefore, the C content is set to 1.00% or less. The C content is preferably 0.80% or less, or 0.70% or less.
[0017]
"Si: 0.50 to 3.00%"
　Si is an element that stabilizes retained austenite. If the Si content is less than 0.50%, the above effect cannot be obtained, the stabilization of retained austenite becomes insufficient, and a desired amount of retained austenite cannot be obtained. Therefore, the Si content is set to 0.50% or more. The Si content is preferably 1.00% or more and 1.40% or more. On the other hand, if the Si content exceeds 3.00%, the amount of ferrite increases and the desired microstructure cannot be obtained. Therefore, the Si content is set to 3.00% or less. The Si content is preferably 2.50% or less, or 2.00% or less.
[0018]
"Mn: more than 3.00% and 5.00% or less"
　Mn is an element that promotes bainite transformation in a low temperature range by lowering the Ms point. If the Mn content is 3.00% or less, a desired amount of high-angle grain boundaries cannot be obtained. Therefore, the Mn content should be more than 3.00%. The Mn content is preferably 3.20% or more and 3.30% or more. On the other hand, when the Mn content exceeds 5.00%, premature breakage tends to occur. Therefore, the Mn content is set to 5.00% or less. The Mn content is preferably 4.50% or less, or 4.00% or less.
[0019]
"Al: 0.100 to 3.000%"
　Al deoxidizes molten steel and suppresses the formation of oxides that act as starting points for fracture, thereby improving deformability and improving the collision characteristics of hot stamped bodies. It is an element that enhances. If the Al content is less than 0.100%, deoxidation is not sufficiently performed, coarse oxides are formed, and the above effects cannot be obtained. Therefore, the Al content is set to 0.100% or more. The Al content is preferably 0.120% or more, 0.200% or more, or 0.300% or more. On the other hand, when the Al content exceeds 3.000%, coarse oxides are formed in the steel, and the impact resistance of the hot stamped body is deteriorated. Therefore, the Al content is set to 3.000% or less. The Al content is preferably 2.500% or less, 2.000% or less, 1.500% or less, or 1.000% or less.
[0020]
"Co: 0.100 to 3.000%"
　Co is an element that promotes bainite transformation in a low temperature range by lowering the Ms point. If the Co content is less than 0.100%, the desired amount of bainite cannot be obtained. Therefore, the Co content is set to 0.100% or more. Co content is preferably 0.110% or more, or 0.120% or more. On the other hand, if the Co content exceeds 3.000%, premature breakage tends to occur. Therefore, the Co content is set to 3.000% or less. The Co content is preferably 2.000% or less, or 1.6000% or less.
[0021]
「Ｐ：０．１００％以下」
　Ｐは、不純物元素であり、粒界に偏析することで破壊の起点となる。そのため、Ｐ含有量は０．１００％以下とする。Ｐ含有量は、好ましくは０．０５０％以下、または０．０３０％以下である。Ｐ含有量の下限は特に限定しないが、０．０００１％未満に低減すると、脱Ｐコストが大幅に上昇し、経済的に好ましくないため、実操業上、０．０００１％を下限としてもよい。
[0022]
「Ｓ：０．１０００％以下」
　Ｓは、不純物元素であり、鋼中に介在物を形成する。この介在物は破壊の起点となるため、Ｓ含有量は０．１０００％以下とする。Ｓ含有量は、好ましくは０．０５００％以下、０．０２００％以下、または０．０１００％以下である。Ｓ含有量の下限は特に限定しないが、０．０００１％未満に低減すると、脱Ｓコストが大幅に上昇し、経済的に好ましくないため、実操業上、０．０００１％を下限としてもよい。
[0023]
「Ｎ：０．０１００％以下」
　Ｎは、不純物元素であり、鋼中に窒化物を形成する。この窒化物は破壊の起点となるため、Ｎ含有量は０．０１００％以下とする。Ｎ含有量は、好ましくは０．００５０％以下、または０．００４０％以下である。Ｎ含有量の下限は特に限定しないが、０．０００１％未満に低減すると、脱Ｎコストが大幅に上昇し、経済的に好ましくないため、実操業上、０．０００１％を下限としてもよい。
[0024]
　本実施形態に係るホットスタンプ成形体の化学組成の残部は、Ｆｅ及び不純物であってもよい。不純物としては、鋼原料もしくはスクラップから及び／又は製鋼過程で不可避的に混入し、本実施形態に係るホットスタンプ成形体の特性を阻害しない範囲で許容される元素が例示される。
[0025]
　本実施形態に係るホットスタンプ成形体は、Ｆｅの一部に代えて、任意元素として、以下の元素を含有してもよい。以下の任意元素を含有しない場合の含有量は０％である。
[0026]
「Ｎｂ：０～０．１５％」
「Ｔｉ：０～０．１５０％」
　ＮｂおよびＴｉは、ホットスタンプ前の加熱において旧オーステナイト粒を細粒化し、オーステナイトからベイナイトまたはマルテンサイトへの変態時に旧オーステナイト粒の変形を抑制することで、大傾角粒界の割合を高める。この効果を確実に発揮させる場合、Ｎｂ：０．０１％以上およびＴｉ：０．０１０％以上のいずれか１種以上を含有することが好ましい。一方、Ｎｂ含有量を０．１５％超、またはＴｉ含有量を０．１５０％超としても上記効果は飽和するので、Ｎｂ含有量は０．１５％以下およびＴｉ含有量は０．１５０％以下とすることが好ましい。
[0027]
"Mo: 0-1.00%"
"Cr: 0-1.00%"
"Cu: 0-1.00%"
"V: 0-1.00%"
"W: 0-1.00%"
"Ni: 0 to 3.00%"
