Patent Application
20250188579
The present invention relates to a hot stamped component.
Priority is claimed on Japanese Patent Application No. 2022-090847, filed Jun. 3, 2022, the content of which is incorporated herein by reference.
In recent years, there has been a demand for a reduction in a weight of a vehicle body for a vehicle from the perspective of environmental protection and resource saving, and a high-strength steel sheet has been applied to vehicle members. Vehicle members are manufactured by press forming, but not only a forming load is increased but also the formability deteriorates as the strength of a steel sheet is increased. For this reason, the formability of a high-strength steel sheet into a member having a complicated shape becomes an issue.
In order to solve this issue, the application of a hot stamping technique in which press forming is performed after a steel sheet is heated up to a high temperature of an austenite range where the steel sheet softens is in progress. Hot stamping is attracting attention as a technique that achieves both the formability of a steel sheet into a vehicle member and strength of a vehicle member by performing hardening of the steel sheet in a die at the same time as press working.
For example, Patent Document 1 discloses a hardenable steel having excellent cold formability that can obtain excellent impact strength and hardness by reheating and quenching the steel.
When a hot stamped component with further improved tensile strength is used as a vehicle member, a greater effect of vehicle weight reduction can be achieved. However, since it is a vehicle member, it may be subjected to bending deformation due to a collision or the like, and therefore the hot stamped component needs to have high bendability. However, Patent Document 1 does not consider bendability.
The present invention has been made in view of the above-mentioned problem, and an object of the present invention is to provide a hot stamped component having high strength and excellent bendability.
The gist of the present invention is as follows.
[1]A hot stamped component according to an aspect of the present invention comprising, as a chemical composition, by mass %:
[2] The hot stamped component according to [1] may comprise, as the chemical composition, by mass %, one or more selected from the group consisting of:
According to the above-described aspects of the present invention, it is possible to provide a hot stamped component having high strength and excellent bendability.
The present inventors found that by controlling a texture of prior austenite and an average value of block sizes of martensite, tempered martensite and bainite in a position at ¼ of a sheet thickness from a surface of a hot stamped component, the bendability of the hot stamped component can be improved. In particularly, the present inventors found that the bendability of a hot stamped component can be improved by controlling not a texture of martensite, tempered martensite, bainite, or the like, which are a microstructure of the hot stamped component but a texture of prior austenite before transformation to martensite, bainite, or the like (i.e., state of austenite at a high temperature of Ar3 point or higher) to be within a specific range.
In addition, the present inventors found that in order to obtain the hot stamped component having the above features, it is particularly effective to strictly control final rolling conditions during hot rolling.
Hereinafter, the hot stamped component according to the present embodiment will be described in detail. First, the reason the chemical composition of the hot stamped component according to the present embodiment is limited will be described.
A limited numerical range described using “to” described below includes a lower limit and an upper limit. Numerical values represented using “less than” or “more than” are not included in a numerical range. All percentages (%) related to the chemical composition mean mass %.
The hot stamped component according to the present embodiment comprises, as a chemical composition, by mass %, C: 0.40% to 0.70%, Si: 0.010% to 3.000%, Mn: 0.10% or more and less than 0.60%, P: 0.100% or less, S: 0.0100% or less, N: 0.0100% or less, O: 0.0200% or less, Al: 0.0010% to 0.5000%, Nb: 0.0010% to 0.1000%, Ti: 0.010% to 0.100%, Cr: 0.010% to 1.000%, Mo: 0.050% to 1.000%, B: 0.0005% to 0.0100%, and a remainder: Fe and impurities.
Each element will be described below.
C is an element that improves the strength of the hot stamped component. When the C content is less than 0.40%, a desired strength of the hot stamped component cannot be obtained. For this reason, the C content is set to 0.40% or more. The C content is preferably more than 0.40%, 0.42% or more or 0.44% or more.
On the other hand, when the C content is more than 0.70%, the strength excessively increases and the bendability of the hot stamped component deteriorates. For this reason, the C content is set to 0.70% or less. The C content is preferably 0.65% or less or 0.60% or less.
Si is an element that improves the strength of the hot stamped component by solid-solution strengthening. When the Si content is less than 0.010%, a desired strength of the hot stamped component cannot be obtained. For this reason, the Si content is set to 0.010% or more. The Si content is preferably 0.100% or more, 0.300% or more or 0.500% or more.
On the other hand, when the Si content is more than 3.000%, the amount of ferrite increases and a desired strength of the hot stamped component cannot be obtained. For this reason, the Si content is set to 3.000% or less. The Si content is preferably 2.000% or less, 1.000% or less or 0.800% or less.
Mn: 0.10% or More and Less than 0.60%
Mn is an element that increases hardenability of steel and increases the strength of the hot stamped component. When the Mn content is less than 0.10%, a desired strength of the hot stamped component cannot be obtained. For this reason, the Mn content is set to 0.10% or more. The Mn content is preferably 0.20% or more or 0.35% or more.
On the other hand, when the Mn content is 0.60% or more, a desired texture of prior austenite cannot be obtained. For this reason, the Mn content is set to less than 0.60%. The Mn content is preferably 0.55% or less or 0.50% or less.
