Specification
Title of the Invention: Hot Stamp Molding
Technical field
[0001]
The present invention relates to a hot-stamped molded product.
This application claims priority based on Japanese Patent Application No. 2020-002409 filed in Japan on January 9, 2020, the content of which is incorporated herein.
Background technology
[0002]
In recent years, there has been a demand for lighter automobile bodies from the perspective of environmental protection and resource saving, and high-strength steel sheets are being applied to automobile parts. 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]
　In order to obtain a higher effect of reducing the weight of automobile bodies, it is necessary to obtain high-strength and excellent resistance to hydrogen embrittlement in automobile parts made by hot-stamping steel sheets.
[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]
Patent Documents 1 to 3 do not consider hydrogen embrittlement resistance.
prior art documents
patent literature
[0008]
Patent Document 1: Japanese Patent Application Laid-Open 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]
An object of the present invention is to provide a hot-stamped article that is excellent in strength and resistance to hydrogen embrittlement.
Means to solve problems
[0010]
The gist of the present invention is as follows.
[1] A hot stamped article according to one aspect of the present invention has a chemical composition, in mass %,
C: more than 0.50%, 1.00% or less,
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 0.150%,
Ti: 0 to 0.150%,
Mo: 0 to 1.00%,
Cr: 0 to 1.00%,
Cu: 0 to 1.00%,
V: 0-1.00%,
W: 0-1.00%,
Ni: 0 to 3.00%,
Mg: 0-1.00%,
Zr: 0 to 1.00%,
Sb: 0 to 1.00%,
Ca: 0-0.10%,
REM: 0-0.30%, and
B: contains 0 to 0.0100%,
The balance consists of Fe and impurities,
The area ratio consists of 20-30% retained austenite, a total of 70-80% bainite and tempered martensite, and less than 5% residual structure,
Among the grain boundaries of the bainite and the tempered martensite, the length of the grain boundary at which the rotation angle is 4° to 12° with the <011> direction as the rotation axis, 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 and the grain boundary length at which the rotation angle is 55° to 75° is 30 % or more.
[2] The hot stamped article according to [1] above, wherein the chemical composition is, in mass%,
Nb: 0.010 to 0.150%,
Ti: 0.010 to 0.150%,
Mo: 0.005 to 1.00%,
Cr: 0.005 to 1.00%,
Cu: 0.001 to 1.00%,
V: 0.0005-1.00%,
W: 0.001-1.00%,
Ni: 0.001 to 3.00%,
Mg: 0.001-1.00%,
Zr: 0.001 to 1.00%,
Sb: 0.001 to 1.00%,
Ca: 0.001 to 0.10%,
REM: 0.001-0.30%, and
B: 0.0005 to 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 above aspect of the present invention, it is possible to obtain a hot-stamped article having excellent strength and resistance to hydrogen embrittlement.
Brief description of the drawing
[0012]
[Fig. 1] Fig. 1 is a diagram showing a test piece used for evaluation of hydrogen embrittlement resistance in Examples.
MODE FOR CARRYING OUT THE INVENTION
[0013]
The inventors have found that the microstructure of the hot-stamped compact contains a predetermined amount of retained austenite, bainite and tempered martensite, and the grain boundaries of said bainite and said tempered martensite <0.11 The length of the grain boundary with a rotation angle of 4° to 12° with the direction as the axis of rotation, the length of the grain boundary with a rotation angle of 49° to 54°, and the grain boundary with a rotation angle of 55° to 75° The ratio of the length of the grain boundary (high-angle grain boundary) at which the rotation angle is 55 ° to 75 ° with respect to the total length of the length (hereinafter sometimes referred to as the high-angle grain boundary) It was found that by setting the content to 30% or more, the hydrogen embrittlement resistance can be improved while ensuring high strength.
[0014]
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. 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. .
[0015]
The hot-stamped molded article according to the present 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.
It should be noted that the numerical limitation range described below with "-" in between includes the lower limit and the upper limit. 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.
[0016]
The hot stamped body according to the present embodiment has a chemical composition in mass% of C: more than 0.50% and 1.00% or less, Si: 0.50 to 3.00%, Mn: 3.00%. super, 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: containing Fe and impurities. Each element will be described in detail below.
[0017]
"C: more than 0.50%, 1.00% or less"
　C is an element that improves the strength of the hot stamped compact. C is also an element that stabilizes retained austenite. If the C content is 0.50% or less, the desired strength cannot be obtained in the hot-stamped product. Therefore, the C content should be more than 0.50%. The C content is preferably 0.52% or more, or 0.54% 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 0.90% or less, 0.80% or less, or 0.70% or less.
[0018]
"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 1.10% 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.
[0019]
"Mn: more than 3.00%, 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 3.10% or more, or 3.20% 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 4.00% or less.
[0020]
"Al: 0.100 to 3.000%"
Al is an element that improves deformability by deoxidizing molten steel and suppressing the formation of oxides that act as starting points for fracture. 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 0.200% 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. Therefore, the Al content is set to 3.000% or less. The Al content is preferably 2.000% or less, 1.500% or less, or 1.000% or less.