　Mo, Cr, Cu, V, W and Ni form a solid solution in prior austenite grains during heating before hot stamping, and have the effect of increasing the strength of the hot stamped compact. . As a result, deformation of prior austenite grains can be suppressed during transformation from austenite to bainite or martensite, and the ratio of high-angle grain boundaries can be increased. To reliably obtain this effect, Mo: 0.005% or more, Cr: 0.005% or more, Cu: 0.001% or more, V: 0.0005% or more, W: 0.001% or more and Ni: It is preferable to contain at least one of 0.001% or more. On the other hand, even if these elements are contained in large amounts, the above effects are saturated, so the Mo content, Cr content, Cu content, V content and W content are each 1.00% or less, and the Ni content is 3.00% or less is preferable.
[0028]
"Mg: 0-1.00%"
"Zr: 0-1.00%"
"Sb: 0-1.00%"
"Ca: 0-0.10%"
"REM: 0-0.30%"
　Mg, Zr, Sb, Ca, and REM are elements that improve deformability by suppressing the formation of oxides that serve as starting points for fracture, and improve the impact resistance of hot stamped bodies. In order to reliably obtain this effect, the content of any one of Mg, Zr, Sb, Ca and REM is preferably 0.001% or more. On the other hand, even if these elements are contained in large amounts, the above effects are saturated, so the Mg content, Zr content, and Sb content are each 1.00% or less, the Ca content is 0.10% or less, and the REM content is The amount is preferably 0.30% or less.
　In this embodiment, REM refers to a total of 17 elements consisting of Sc, Y and lanthanoids, and the content of REM refers to the total content of these elements.
[0029]
"B: 0 to 0.0100%"
　B is an element that segregates at prior austenite grain boundaries to suppress the formation of ferrite and pearlite. To ensure this effect, the B content is preferably 0.0005% or more. On the other hand, even if the B content exceeds 0.0100%, the above effect is saturated, so the B content is preferably 0.0100% or less.
[0030]
　The chemical composition of the hot-stamped body described above may be measured by a general analytical method. For example, it may be measured using ICP-AES (Inductively Coupled Plasma-Atomic Emission Spectrometry). Incidentally, C and S may be measured using the combustion-infrared absorption method, and N may be measured using the inert gas fusion-thermal conductivity method. When the surface of the hot stamped body is provided with a plating layer, the chemical composition may be analyzed after removing the plating layer by mechanical grinding.
[0031]
　Next, the microstructure of the hot stamped body according to this embodiment will be described.
　The hot stamped body according to the present embodiment has an area ratio of 10% or more and less than 20% retained austenite, 5 to 15% fresh martensite, and a total of 65 to 85% bainite and tempered martensite. and a residual structure of less than 5%, and the length of the grain boundary at which the rotation angle is 4° to 12° with the <011> direction as the rotation axis among the grain boundaries of the bainite and the tempered martensite and the length of the grain boundary with a rotation angle of 49 ° to 54 ° and the length of the grain boundary with a rotation angle of 55 ° to 75 ° (large tilt grain boundary), relative to the rotation It has a microstructure in which the proportion of the length of grain boundaries with angles of 55° to 75° is 30% or more.
[0032]
　In this embodiment, the depth position of 1/4 of the plate thickness from the surface of the hot stamped product (region of 1/8 of the plate thickness from the surface to 3/8 of the plate thickness from the surface) Define microstructure. This depth position is the midpoint between the surface of the hot stamped body and the thickness center position, and the microstructure at this position represents the steel structure of the hot stamped body (average of the entire hot stamped body This is because it shows a fine microstructure).