P decreases the strength of the grain boundaries by segregating in the grain boundaries. As a result, P deteriorates the bendability of the hot stamped component. When the P content is more than 0.100%, the bendability of the hot stamped component deteriorates significantly. For this reason, the P content is set to 0.100% or less. The P content is preferably 0.050% or less or 0.010% or less.
The lower limit of the P content may be 0%. However, when the P content is reduced to less than 0.0001%, the dephosphorization cost increases significantly, which is not preferable economically. For this reason, the P content may be set to 0.0001% or more.
S forms inclusions in steel. When the S content is more than 0.0100%, the bendability of the hot stamped component deteriorates significantly. For this reason, the S content is set to 0.0100% or less. The S content is preferably 0.0080% or less, 0.0050% or less or 0.0030% or less.
The lower limit of the S content may be 0%. However, when the S content is reduced to less than 0.0001%, the desulfurization cost increases significantly, which is not preferable economically. For this reason, the S content may be set to 0.0001% or more.
N forms nitrides in steel. When the N content is more than 0.0100%, the bendability of the hot stamped component deteriorates significantly. For this reason, the N content is set to 0.0100% or less. The N content is preferably 0.0080% or less, 0.0060% or less or 0.0040% or less.
The lower limit of the N content may be 0%. However, when the N content is reduced to less than 0.0001%, the denitrification cost increases significantly, which is not preferable economically. For this reason, the N content may be set to 0.0001% or more.
O forms coarse oxides when a large amount of O is comprised in steel. When the O content is more than 0.0200%, the bendability of the hot stamped component deteriorates significantly. For this reason, the O content is set to 0.0200% or less. The O content is preferably 0.0100% or less, 0.0070% or less, 0.0040% or less or 0.0030% or less.
The O content may be 0%. However, in order to disperse many oxides during deoxidizing of molten steel, the O content may be set to 0.0005% or more.
Al is an element having an effect of deoxidizing molten steel and achieving soundness of the steel (minimizing the occurrence of defects such as blowholes in steel). When the Al content is less than 0.0010%, deoxidation is not sufficiently performed, and coarse oxides are generated. As a result, the bendability of the hot stamped component deteriorates. For these reasons, the Al content is set to 0.0010% or more. The Al content is preferably 0.0050% or more, 0.0100% or more or 0.0300% or more.
On the other hand, when the Al content is more than 0.5000%, coarse oxides are generated in steel. As a result, the bendability of the hot stamped component deteriorates significantly. For this reason, the Al content is set to 0.5000% or less. The Al content is preferably 0.4000% or less, 0.3000% or less, or 0.2000% or less or 0.1000% or less.
Nb is an element that forms carbonitrides in steel and improves the strength of the hot stamped component by precipitation strengthening. When the Nb content is less than 0.0010%, a desired strength of the hot stamped component cannot be obtained. For this reason, the Nb content is set to 0.0010% or more. The Nb content is preferably 0.0050% or more, 0.0100% or more or 0.0200% or more.
On the other hand, when the Nb content is more than 0.1000%, many carbonitrides are generated in steel, and the bendability of the hot stamped component deteriorates. For this reason, the Nb content is set to 0.1000% or less. The Nb content is preferably 0.0800% or less or 0.0600% or less.
Ti is an element that forms carbonitrides in steel and improves the strength of the hot stamped component by precipitation strengthening. When the Ti content is less than 0.010%, a desired strength of the hot stamped component cannot be obtained. For this reason, the Ti content is set to 0.010% or more. The Ti content is preferably 0.020% or more or 0.025% or more.
On the other hand, when the Ti content is more than 0.100%, many carbonitrides are generated in steel, and the bendability of the hot stamped component deteriorates. For this reason, the Ti content is set to 0.100% or less. The Ti content is preferably 0.080% or less, 0.060% or less or 0.050% or less.
Cr is an element that increases the strength of the hot stamped component by dissolving in prior austenite grains during heating before hot stamping. When the Cr content is less than 0.010%, a desired strength of the hot stamped component cannot be obtained. For this reason, the Cr content is set to 0.010% or more. The Cr content is preferably 0.100% or more, 0.150% or more or 0.200% or more.
On the other hand, when the Cr content is more than 1.000%, a desired texture of prior austenite cannot be obtained. For this reason, the Cr content is set to 1.000% or less. The Cr content is preferably 0.700% or less, 0.500% or less or 0.400% or less.
Mo is an element that increases the strength of the hot stamped component by dissolving in prior austenite grains during heating before hot stamping. When the Mo content is less than 0.050%, a desired strength of the hot stamped component cannot be obtained. For this reason, the Mo content is set to 0.050% or more. The Mo content is preferably 0.100% or more or 0.150% or more.
On the other hand, when the Mo content is more than 1.000%, a desired texture of prior austenite cannot be obtained. For this reason, the Mo content is set to 1.000% or less. The Mo content is preferably 0.800% or less, 0.600% or less or 0.400% or less.
B is an element that improves the hardenability of steel. When the B content is less than 0.0005%, a desired strength of the hot stamped component cannot be obtained. For this reason, the B content is set to 0.0005% or more. The B content is preferably 0.0020% or more or 0.0030% or more.