[0021]
"Co: 0.100 to 3.000%"
Co is an element that promotes bainite transformation in the 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. Co content is preferably 2.000% or less, 1.500% or less, 1.000% or less, 0.500% or less, or 0.200% or less.
[0022]
"P: 0.100% or less"
　P is an impurity element, and segregates at the grain boundary to become the starting point of fracture. Therefore, the P content is set to 0.100% or less. The P content is preferably 0.050% or less, or 0.020% or less. The lower limit of the P content is not particularly limited, but if it is reduced to less than 0.0001%, the cost of removing P increases significantly, which is not economically preferable.
[0023]
"S: 0.1000% or less"
S is an impurity element and forms inclusions in steel. Since this inclusion becomes a starting point of fracture, the S content is made 0.1000% or less. The S content is preferably 0.0500% or less, 0.0100% or less, or 0.0050% or less. The lower limit of the S content is not particularly limited, but if it is reduced to less than 0.0001%, the desulfurization cost will increase significantly, which is not economically preferable.
[0024]
"N: 0.0100% or less"
N is an impurity element and forms nitrides in steel. The N content is set to 0.0100% or less because this nitride becomes a starting point of fracture. The N content is preferably 0.0060% or less, or 0.0050% or less. The lower limit of the N content is not particularly limited, but if it is reduced to less than 0.0001%, the de-N cost will increase significantly,Since it is economically unfavorable, 0.0001% may be set as the lower limit in actual operation.
[0025]
The rest of the chemical composition of the hot stamped compact according to this embodiment may be Fe and impurities. Examples of impurities include elements that are inevitably mixed from steel raw materials or scraps and/or during the steelmaking process and that are permissible within a range that does not impair the properties of the hot-stamped article according to the present embodiment.
[0026]
The hot-stamped compact according to the present embodiment may contain the following elements as arbitrary elements instead of part of Fe. The content is 0% when the following optional elements are not contained.
[0027]
"Nb: 0 to 0.150%"
"Ti: 0 to 0.150%"
Nb and Ti refine the prior austenite grains during heating before hot stamping and suppress the deformation of prior austenite during the transformation from austenite to bainite or martensite, thereby increasing the ratio of large-angle grain boundaries. In order to ensure this effect, the content of either one of Nb and Ti is preferably 0.010% or more. On the other hand, even if the content of any one of Nb and Ti exceeds 0.150%, the above effect is saturated, so the content of Nb and Ti is preferably 0.150% or less.
[0028]
"Mo: 0 to 1.00%"
"Cr: 0 to 1.00%"
"Cu: 0 to 1.00%"
"V: 0-1.00%"
"W: 0-1.00%"
"Ni: 0 to 3.00%"
Mo, Cr, Cu, V, W and Ni have the effect of increasing the strength of the hot-stamped compact by forming a solid solution in the prior austenite grains during heating before hot-stamping. 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.
[0029]
"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%"
　Mg, Zr, Sb, Ca and REM improve deformability by suppressing the formation of oxides that act as starting points for fracture. To reliably obtain this effect, the content of at least 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 1.00% or less, the Ca content is 0.10% or less, and the REM content is 1.00% or less. 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.
[0030]
"B: 0 to 0.0100%"
B is an element that segregates at prior austenite grain boundaries and suppresses 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.
[0031]
The chemical composition of the hot stamped body described above can 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.
[0032]
Next, the microstructure of the hot stamped body according to this embodiment will be described.
The hot stamped body according to the present embodiment is composed of 20 to 30% retained austenite, 70 to 80% bainite and tempered martensite in total, and less than 5% residual structure in terms of area ratio. Among the grain boundaries of the bainite and the 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 grain boundary length at which the rotation angle is 55° to 75° with respect to the total length of the boundary length and the grain boundary length at which the rotation angle is 55° to 75° (high angle grain boundary) It has a microstructure with a thickness ratio of 30% or more.
[0033]
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).
[0034]
"Retained austenite: 20 to 30%"
　Retained austenite improves the resistance to hydrogen embrittlement in hot stamped compacts. If the retained austenite is less than 20%, desired hydrogen embrittlement resistance cannot be obtained. Therefore, retained austenite is set to 20% or more. Preferably it is 22% or more. On the other hand, if the retained austenite exceeds 30%, the desired strength cannot be obtained. Therefore, retained austenite is set to 30% or less. Preferably, it is 27% or less.
[0035]
"Bainite and tempered martensite: 70-80% in total"
By including the desired amount of bainite and tempered martensite, the hydrogen embrittlement resistance of the hot stamped product is improved. If the sum of bainite and tempered martensite is less than 70% or more than 80%, desired hydrogen embrittlement resistance cannot be obtained. Therefore, bainite and tempered martensite should be 70 to 80% in total. The lower limit is preferably 72% or more. Moreover, the upper limit is preferably 77% or less.
[0036]
"Remaining tissue: less than 5%"
Fresh martensite, ferrite, pearlite, and granular bainite may be contained as residual structures in the microstructure of the hot-stamped product according to the present embodiment. If the residual structure has a high area ratio, desired strength and resistance to hydrogen embrittlement cannot be obtained. Therefore, the residual tissue should be less than 5%. It is preferably 4% or less, 3% or less, 2% or less, or 1% or less.