[0033]
"Retained austenite: 10% or more and less than 20%"
　By containing a predetermined amount of retained austenite, the hot stamped compact has improved strain dispersion characteristics. If the retained austenite is less than 10% or more than 20%, desired strain dispersion characteristics cannot be obtained. Therefore, the retained austenite should be 10% or more and less than 20%.
[0034]
"Fresh martensite: 5-15%"
　Fresh martensite improves the strength of the hot stamped product. If the fresh martensite content is less than 5%, desired strain dispersion characteristics cannot be obtained. Therefore, fresh martensite should be 5% or more. Preferably it is 7% or more. On the other hand, if the fresh martensite content exceeds 15%, the maximum bending angle of the hot-stamped article is lowered, that is, the bendability is lowered. Therefore, fresh martensite should be 15% or less. Preferably, it is 12% or less.
[0035]
"Bainite and tempered martensite: 65-85% in total"
　Bainite and tempered martensite improve the strength of hot stamped compacts. If the sum of bainite and tempered martensite is less than 65%, the desired strength cannot be obtained. Therefore, the total content of bainite and tempered martensite is set to 65% or more. Preferably it is 70% or more. On the other hand, if the total content of bainite and tempered martensite exceeds 85%, desired strain dispersion characteristics cannot be obtained. Therefore, the total content of bainite and tempered martensite is 85% or less. Preferably it is 80% or less.
[0036]
「残部組織：５％未満」
　本実施形態に係るホットスタンプ成形体のミクロ組織中には、残部組織として、フェライト、パーライトおよびグラニュラーベイナイトが含まれる場合がある。残部組織の面積率が高いと、所望の強度および衝突特性を得ることができない。そのため、残部組織は５％未満とする。好ましくは４％以下、３％以下、２％以下、または１％以下である。
[0037]
「残留オーステナイト、並びにベイナイトおよび焼き戻しマルテンサイトの面積率の測定」
　ホットスタンプ成形体の端面から５０ｍｍ以上離れた任意の位置（この位置から採取できない場合は端部を避けた位置）から表面に垂直な断面（板厚断面）が観察できるようにサンプルを切り出す。サンプルの大きさは、測定装置にもよるが、圧延方向に１０ｍｍ程度観察できる大きさとする。
[0038]
　After polishing the cross section of the above sample using silicon carbide paper of #600 to #1500, a diamond powder with a particle size of 1 to 6 μm is dispersed in a dilute solution such as alcohol or pure water to finish it to a mirror surface. . Next, the sample is polished for 8 minutes with colloidal silica containing no alkaline solution at room temperature to remove strain introduced into the surface layer of the sample. At an arbitrary position in the longitudinal direction of the sample cross section, electron backscattering at a measurement interval of 0.1 μm in a region of 50 μm in length, 1/8 of the plate thickness from the surface to 3/8 of the plate thickness from the surface Crystal orientation information is obtained by measurement using a diffraction method. For the measurement, an EBSD apparatus composed of a thermal field emission scanning electron microscope (JSM-7001F manufactured by JEOL) and an EBSD detector (DVC5 type detector manufactured by TSL) is used. At this time, the degree of vacuum in the EBSD apparatus is 9.6×10 −5 Pa or less, the acceleration voltage is 15 kV, the irradiation current level is 13, and the electron beam irradiation level is 62. The obtained crystal orientation information is used to calculate the area ratio of retained austenite using the "Phase Map" function installed in the software "OIM Analysis (registered trademark)" attached to the EBSD analysis device. A crystal structure of fcc is determined to be retained austenite.
[0039]
　Next, those with a crystal structure of bcc are determined to be bainite, tempered martensite, fresh martensite, granular bainite and ferrite, and these regions are analyzed using the software "OIM Analysis (registered trademark)" attached to the EBSD analysis device. By using the "Grain Average Misorientation" function installed in , the area with a Grain Average Image Quality value of less than 60000 is determined as bainite, tempered martensite, and fresh martensite, and the total area ratio of these is calculated. , to obtain the area ratio of the sum of "bainite, tempered martensite, and fresh martensite". "Bainite and tempered martensite" is obtained by subtracting the area ratio of fresh martensite obtained by the method described below from the total area ratio of "bainite, tempered martensite and fresh martensite" obtained by the above method. Get the total area ratio of
[0040]
"Measurement of area ratio of fresh martensite and residual structure"
　Cross section perpendicular to the surface (plate Cut out the sample so that the thick section) can be observed. Although the size of the sample depends on the measuring device, it should be a size that allows observation of about 10 mm in the rolling direction.