On the other hand, when the B content is more than 0.0100%, coarse intermetallic compounds are formed in the hot stamped component. As a result, the bendability of the hot stamped component deteriorates. For this reason, the B content is set to 0.0100% or less. The B content is preferably 0.0080% or less, 0.0060% or less or 0.0040% or less.
The hot stamped component may comprise the following elements as optional elements in place of a part of Fe. The content of the following optional elements obtained when the following optional elements are not contained is 0%.
Co is an element that improves strength of the hot stamped component by solid-solution strengthening. In order to reliably obtain the effect, the Co content is preferably set to 0.01% or more, and more preferably set to 0.05% or more.
On the other hand, since the above effect will be saturated even if a large amount is comprised, the Co content is set to 3.00% or less. If necessary, the Co content may be limited to 2.00% or less, 1.50% or less, 1.00% or less or 0.50% or less.
Ni has an effect of increasing strength of the hot stamped component by dissolving in prior austenite grains during heating before hot stamping. In order to reliably obtain the effect, the Ni content is preferably set to 0.01% or more.
On the other hand, since the above effect will be saturated even if a large amount is comprised, the Ni content is set to 3.00% or less. If necessary, the Ni content may be limited to 2.00% or less, 1.50% or less, 1.00% or less or 0.50% or less.
Cu has an effect that increases the strength of the hot stamped component by dissolving in prior austenite grains during heating before hot stamping. In order to reliably obtain the effect, the Cu content is preferably set to 0.01% or more, and more preferably set to 0.05% or more.
On the other hand, since the above effect will be saturated even if a large amount is comprised, the Cu content is set to 3.00% or less. If necessary, the Cu content may be limited to 2.00% or less, 1.50% or less, 1.00% or less or 0.50% or less.
V has an effect that forms carbonitrides in steel and improves the strength of the hot stamped component by precipitation strengthening. In order to reliably obtain the effect, the V content is preferably set to 0.01% or more, and more preferably set to 0.05% or more.
On the other hand, when the V content is more than 3.00%, a lot of coarse carbonitrides is generated in steel. As a result, the bendability of the hot stamped component deteriorates. For this reason, the V content is set to 3.00% or less. If necessary, the V content may be limited to 2.00% or less, 1.50% or less, 1.00% or less or 0.50% or less.
W has an effect of improving the strength of the hot stamped component. In order to reliably obtain the effects, the W content is preferably set to 0.01% or more, and more preferably set to 0.05% or more.
On the other hand, since the above effect will be saturated even if a large amount is comprised, the W content is set to 3.00% or less. If necessary, the W content may be limited to 2.00% or less, 1.50% or less, 1.00% or less or 0.50% or less.
Ca is an element that suppresses generation of carbides that become starting points for fracture, and contributes for improvement of the bendability of the hot stamped component. In order to reliably obtain the effect, the Ca content is preferably set to 0.0001% or more, and more preferably set to 0.0010% or more.
On the other hand, since the above effect will be saturated even if a large amount is comprised, the Ca content is set to 0.1000% or less. If necessary, the Ca content may be limited to 0.0500% or less, 0.0200% or less, 0.0100% or less or 0.0060% or less.
Mg refines the microstructure due to formation of oxides and sulfides in molten steel, suppressing formation of a coarse MnS, and dispersing a lot of fine oxides. As a result, Mg contributes for improvement of the bendability of the hot stamped component. In order to reliably obtain these effects, the Mg content is preferably set to 0.0001% or more, and more preferably set to 0.0010% or more.
On the other hand, since the above effect will be saturated even if a large amount is comprised, the Mg content is set to 1.0000% or less. If necessary, the Mg content may be limited to 0.0500% or less, 0.0200% or less, 0.0100% or less or 0.0060% or less.
REM suppresses generation of coarse oxides. As a result, REM contributes for improvement of the bendability of the hot stamped component. In order to reliably obtain the effect, the REM content is preferably set to 0.0001% or more, and more preferably set to 0.0010% or more.
On the other hand, since the above effect will be saturated even if a large amount is comprised, the REM content is set to 1.0000% or less. If necessary, the REM content may be limited to 0.0500% or less, 0.0200% or less, 0.0100% or less or 0.0060% or less.
In the present embodiment, REM refers to a total of 17 elements that are composed of Sc, Y and lanthanoid, and the REM content refers to the total content of these elements.
Sb suppresses generation of coarse oxides. As a result, Sb contributes for improvement of the bendability of the hot stamped component. In order to reliably obtain the effect, the Sb content is preferably set to 0.001% or more.
On the other hand, since the above effect will be saturated even if a large amount is comprised, the Sb content is set to 1.000% or less. If necessary, the Sb content may be limited to 0.500% or less, 0.200% or less, 0.100% or less or 0.050% or less.
Sn suppresses generation of coarse oxides. As a result, Sn contributes for improvement of the bendability of the hot stamped component. In order to reliably obtain the effect, the Sn content is preferably set to 0.001% or more.
On the other hand, since the above effect will be saturated even if a large amount is comprised, the Sn content is set to 1.000% or less. If necessary, the Sn content may be limited to 0.500% or less, 0.200% or less, 0.100% or less or 0.050% or less.