[0037]
"Retained Austenite and Determination of Area Ratio of Bainite and Tempered Martensite"
A sample is cut from an arbitrary position 50 mm or more away from the end face of the hot stamped body (a position that avoids the end if it cannot be sampled from this position) so that a cross section (thickness cross section) perpendicular to the surface 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.
[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.
[0039]
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.
[0040]
Next, those with a crystal structure of bcc are judged to be bainite, tempered martensite, fresh martensite, granular bainite and ferrite, and the software "OIM Analysis (registered trademark)" attached to the EBSD analysis device is analyzed for these regions. By using the "Grain Average Misorientation" function installed in the , areas with a Grain Average Image Quality value of less than 60000 are 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
[0041]
"Measurement of area ratio of residual tissue"
A sample is cut from an arbitrary position 50 mm or more away from the end face of the hot stamped body (a position that avoids the end if it cannot be sampled from this position) so that a cross section (thickness cross section) perpendicular to the surface 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.
[0042]
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.
[0043]
The area where cementite is precipitated in lamellar form inside the grain is judged 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.
[0044]
"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° Percentage of: 30% or more”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. The high-angle grain boundaries are highly effective in suppressing the propagation of cracks caused by hydrogen. Embrittlement properties cannot be obtained. Therefore, the ratio of the length of the high-angle grain boundaries is set to 30% or more. It is preferably 35% or more and 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.
[0045]
"Method for measuring the length ratio of high-angle grain boundaries"
A sample is cut from a position 50 mm or more away from the end face of the hot stamped body (a position that avoids the end if it cannot be sampled from this position) so that a cross section (thickness cross section) perpendicular to the surface 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.
[0046]
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 .
[0047]
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.
[0048]
The above grain boundary length 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.
[0049]
"Average dislocation density: 4.0 × 10 15 m/m 2 or more"
The hot-stamped article 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, i.e., 20-30% retained austenite, 70-80% bainite and tempered martensite in total, and less than 5% residual structure in terms of area fraction 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 crystal grains, and the rotation angle is 49 ° to The grain boundary length at which the rotation angle is 55° to 75° relative to the total length of the grain boundary length at which the rotation angle is 54° and the grain boundary length at which the rotation angle is 55° to 75° of 30% or more, the average dislocation density necessarily becomes 4.0×10 15 m/m 2 or more.
[0050]
"Measurement of Average Dislocation Density"
A sample is cut out from an arbitrary position that is 50 mm or more away from the end face of the hot stamped product (a position that avoids the end if it cannot be collected 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.
[0051]
"Lath width of crystal grains having a body-centered structure: 200 nm or less"
In the hot-stamped body 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, i.e., 20-30% retained austenite, 70-80% bainite and tempered martensite in total, and less than 5% residual structure in terms of area fraction 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 crystal grains, and the rotation angle is 49 ° to The grain boundary length at which the rotation angle is 55° to 75° relative to the total length of the grain boundary length at which the rotation angle is 54° and the grain boundary length at which the rotation angle is 55° to 75° is 30% or more, the lath width of crystal grains having a body-centered structure is inevitably 200 nm or less.
[0052]
If the lath width of the crystal grains having a body-centered structure is 200 nm or less, the effect of refining the crystal grains can be obtained, and the 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.

The scope of the claims

[Claim 1]
The chemical composition, in mass%,
C: more than 0.50%, 1.00% or less,
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 0.150%,
Ti: 0 to 0.150%,
Mo: 0 to 1.00%,
Cr: 0 to 1.00%,
Cu: 0 to 1.00%,
V: 0-1.00%,
W: 0-1.00%,
Ni: 0 to 3.00%,
Mg: 0-1.00%,
Zr: 0 to 1.00%,
Sb: 0 to 1.00%,
Ca: 0-0.10%,
REM: 0-0.30%, and
B: contains 0 to 0.0100%,
The balance consists of Fe and impurities,
The area ratio consists of 20-30% retained austenite, a total of 70-80% bainite and tempered martensite, and less than 5% residual structure,
Among the grain boundaries of the bainite and the tempered martensite, the length of the grain boundary at which the rotation angle is 4° to 12° with the <011> direction as the rotation axis, 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 and the grain boundary length at which the rotation angle is 55° to 75° is 30 % or more
A hot-stamped article characterized by:
[Claim 2]
The chemical composition, in mass%,
Nb: 0.010 to 0.150%,
Ti: 0.010 to 0.150%,
Mo: 0.005 to 1.00%,
Cr: 0.005 to 1.00%,
Cu: 0.001 to 1.00%,
V: 0.0005-1.00%,
W: 0.001-1.00%,
Ni: 0.001 to 3.00%,
Mg: 0.001-1.00%,
Zr: 0.001 to 1.00%,
Sb: 0.001 to 1.00%,
Ca: 0.001 to 0.10%,
REM: 0.001-0.30%, and
B: 0.0005 to 0.0100%
2. The hot-stamped article according to claim 1, comprising one or more of the group consisting of:
drawing