[0041]
　After polishing the cross section of the above sample using #600 to #1500 silicon carbide paper, a mirror finish is achieved using a liquid in which diamond powder with a particle size of 1 to 6 μm is dispersed in a diluted solution such as alcohol or pure water. , Nital etching. Then, in an arbitrary position in the longitudinal direction of the sample cross section, a thermal field emission scanning electron microscope ( Photographs of multiple fields of view are taken using JSM-7001F manufactured by JEOL. An equidistant grid is drawn on the photograph to identify the tissue at the grid points. The area ratio of each tissue is obtained by calculating the number of grid points corresponding to each tissue and dividing it by the total number of grid points. The larger the total number of grid points, the more accurately the area ratio can be calculated. In this embodiment, the grid spacing is 2 μm×2 μm, and the total number of grid points is 1,500.
[0042]
　A region in which cementite is precipitated in a lamellar shape within grains is determined to be pearlite. A region with low brightness and no substructure is judged to be ferrite. Regions with high brightness and in which the substructure is not revealed by etching are judged to be fresh martensite and retained austenite. A region that does not correspond to any of the above is determined to be granular bainite. The area ratio of fresh martensite is obtained by subtracting the area ratio of retained austenite obtained by the above EBSD analysis from the area ratio of fresh martensite and retained austenite obtained from the photograph.
[0043]
"Among the grain boundaries of bainite and tempered martensite, the length of the grain boundary with a rotation angle of 4° to 12° with the <011> direction as the rotation axis, and the grain with a rotation angle of 49° to 54° The length of the grain boundary with a rotation angle of 55° to 75° (high angle grain boundary) with respect to the total length of the boundary length and the length of the grain boundary with a rotation angle of 55° to 75° ratio: 30% or more”
　The high angle grain boundary is the grain boundary with the highest angle among the grain boundaries included in the grains of bainite and tempered martensite. The high-angle grain boundaries are highly effective in suppressing the propagation of cracks generated at the time of collision. If the proportion of the length of the high-angle grain boundaries is less than 30%, the hot-stamped compact cannot obtain the desired collision characteristics. Therefore, the ratio of the length of the high-angle grain boundaries is set to 30% or more. Preferably it is 40% or more. Although the upper limit of the ratio of the length of the high-angle grain boundaries is not specified, the substantial upper limit is 90% according to the chemical composition and the manufacturing method according to the present embodiment.
[0044]
"Method for measuring the length ratio of high-angle grain boundaries"
　A cross section perpendicular to the surface (plate Cut out the sample so that the thick section) can be observed. The sample should have a length that allows observation of about 10 mm in the rolling direction, depending on the measuring device. For the cut sample, the depth position of 1/4 of the plate thickness (1/8 of the plate thickness from the surface to 3/8 of the plate thickness from the surface) is analyzed by EBSD at a measurement interval of 0.1 μm. to obtain crystal orientation information. Here, the EBSD analysis uses an EBSD device composed of a thermal field emission scanning electron microscope (JSM-7001F manufactured by JEOL) and an EBSD detector (DVC5 type detector manufactured by TSL), and an electron beam irradiation level of 62. implement.
[0045]
　Next, for the obtained crystal orientation information, using the "Grain Average Image Quality" function installed in the software "OIM Analysis (registered trademark)" attached to the EBSD analysis device, the Grain Average Image Quality value is 60000. Areas below are determined to be bainite, tempered martensite, and fresh martensite crystal grains, and among the grain boundaries of these crystal grains, the grain boundaries of the bainite and tempered martensite crystal grains are oriented in the <011> direction. The length of the grain boundary with a rotation angle of 4° to 12° as the rotation axis, the length of the grain boundary with a rotation angle of 49° to 54°, and the length of the grain boundary with a rotation angle of 55° to 75°. is calculated, and the ratio of the length of the grain boundary at which the rotation angle is 55° to 75° is calculated with respect to the total value of the length of each grain boundary. As a result, the length of the grain boundary at which the rotation angle is 4° to 12° with the <011> direction as the rotation axis of the bainite and tempered martensite crystal grains, and the grain boundary at which the rotation angle is 49° to 54° Grain boundaries with a rotation angle of 55 ° to 75 ° (high angle grain boundaries) with respect to the total length of the length and the length of the grain boundaries with a rotation angle of 55 ° to 75 ° (high angle grain boundaries) Get the length ratio of .
[0046]
　In addition, photographs were taken by the same method as the method for measuring the area ratio of the residual structure, and fresh martensite was discriminated from the crystal grains of bainite, tempered martensite, and fresh martensite, and bainite, tempered martensite, and Fresh martensite should be excluded from fresh martensite crystal grains. The reason why the grain boundaries of the crystal grains of fresh martensite are not included in the measurement of the high-angle grain boundaries is that fresh martensite has a high hardness and becomes a starting point of fracture.