Zr suppresses generation of coarse oxides. As a result, Zr contributes for improvement of the bendability of the hot stamped component. In order to reliably obtain the effect, the Zr content is preferably set to 0.001% or more.
On the other hand, since the above effect will be saturated even if a large amount is comprised, the Zr content is set to 1.000% or less. If necessary, the Zr content may be limited to 0.500% or less, 0.200% or less, 0.100% or less or 0.050% or less.
As refines the prior austenite grains by lowering an austenite single-phase transformation temperature. As a result, As contributes for improvement of the bendability of the hot stamped component. In order to reliably obtain the effect, the As content is preferably set to 0.001% or more.
On the other hand, since the above effect will be saturated even if a large amount is comprised, the As content is set to 0.100% or less. If necessary, the As content may be limited to 0.500% or less, 0.200% or less, 0.100% or less or 0.050% or less.
The remainder of the chemical composition of the hot stamped component may be Fe and impurities. Elements which are unavoidably mixed from a steel raw material or scrap and/or during the manufacture of steel and are allowed in a range where the properties of the hot stamped component according to the present embodiment do not deteriorate are exemplary examples of the impurities.
The above-mentioned chemical composition of the hot stamped component may be measured by an ordinary analysis method. For example, the chemical composition may be measured using inductively coupled plasma-atomic emission spectrometry (ICP-AES). C and S may be measured using a combustion-infrared absorption method, N may be measured using an inert gas fusion-thermal conductivity method, and O may be measured using an inert gas fusion-nondispersive infrared absorption method.
When a plating layer or a coating film is provided on the surface of the hot stamped component, the chemical composition is analyzed after the plating layer or the coating film is removed by mechanical grinding.
Next, the microstructure of the hot stamped component according to the present embodiment will be described.
In the hot stamped component according to the present embodiment, in a position at ¼ of a sheet thickness from a surface, in a texture of prior austenite, a maximum value of pole densities of an orientation group expressed by Euler angles of Φ=60° to 90°, φ1=60° to 90°, and φ2=450 is 3.0 or more, an average value of block sizes of martensite, tempered martensite and bainite is 1.20 μm or less.
In the present embodiment, the microstructure is specified in the position at ¼ of the sheet thickness from the surface of the hot stamped component (in a region from a depth of ⅛ of the sheet thickness from the surface to a depth of ⅜ of the sheet thickness from the surface). The reason therefor is that the microstructure at this position indicates a typical microstructure of the hot stamped component.
Note that when the hot stamped component has the plating layer or the coating film on the surface thereof, the “surface” refers to the interface of the plating layer or the coating film and the base steel sheet.
In texture of prior austenite, maximum value of pole densities of orientation group expressed by Euler angles of Φ=60° to 90°, φ1=60° to 90°, and φ2=45°: 3.0 or more
The present inventors obtained the following findings about a texture of prior austenite.
By developing the texture of prior austenite, it is possible to alleviate a strain concentration introduced by bending deformation. As a result, an increase of a load in an initial stage of the bending deformation is reduced and the bendability of the hot stamped component can be increased.
In the texture of prior austenite, when the maximum value of pole densities of the orientation group expressed by Euler angles of Φ=60° to 90°, φ1=60° to 90°, and φ2=45° (hereinafter, it may be referred as the pole density in the texture of prior austenite) is less than 3.0, a desired bendability of the hot stamped component cannot be obtained. For this reason, the maximum value of the pole densities of the orientation group in the texture of prior austenite is set to 3.0 or more. It is preferably 5.0 or more.
The upper limit is not particularly limited, but the maximum value of the pole densities of the orientation group in the texture of prior austenite may be set to 50.0 or less, 20.0 or less, 15.0 or less or 10.0 or less.
The pole density in the texture of prior austenite is measured by the following method.
The pole density of the texture of prior austenite is measured using an EBSD analyzer including a thermal field emission type scanning electron microscope and an EBSD detector, and the software “OIM Analysis (registered trademark)” attached to the EBSD analyzer. The pole density of the texture of prior austenite can be obtained by using the orientation data measured by the EBSD (Electron Back Scattering Diffraction) method and an orientation distribution function (ODF) that displays the three-dimensional texture calculated by computing, using spherical harmonics.
For a sample to be subjected to analysis by the EBSD method, a cross section parallel to a rolling direction and perpendicular to a sheet surface is mechanically polished, and strain is removed by chemical polishing or electrolytic polishing. Using this sample, EBSD measurement is performed at the position at ¼ of the sheet thickness from the surface (in the region from the depth of ⅛ of the sheet thickness from the surface to the depth of ⅜ of the sheet thickness from the surface), with a measurement range of 150 μm in length and a region of 50 μm in the sheet thickness direction and measurement intervals of 0.2 μm. For the measurement, an EBSD analyzer including a thermal field emission type scanning electron microscope and an EBSD detector may be used, for example, an EBSD analyzer including JSM-7001F manufactured by JEOL Ltd. and DVC5-type detector manufactured by TSL Solutions may be used. In this case, the degree of vacuum in the EBSD analyzer may be set to 9.6×10−5 Pa or less, the acceleration voltage may be set to 15 kV and the irradiation current level may be set to 13.