[0047]
　The length of the grain boundary can be easily calculated using, for example, the "Inverse Pole Figure Map" and "Axis Angle" functions installed in the software "OIM Analysis (registered trademark)" attached to the EBSD analysis device. It is possible to With these functions, the total length of the grain boundaries can be calculated by specifying a specific rotation angle with an arbitrary direction as the axis of rotation for bainite and tempered martensite grains. The above analysis was carried out for all crystal grains contained in the measurement area, and the lengths of the three types of grain boundaries described above were measured with the <011> direction of the grain boundaries of the bainite and tempered martensite grains as the rotation axis. Just calculate.
[0048]
“Average Dislocation Density: 4.0×10 15 m/m 2 or More”
　The hot-stamped product according to the present embodiment may have an average dislocation density of 4.0×10 15 m/m 2 or more. Having the above-mentioned chemical composition and the above-mentioned microstructure, that is, 10% or more and less than 20% of retained austenite, 5 to 15% of fresh martensite, and 65 to 85% of bainite in total in terms of area ratio and tempered martensite and a residual structure of less than 5%, and the rotation angle is 4° to 12° with the <011> direction of the grain boundaries of the bainite and the tempered martensite as the rotation axis. The rotation angle is If you have a microstructure with a grain boundary length ratio of 30% or more where the grain boundaries are between 55° and 75°, the average dislocation density will necessarily be greater than or equal to 4.0×10 15 m/m 2 .
[0049]
[Measurement of Average Dislocation Density] A
　sample is cut from an arbitrary position 50 mm or more away from the end face of the hot stamped body (a position avoiding the end if it cannot be sampled from this position). The size of the sample is about 20 mm square, although it depends on the measuring device. A mixed solution of 48% by volume distilled water, 48% by volume hydrogen peroxide, and 4% by volume hydrofluoric acid is used to reduce the thickness of the sample. At this time, the thickness of the front surface and the back surface of the sample is reduced by the same thickness, and the depth position of 1/4 of the plate thickness from the sample surface before decompression (1/8 of the plate thickness from the surface to the plate thickness from the surface) 3/8 depth region) are exposed. X-ray diffraction measurements are performed on this exposed surface to identify multiple diffraction peaks of the body-centered cubic lattice. By analyzing the average dislocation density from the half widths of these diffraction peaks, the average dislocation density of the surface layer region is obtained. As for the analytical method, the modified Williamson-Hall method described in "T. Ungar et al., Journal of Applied Crystallography, 1999, Vol. 32, pp. 992-1002" is used.
[0050]
[Lath width of crystal grains having a body-centered structure: 200 nm or less
　] In the hot stamped product according to the present embodiment, the lath width of crystal grains having a body-centered structure may be 200 nm or less. Having the above-mentioned chemical composition and the above-mentioned microstructure, that is, 10% or more and less than 20% of retained austenite, 5 to 15% of fresh martensite, and 65 to 85% of bainite in total in terms of area ratio and tempered martensite and a residual structure of less than 5%, and the rotation angle is 4° to 12° with the <011> direction of the grain boundaries of the bainite and the tempered martensite as the rotation axis. The rotation angle is If the microstructure has a grain boundary length ratio of 30% or more where the angle is 55° to 75°, the lath width of crystal grains having a body-centered structure is inevitably 200 nm or less.
[0051]
　If the lath width of crystal grains having a body-centered structure is 200 nm or less, the effect of refining crystal grains can be obtained, and desired tensile strength can be obtained. It is preferably 180 nm or less. Since the smaller the lath width, the better, the lower limit is not particularly defined.
[0052]
"Measurement of the lath width of crystal grains with a body-centered structure"
　A cross section perpendicular to the surface (plate Cut out the sample so that the thick section) can be observed. The sample should have a length that allows observation of about 10 mm in the rolling direction, depending on the measuring device. For the cut sample, the depth position of 1/4 of the plate thickness (1/8 of the plate thickness from the surface to 3/8 of the plate thickness from the surface) is analyzed by EBSD at a measurement interval of 0.1 μm. to obtain crystal orientation information. Here, the EBSD analysis uses an EBSD device composed of a thermal field emission scanning electron microscope (JSM-7001F manufactured by JEOL) and an EBSD detector (DVC5 type detector manufactured by TSL), and an electron beam irradiation level of 62. implement.
[0053]
　Next, using the "Invere Pole Figure" function installed in the software "OIM Analysis (registered trademark)" attached to the EBSD analysis device for the obtained crystal orientation information, only the crystal grains having a body-centered structure were analyzed. An Invere Pole Figure image is drawn, a crystal grain with a crystal orientation difference of 8° or less is regarded as one lath (generally called a block, but expressed as a lath in this embodiment), and the length of the short axis direction of the lath is to measure. By measuring the length of 20 or more laths in the minor axis direction and calculating the average value thereof, the lath width of the crystal grain having a body-centered structure is obtained.
[0054]
[Thickness and Tensile Strength
　] The thickness of the hot-stamped body according to the present embodiment is not particularly limited, but it is preferably 0.5 to 3.5 mm from the viewpoint of weight reduction of the vehicle body. Further, from the viewpoint of reducing the weight of the vehicle body, the tensile strength of the hot-stamped product is preferably 1500 MPa or more. More preferably, it is 1800 MPa or more and 2000 MPa or more. Although the upper limit of the tensile strength is not particularly defined, it may be 2600 MPa or less.
[0055]
"Plating layer"
　The hot stamped article according to the present embodiment may have a plating layer formed on its surface for the purpose of improving corrosion resistance. The plating layer may be either an electroplating layer or a hot dipping layer. The electroplated layer includes, for example, an electrogalvanized layer, an electroplated Zn—Ni alloy layer, and the like. The hot-dip plating layer is, for example, a hot-dip galvanized layer, an alloyed hot-dip galvanized layer, a hot-dip aluminum plating layer, a hot-dip Zn--Al alloy plating layer, a hot-dip Zn--Al--Mg alloy-plating layer, or a hot-dip Zn--Al--Mg--Si. Including alloy plating layer, etc. The coating amount of the plating layer is not particularly limited, and a general coating amount may be used.
[0056]
[Manufacturing Method of Hot Stamped Body]
　Next, a preferred method of manufacturing the hot stamped body according to the present embodiment will be described.
　The hot-stamped product according to the present embodiment is obtained by hot-stamping a cold-rolled steel sheet produced by a conventional method or a cold-rolled steel sheet having a plating layer on the surface, and holding it in a low-temperature region after hot-stamping. After that, it can be manufactured by cooling.
[0057]
“Heating and holding before hot stamping” Before
　hot stamping, it is preferable to hold in the temperature range of 800 to 1000° C. for 60 to 600 seconds. If the heating temperature is less than 800° C. or the holding time is less than 60 seconds, sufficient austenitization cannot be achieved, and the desired amount of bainite and tempered martensite may not be obtained in the hot stamped compact. If the heating temperature exceeds 1000° C. or the holding time exceeds 600 seconds, the transformation to bainite and tempered martensite is delayed due to coarsening of the austenite grain size, and the desired amount of bainite and tempered martensite cannot be obtained. Sometimes.
[0058]
　The average heating rate during heating may be 0.1° C./s or more and 200° C./s or less. The average heating rate referred to here is a value obtained by dividing the temperature difference between the steel sheet surface temperature at the start of heating and the holding temperature by the time difference from the start of heating until reaching the holding temperature. Further, in the holding described above, the steel sheet temperature may be varied within the temperature range of 800 to 1000° C., or may be kept constant.
[0059]
　Heating methods before hot stamping include heating with an electric furnace or gas furnace, flame heating, electrical heating, high-frequency heating, induction heating, and the like.
[0060]
"Cooling after Hot Stamping" Hot
　stamping is performed after the heating and holding described above. After hot stamping, it is preferable to cool down to a temperature range of 150 to 300° C. at an average cooling rate of 1.0 to 100° C./s. In the cooling after hot stamping, if the cooling stop temperature is less than 150° C., the introduction of lattice defects is excessively promoted, and the desired dislocation density may not be obtained. If the cooling stop temperature is higher than 300° C., the hardness of the prior austenite grains becomes low, and a desired amount of high-angle grain boundaries may not be formed. Also, if the average cooling rate is less than 1.0° C./s, the transformation to ferrite, granular bainite, and pearlite is accelerated, and the desired amounts of bainite and tempered martensite may not be obtained. When the average cooling rate is more than 100 ° C./s, the driving force for the transformation to tempered martensite and bainite is large, the effect of relaxing the strain introduced by the transformation is small, and the desired amount of large-angle grain boundaries becomes difficult to obtain. Here, the average cooling rate is a value obtained by dividing the temperature difference between the steel sheet surface temperature at the start of cooling and the cooling stop temperature by the time difference from the start of cooling to the stop of cooling.
[0061]
"Low temperature maintenance"
　It is preferable to carry out low temperature maintenance in the temperature range of 150 to 300°C for 1.0 to 50 hours. During the low temperature hold, carbon partitions from austenite-transformed martensite to untransformed austenite. The carbon-enriched austenite does not transform into martensite and remains as retained austenite even after the cooling after the low-temperature holding is finished. Further, by holding the steel at a low temperature under the above conditions, the carbon-enriched austenite has a high hardness, so that the ratio of the high-angle grain boundaries can be increased.