The orientation of prior austenite is measured by the following method. The orientation of prior austenite is calculated by the method described in Non-Patent Document 1, and the orientation of the prior austenite in each coordinate of the EBSD-measured region is specified. Next, an orientation map of prior austenite is created using the “Inverse Pole Figure” function installed in the software “OIM Analysis (registered trademark)” attached to the EBSD analyzer. Based on the orientation map, the maximum value of pole densitis of an orientation group within the ranges of Φ=60° to 90°, φ1=60° to 90° in section of φ2=45° is calculated. As a result, the maximum value of the pole densitis of the orientation group expressed by Euler angles of 0=60° to 90°, φ1=60° to 90°, and φ2=450 is obtained.
Analyses of a texture using the Euler angles (φ1, Φ, φ2) are widely performed. For example, the definition of the Euler angles (φ1, Φ, φ2) is described in Hiroshi Inoue: “Lecture (Easy Material Analysis Techniques)—Three-dimensional Orientation Analysis of Texture”, Light Metals, Vol. 41, No. 6 (1992), 358. By performing analysis using the above-mentioned software, even a person who does not fully understand the definition of the Euler angles (φ1, Φ, φ2) can easily calculate the maximum value of the pole densitis of the orientation group within the ranges of Φ=60° to 90°, φ1=60° to 90° in section of φ2=45°.
Average value of block sizes of martensite, tempered martensite and bainite: 1.20 μm or less
When the average value of block sizes of martensite, tempered martensite and bainite is more than 1.20 μm, a desired bendability of the hot stamped component cannot be obtained. For this reason, the average value of block sizes of martensite, tempered martensite and bainite is set to 1.20 μm or less. It is preferably 1.00 μm or less, and more preferably 0.90 μm or less.
The lower limit is not particularly limited, but it may be set to 0.30 μm or more, 0.40 μm or more or 0.50 μm or more.
The average value of block sizes of martensite, tempered martensite and bainite is measured by the following method.
A sample is cut out from an arbitrary position away from an end surface of the hot stamped component by a distance of 50 mm or more (a position that possibly avoids an end portion in a case where the sample cannot be collected at this position) so that a sheet thickness cross section parallel to the rolling direction can be observed. The size of the sample depends on a measurement device, but is set to a size that can be observed by at least about 10 mm in the rolling direction.
After polishing the cross section of the above sample using silicon carbide paper of #600 to #1500, the cross section is mirror-finished using liquid in which diamond powder having a grain size in the range of 1 to 6 μm is dispersed in a diluted solution of alcohol or the like or pure water. Next, the observation surface is finished by electrolytic polishing. Using this sample, in a position at ¼ of the sheet thickness from the surface (a region from a depth of ⅛ of the sheet thickness from the surface to a depth of ⅜ of the sheet thickness from the surface), an orientation information is obtained by measurement using an electron backscatter diffraction method with a measurement range of 150 μm in length and a region of 50 μm in the sheet thickness direction and measurement intervals of 0.2 μm. For the measurement, an EBSD analyzer including a thermal field emission type scanning electron microscope and an EBSD detector may be used, for example, an EBSD analyzer including JSM-7001F manufactured by JEOL Ltd. and DVC5-type detector manufactured by TSL Solutions may be used. In this case, the degree of vacuum in the EBSD analyzer may be set to 9.6×10−5 Pa or less, the acceleration voltage may be set to 15 kV and the irradiation current level may be set to 13.
In the obtained orientation information, using “Phase Map” function installed in the software “OIM Analysis (registered trademark)” attached to the EBSD analyzer, a region where a crystal structure is fcc is extracted. In these regions, using “Grain Average Misorientation” function installed in the software “OIM Analysis (registered trademark)” attached to the EBSD analyzer, under the condition that boundary with a crystal misorientation of 5° or more is regarded as the grain boundary, regions where the grain average misorientation is more than 0.5° are extracted as martensite, tempered martensite and bainite. For the obtained region, under the condition that boundary with a crystal misorientation of 150 or more is regarded as the grain boundary, the average value of block sizes of martensite, tempered martensite and bainite is obtained by obtaining the value calculated by the Number method using the “Grain Size (diameter)” function.
Note that the rolling direction of the hot stamped component is determined by the following method.
First, a sample is collected so that a sheet thickness cross section of the hot stamped component can be observed. The sheet thickness cross section of the collected sample is finished by mirror polishing, and then observed with an optical microscope. The observation area is width of 500 μm and full of the sheet thickness, and the areas with low brightness are determined as inclusions. Next, using the sheet thickness cross section initially observed by the above method as a reference, in the range of 0° to 180° with the sheet thickness direction as the axis, the cross-sectional observations of the plane parallel to the plane rotated in 5° increments are performed in the same way as the above method. The average values of the lengths of the long axes of inclusions in each cross section are calculated respectively, and a direction parallel to the long axes of the inclusions in the cross section in which the average value of the length of the long axes of the inclusions is maximum is determined as the rolling direction.
Note that when the rolling direction of the hot stamped component is known in advance, the rolling direction of the hot stamped component may be determined without using the above-mentioned determination method.