[0062]
　If the holding temperature is less than 150°C or the holding time is less than 1.0 hour, carbon will not be sufficiently distributed from martensite to untransformed austenite, and the desired amount of retained austenite may not be obtained. In addition, the ratio of high-angle grain boundaries is reduced. If the holding temperature is higher than 300° C., the hardness of the prior austenite grains may be lowered, and a desired amount of large-angle grain boundaries may not be obtained. If the holding time exceeds 50 hours, it may not be possible to obtain the desired fresh martensite. In the low-temperature holding, the steel sheet temperature may be varied within the temperature range of 150 to 300° C., or may be kept constant.
[0063]
　The low-temperature holding is not particularly limited, but for example, the steel sheet after hot stamping may be conveyed to a heating furnace.
[0064]
　Note that if the material is heated to a temperature range of 300° C. or higher after hot stamping and cooling, but before holding at a low temperature, bainite is formed, and as a result, it becomes impossible to obtain a desired amount of large-angle grain boundaries. Therefore, when manufacturing the hot-stamped product according to the present embodiment, it is not desirable to heat the product to a temperature range of 300° C. or higher after cooling by hot stamping and before holding at a low temperature.
[0065]
"Cooling after holding at low temperature" After holding at
　low temperature, it is preferable to cool to 80°C or less at an average cooling rate of 1.0 to 100°C/s. If the average cooling rate is less than 1.0°C/s or the cooling stop temperature is more than 80°C, the retained austenite may decompose and the desired amount of retained austenite may not be obtained. If the average cooling rate exceeds 100°C/s, the cooling device will be overloaded. The average cooling rate here is a value obtained by dividing the temperature difference between the steel sheet surface temperature at the start of cooling after low-temperature holding and the cooling stop temperature by the time difference from the start of cooling to the end of cooling.
Example
[0066]
　Next, examples of the present invention will be described. The conditions in the examples are one example of conditions adopted for confirming the feasibility and effect of the present invention, and the present invention is based on this one example of conditions. It is not limited. Various conditions can be adopted in the present invention as long as the objects of the present invention are achieved without departing from the gist of the present invention.
[0067]
　Steel slabs produced by casting molten steel having chemical compositions shown in Tables 1 and 2 were subjected to hot rolling and cold rolling, and if necessary, were plated to obtain cold-rolled steel sheets. Next, hot-stamped bodies shown in Tables 3-5 were produced from the cold-rolled steel sheets under the conditions shown in Tables 3-5.
[0068]
　The average heating rate in heating before hot stamping was 0.1 to 200 ° C./s, cooling after hot stamping was performed to a temperature range of 150 to 300 ° C., and cooling after low temperature holding was performed to 80 ° C. or less. . In addition, production No. in Table 3. 18 is a hot-dip aluminum plating layer; 19 was given a hot dip galvanized layer.
[0069]
　Production No. in Table 5. No. 57 was held in a temperature range of 300 to 560° C. for 30 seconds after hot stamping and cooling, and before low temperature holding, and then the low temperature holding shown in Table 5 was performed.
[0070]
　Underlines in the table indicate that they are outside the scope of the present invention, that they are outside the preferred manufacturing conditions, or that their characteristic values ​​are unfavorable. γr in Tables 3 to 5 indicates retained austenite, FM indicates fresh martensite, B indicates bainite, and TM indicates tempered martensite.
[0071]
　Regarding the microstructure of the hot-stamped compact, the measurement of the area ratio of each structure, the measurement of the length ratio of the large-angle grain boundaries, the measurement of the dislocation density, and the measurement of the lath width of the crystal grains having a body-centered structure are performed as described above. It was carried out according to the measurement method. Moreover, the mechanical properties of the hot-stamped product were evaluated by the following methods.
[0072]
"Tensile strength"
　The tensile strength of the hot stamped body is measured by preparing a No. 5 test piece described in JIS Z 2241: 2011 from an arbitrary position of the hot stamped body, and using the test method described in JIS Z 2241: 2011. sought according to The crosshead speed was set to 3 mm/min. When the tensile strength was 1500 MPa or more, the strength was judged to be excellent, and it was judged to be acceptable.
[0073]
"Collision property (strain dispersion property evaluation)" In evaluating
　the collision property (strain dispersion property and bendability) of the hot stamped body, in this example, the VDA standard (VDA238-100) specified by the German Automobile Manufacturers Association based on the maximum bending angle and the deformation area at a bending angle of 40°. The VDA test was performed under the following conditions.
　In this example, when the maximum bending angle obtained by the VDA test is 60° or more, it is judged to be excellent in bendability and is judged to be acceptable. Judged as qualified.
[0074]
　Test piece dimensions: 60 mm (rolling direction) x 30 mm (direction parallel to the plate width direction)
　Test piece plate thickness: 1.01 to 1.05 mm (front and back surfaces are ground by the same amount)
　Bending ridge line: parallel to the plate width direction Direction
　test method: Roll support, punch pushing
　Roll diameter: φ30
　mm Punch shape: Tip R = 0.4 mm
　Distance between rolls: 2.0 x plate thickness (mm) + 0.5 mm
　Pushing speed: 20 mm/min
　Testing machine: Shimadzu Corporation AG -100KNI
[0075]
　Strain dispersion properties were evaluated in the deformation region at a bending angle of 40° after the VDA bending test. 10 grids in the width direction×20 grids in the length direction (total 200 grids) with a spacing of 100 μm were marked on the central portion of the surface of the test piece before the VDA test by laser irradiation. A VDA test was performed under the same test conditions as above, and the test was stopped when the bending angle reached 40°. Using a laser microscope, the inter-grid distance in the direction perpendicular to the bending ridge was measured for each grid, and the value obtained by dividing it by 100 μm was calculated to obtain the deformation amount of each grid. The length of the deformed region was obtained by calculating the total length of the inter-grid distances in the direction perpendicular to the bent ridgeline of the grid with a deformation amount of 1.05 or more. In the present embodiment, when the length of the deformation region is 500 μm or more, it is judged to be excellent in strain dispersion characteristics and is judged to be acceptable, and when the length of the deformation region is less than 500 μm, the strain dispersion characteristics are inferior. It was judged that it was unsatisfactory.
[0076]
　Looking at Tables 3-5, it can be seen that hot stamped bodies whose chemical compositions and microstructures are within the scope of the present invention have excellent strength and impact properties.
　On the other hand, it can be seen that hot-stamped articles having chemical compositions and microstructures outside the scope of the present invention are inferior in one or more of strength and impact resistance.
[0077]
[table 1]

[0078]
[Table 2]

[0079]
[Table 3]

[0080]
[Table 4]

[0081]
[Table 5]

Industrial applicability
[0082]
　According to the aspect of the present invention, it is possible to obtain a hot-stamped article having excellent strength and impact resistance.

The scope of the claims

[Claim 1]
　The chemical composition is mass %,
C: 0.15 to 1.00%,
Si: 0.50 to 3.00%,
Mn: more than 3.00%, 5.00% or less,
Al: 0.100 to 3.000%,
Co: 0.100 to 3.000%,
P: 0.100% or less,
S: 0.1000% or less,
N: 0.0100% or less,
Nb: 0 to 0.15%,
Ti : 0 to 0.150%
Mo: 0 to 1.00%
Cr: 0 to 1.00%
Cu: 0 to 1.00%
V: 0 to 1.00%
W: 0 to 1.00% 00%,
Ni: 0 to 3.00%,
Mg: 0 to 1.00%,
Zr: 0 to 1.00%,
Sb: 0 to 1.00%,
Ca: 0 to 0.10%,
REM: 0 to 0.30%, and
B: 0 to 0.0100%, the
balance being Fe and impurities,
　In terms of area ratio, it consists of retained austenite of 10% or more and less than 20%, fresh martensite of 5 to 15%, bainite and tempered martensite of 65 to 85% in total, and residual structure of less than 5%. ,
　the length of the grain boundary at which the rotation angle is 4° to 12° with the <011> direction as the rotation axis of the grain boundary of the bainite and the tempered martensite, and the rotation angle is 49° to 54°. The ratio of the grain boundary length at which the rotation angle is 55° to 75° to the total length of the grain boundary length at which the rotation angle is 55° to 75°
A hot-stamped article characterized by having a microstructure of 30% or more .
[Claim 2]
　The chemical composition is, in mass %,
Nb: 0.01 to 0.15%,
Ti: 0.010 to 0.150%,
Mo: 0.005 to 1.00%,
Cr: 0.005 to 1.0%. 00%,
Cu: 0.001 to 1.00%,
V: 0.0005 to 1.00%,
W: 0.001 to 1.00%,
Ni: 0.001 to 3.00%,
Mg: 0 .001-1.00%,
Zr: 0.001-1.00%,
Sb: 0.001-1.00%,
Ca: 0.001-0.10%,
REM: 0.001-0.30 %, and B: one or more selected from the group consisting of
0.0005 to 0.0100% .