The microstructure of the hot stamped component is not particularly limited as long as a desired strength and bendability can be obtained. For example, the microstructure may consist of, by area %, a total of 90% or more of martensite, bainite and tempered martensite, and 10% or less of ferrite and residual austenite.
The area ratios of each structure are measured by the following method.
A sample is cut out from an arbitrary position away from an end surface of the hot stamped component by a distance of 50 mm or more (a position that possibly avoids an end portion in a case where a sample cannot be collected at this position) so that a sheet thickness cross section parallel to the rolling direction can be observed. The size of the sample depends on a measurement device, but is set to a size that can be observed by at least about 10 mm in the rolling direction.
After polishing the cross section of the sample using silicon carbide paper of #600 to #1500, the cross section is mirror-finished using liquid in which diamond powder having a grain size in the range of 1 to 6 μm is dispersed in a diluted solution of alcohol or the like or pure water. Next, the observation surface is finished by electrolytic polishing. At an arbitrary position on the cross section of the sample in a longitudinal direction, for a region which has a length of 50 μm and is present in a region from the depth of ⅛ of the sheet thickness from the surface to the depth of ⅜ of the sheet thickness from the surface, an orientation information is obtained by measurement using the electron backscatter diffraction method with measurement intervals of 0.1 μm. For the measurement, an EBSD analyzer including a thermal field emission type scanning electron microscope and an EBSD detector may be used, for example, an EBSD analyzer including JSM-7001F manufactured by JEOL Ltd. and DVC5-type detector manufactured by TSL Solutions may be used. In this case, a degree of vacuum in the EBSD analyzer may be set to 9.6×10−5 Pa or less, the acceleration voltage may be set to 15 kV and the irradiation current level may be set to 13.
Using the obtained crystal structure information and the “Phase Map” function installed in the software “OIM Analysis (registered trademark)” attached to the EBSD analyzer, a region where a crystal structure is fcc is determined as residual austenite. The ratio of the residual austenite is calculated, thereby obtaining the area ratio of the residual austenite. Next, in the regions where the crystal structure is bcc is determined as bainite, tempered martensite, martensite and ferrite. For these regions, using the “Grain Average Misorientation” function installed in the software “OM Analysis (registered trademark)” attached to the EBSD analyzer, under the condition that boundary with a crystal misorientation of 5° or more is regarded as the grain boundary, regions where the grain average misorientation is 0.50 or less are extracted as ferrite. The area ratio of the extracted ferrite is calculated, thereby obtaining the area ratio of ferrite.
Subsequently, the area ratio of the remaining region (the region where “Grain Average Misorientation” is more than 0.5°) is regarded as the area ratio as martensite, tempered martensite and bainite.
The hot stamped component may have a plating layer or a coating film on the surface. By having the plating layer or the coating film on the surface, corrosion resistance can be improved after hot stamping. Examples of the plating layer include an aluminum plating layer, aluminum-galvanized layer, aluminum-silicon plating layer, hot-dip galvanized layer, electrogalvanized layer, galvannealed layer, zinc-nickel plating layer, aluminum-magnesium-zinc-based plating layer.
The sheet thickness of the hot stamped component according to the present embodiment is not particularly limited, but it is preferably set to 0.5 to 3.5 mm from the perspective of reducing the weight of a vehicle body or the like.
It is not specifically necessary to limit the shape of the hot stamped component. For example, the hot stamped component may have a flat sheet shape, a curved shape, or a three-dimensional shape such as a hat shape.
The hot stamped component according to the present embodiment preferably have a tensile strength of 2300 MPa or more. The tensile strength is more preferably 2400 MPa or more, and even more preferably 2500 MPa or more. It is not necessary to limit the upper limit of the tensile strength, if necessary, the tensile strength may be set to 3000 MPa or less or 2800 MPa or less.
The tensile strength is obtained according to the test method described in JIS Z 2241:2011 by producing a No. 5 test piece described in JIS Z 2241:2011 from a flat position of the hot stamped component. A crosshead speed is set to 1 mm/min.
When the hot stamped component according to the present embodiment has a flat sheet shape (has no curved portion, etc.), a load at a ½ stroke of a stroke at the maximum load is preferably 8050 N or more. It is more preferably 8100 N or more, and even more preferably 8150 N or more. However, these standards are based on the case where the sheet thickness of the hot stamped component is 1.6 mm.
The load at the ½ stroke is obtained by performing a bending test under the following conditions based on the VDA standard (VDA238-100: 2017-04) specified by the Verband der Automobilindustrie and obtaining the load at the ½ stroke of the stroke at the maximum load.
When the sheet thickness of the hot stamped component is more than 1.6 mm, the bending test is performed after reducing the sheet thickness to 1.6 mm.
When the sheet thickness of the hot stamped component is less than 1.6 mm, where t is the sheet thickness of the hot stamped component, the load at the ½ stroke of the stroke at the maximum load is preferably 8050×t/1.6 (N) or more.
Note that the load at the ½ stroke of the stroke at the maximum load (however, when the sheet thickness of the hot stamped component is less than 1.6 mm, the value obtained by multiplying the load at the ½ stroke by 1.6/t (t is the sheet thickness in mm)) rarely exceeds 8500 N, 8300 N or 8200 N.
Next, a steel sheet for hot stamping for obtaining the hot stamped component according to the present embodiment will be described.
The steel sheet for hot stamping has the above-described chemical composition. The microstructure of the steel sheet for hot stamping is not particularly limited as long as a desired strength and bendability are obtained after hot stamping. For example, the microstructure may consist of, by area %, ferrite: 0% to 90%, bainite and martensite: 0% to 100%, pearlite: 0% to 80%, and residual austenite: 0% to 5%.
Further, the steel sheet for hot stamping may have a plating layer or a coating film on the surface. By having the plating layer or the coating film on the surface, corrosion resistance can be improved after hot stamping. Examples of the plating layer include an aluminum plating layer, aluminum-galvanized layer, aluminum-silicon plating layer, hot-dip galvanized layer, electrogalvanized layer, galvannealed layer, zinc-nickel plating layer, aluminum-magnesium-zinc-based plating layer.
A manufacturing method to obtain the steel sheet for hot stamping for obtaining the hot stamped component according to the present embodiment will be described. In order to obtain the above-described hot stamped component, it is particularly effective to control the finish rolling conditions during hot rolling in the manufacturing method of the steel sheet for hot stamping.
In the finish rolling, it is preferable to perform a rolling at one stand before a final stand and a rolling at the final stand with a rolling reduction of 50% or more respectively. By performing the rolling at one stand before the final stand and the rolling at the final stand with the rolling reduction of 50% or more, it is possible to control prior austenite with a desired texture.
Note that the rolling reduction here can be expressed as (1−t1/t0)×100(%), where t0 is an inlet sheet thickness and t1 is an outlet sheet thickness of each stand.
After the completion of the finish rolling (after the rolling of the final stand), it is preferable to start cooling after a lapse of 5.0 seconds or more. By elapsing 5.0 seconds or more before starting cooling, granular austenite grains can be generated. As a result, austenite grains with a flat shape are reduced, and granular austenite grains can be sufficiently secured.
Note that the cooling here does not include air cooling (cooling at an average cooling rate of slower than 10° C./s), but includes, for example, such as water cooling at an average cooling rate of 10° C./s or faster. The cooling stop temperature is preferably 550° C. to 650° C.
By the cooling after the finish rolling, austenite transforms into ferrite and pearlite. At this time, pearlite transformation progresses from the grain boundaries of the prior austenite grains. Pearlite having a specific texture is generated by transformation from austenite grains having a specific texture.
In addition, in order to soften the hot-rolled steel sheet, a coil after coiling may be subjected to softening heat treatment. The method of the softening heat treatment is not particularly limited, and an ordinary conditions may be used.
The total reduction during cold rolling is preferably set to 50% or less. The total reduction here can be expressed as (1−t3/t2)×100(%), where t3 is the sheet thickness after the cold rolling and t2 is the sheet thickness before the cold rolling.
A hot stamped component according to the present embodiment is obtained by hot stamping the steel sheet for hot stamping manufactured by the above-described method. As the hot stamping conditions, for example, it is preferable to heat the steel sheet for hot stamping to a temperature range of 800° C. to 1000° C. and hold in this temperature range for 60 to 1200 seconds.
By heating during hot stamping, a reverse transformation from pearlite to austenite is caused. Because pearlite has a specific texture, the texture of the austenite generated by the reverse transformation develops. By cooling after hot stamping, a transformation from austenite to martensite is caused. When the final structure becomes martensite, the texture of austenite is preserved. Therefore, the texture of the prior austenite remains developed in the structure after hot stamping.
When the heating temperature is lower than 800° C. or the holding time is shorter than 60 seconds, austenitization becomes insufficient, and the bendability may deteriorate or a desired strength may not be obtained in the hot stamped component. On the other hand, when the heating temperature is higher than 1000° C. or the holding time is longer than 1200 seconds, the grains of prior austenite grow excessively, and the bendability may deteriorate or a desired strength may not be obtained in the hot stamped component.
A heating atmosphere is, for example, such as the atmosphere, a gas combustion atmosphere with a controlled ratio of air and fuel, or a nitrogen atmosphere, and the dew point of these gases may be controlled.
After holding in the temperature range, hot stamping is performed. After hot stamping, cooling may be performed to a temperature range of 250° C. or lower at an average cooling rate of 20° C./s or faster.
Examples of heating methods before hot stamping include heating using an electric furnace and a gas furnace, a flame heating, an electrical heating, a high-frequency heating, and an induction heating.
By the above methods, the hot stamped component according to the present embodiment is obtained. A tempering treatment at 130° C. to 600° C. may be performed after hot stamping for softening, or a baking hardening treatment after painting may be performed. In addition, a portion of the hot stamped component may be tempered by laser irradiation or the like to provide a partially softened region.
Next, examples of the present invention will be described. Conditions in the examples are one example of conditions employed to confirm the feasibility and effects of the present invention, but the present invention is not limited to these examples. The present invention may employ various conditions to achieve the object of the present invention without departing from the scope of the present invention.
Slabs manufactured by casting molten steel having a chemical composition shown in Tables 1A to 1F were held in a temperature range of 1200° C. or higher for 20 minutes or longer, and then subjected to hot rolling, coiling, and cold rolling. The final rolling was performed under conditions shown in Tables 2A to 2E.
Note that after the completion of the finish rolling, the average cooling rate of cooling after a lapse of 5.0 seconds or more was 10° C./s or faster, and the cooling stop temperature was 550° C. to 650° C. In addition, the total reduction of cold rolling was 50% or less.
The obtained steel sheets for hot stamping were subjected to hot stamping under the conditions shown in Tables 2A to 2E, and then cooled to the temperature range of 250° C. or lower at an average cooling rate of 20° C./s or faster. As a result, the hot-stamping formed bodies shown in Tables 3A to 3G were obtained.
However, for some examples, as described in the tables, plating or heating treatment for softening were performed.
The underlines in the tables indicate that it is outside the scope of the present invention, falls outside the preferable manufacturing conditions, or the characteristic value is not preferable.
The microstructure of the hot stamped component according to the present invention consisted of, by area %, a total of 90% or more of martensite, bainite and tempered martensite, and 10% or less of ferrite and residual austenite. In addition, the sheet thickness of the hot stamped component according to the present invention was 0.5 to 3.5 mm.
Measurements of the microstructure of the hot stamped component and the measurement of the mechanical properties of the hot stamped component were performed by the above-described methods.
The bending test according to the VDA standard (VDA238-100: 2017-04) is widely performed on components for vehicle, but the bending test targets only flat sheet. Therefore, this VDA standard cannot evaluate the bendability of the hot stamped component with shapes other than flat sheet shape. On the other hand, when the hot stamped component has a bent portion, the bend portion is affected by such as the curvature of the bent portion. For this reason, the inventors considered that it is appropriate to evaluate the bendability according to this VDA standard using a hot stamped component with a flat sheet shape as a test material. Therefore, the bending test was performed on a hot stamped component with a flat sheet shape obtained by hot stamping without bending (using a die that can obtain a hot stamped component without a bent portion). In addition, since the rolling direction of the hot stamped component was known in advance, the rolling direction of the hot stamped component was determined without determining of the rolling direction by evaluation using the above-mentioned determination method. For the bending test machine, a SHIMADZU AUTOGRAPH 20 kN was used.
When the tensile strength TS was 2300 MPa or more, it was determined as having high strength and acceptable, and when the tensile strength TS was less than 2300 MPa, it was determined as not having high strength and unacceptable.
When the load at the ½ stroke of the stroke at the maximum load was 8050 N or more, it was determined as having excellent bendability and acceptable. On the other hand, when the load at the ½ stroke of the stroke at the maximum load was less than 8050 N, it was determined as not having excellent bendability and unacceptable. However, in a case where the sheet thickness of the hot stamped component was less than 1.6 mm, where t was the sheet thickness of the hot stamped component, when the load at the ½ stroke of the stroke at the maximum load was 8050×t/1.6 (N) or more, it was determined as having excellent bendability and acceptable. On the other hand, when the load at the ½ stroke of the stroke at the maximum load was less than 8050×t/1.6 (N), it was determined as not having excellent bendability and unacceptable. Note that in a case where the sheet thickness of the hot stamped component was less than 1.6 mm, the value obtained by multiplying the load at the ½ stroke by 1.6/t (t is the sheet thickness in mm) was mentioned in the “Load at ½ stroke” in Tables 3 A to 3G.
0.38
0.72
0.008
3.200
0.05
0.84
0.120
0.0134
0.0121
0.0240
0.0006
0.6200
0.0008
0.1330
0.007
0.121
0.007
1.220
0.020
1.230
0.0004
0.0115
17
18
24
30
36
42
48
49
59
60
69
70
105
20
10
20
20
30
30
40
30
30
40
40
40
20
20
0.4
2.2
4.1
745
1023
1258
2083
7826
2214
17
17
2216
18
18
2187
24
24
2.1
8004
30
30
2.7
7832
36
36
2.8
7838
42
42
1.7
7972
48
48
2.2
7885
49
49
2.4
7913
59
59
2.6
7838
60
60
2175
69
69
1.9
7962
70
70
2226
78
78
2.3
7953
79
79
2298
2.6
7963
2244
2.2
7889
2209
105
105
2.1
7859
152
1.9
7964
153
1.8
7965
154
2.2
7982
155
2.4
8002
156
2.7
8011
157
2.8
8039
158
2.2
7995
159
2.5
8011
160
2.3
7896
161
2.1
7923
162
2.4
7984
171
2.5
2265
7898
175
1.9
1.31
2156
7762
2.4
2259
7930
180
2.2
1.27
2203
7991
From Tables 3A to 3G, it can be seen that the hot-stamping formed bodies according to the present invention examples had high strength and excellent bendability.
On the other hand, it can be seen that in the hot-stamping formed bodies according to comparative examples, one or more of the properties deteriorated.
According to the above-described aspects of the present invention, it is possible to provide a hot stamped component having high strength and excellent bendability.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2022-090847 | Jun 2022 | JP | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/JP2023/020238 | 5/31/2023 | WO |
