[0001]The present invention relates to a hot-stamped part and a manufacturing method thereof. Specifically, the present invention relates to a hot-stamped part having excellent strength and ductility, which contributes to a reduction in the weight of a vehicle body and an improvement in collision safety, and a manufacturing method thereof.
Priority is claimed on Japanese Patent Application No. 2019-096625, filed May 23, 2019, the content of which is incorporated herein by reference. [Background Art]
[0002]
In recent years, due to the demand for a reduction in the weight of a vehicle body and an improvement in collision safety, a high strength steel sheet has been applied to vehicle body components for vehicles. Since vehicle body components are formed by press forming, an improvement in press formability, particularly an improvement in shape fixability is an issue. Therefore, as a method for manufacturing a vehicle body component having excellent shape accuracy and high strength, a hot stamping method has attracted attention.
[0003]
Furthermore, in recent years, a technique for applying a tailored blank to a hot stamping method has been studied. A tailored blank is obtained by joining steel sheets having different sheet thicknesses, chemical compositions, microstructures, or
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the like by welding. In the tailored blank, the properties in one of the joined steel sheets can be partially changed. For example, a certain part is given high strength to suppress deformation in that part, and another part is given low strength to deform that part, whereby an impact can be absorbed.
[0004]
As the technique for applying the tailored blank to the hot stamping method, there is a technique of using a tailored blank obtained by joining a steel sheet that has low strength after hot stamping and a steel sheet that has high strength after hot stamping together by welding. As the steel sheet that has high strength after hot stamping, for example, a steel sheet disclosed in Patent Document 1 can be used. As the steel sheet that has low strength after hot stamping, the chemical composition of steel may be adjusted so as to achieve low strength after cooling a die in the hot stamping.
[0005]
As one of the kinds of steel applied to the tailored blank, there is ultra low carbon steel. The ultra low carbon steel has a low carbon content and thus has a characteristic that high-strengthening is difficult even if rapid cooling is performed after heating. Patent Document 2 discloses that ultra low carbon steel is used as a low strength material in a hot stamping method. Patent Document 2 discloses a technique in which a steel sheet is heated to a temperature of an AC3 or higher and thereafter hot-stamped to achieve a microstructure having bainite and bainitic ferrite as a primary phase, thereby improving local deformability. Patent Document 2 discloses that with this technique, fracture is less likely to occur when a vehicle body component is deformed in a bending mode at the time of a collision and thus impact absorbability is excellent due to plastic deformation.
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[0006]
In a case of using the ultra low carbon steel as the low strength material, there are cases where deformation is concentration due to a collision, fracture occurs when large tensile deformation is received instead of a bending mode, and the energy absorption ability of a component is significantly reduced. Therefore, the ultra low carbon steel used as the low strength material for the tailored blank is required to have excellent ductility after hot stamping. [Prior Art Document] [Patent Document]
[0007]
[Patent Document 1] Japanese Unexamined Patent Application, First Publication No. 2004-197213
[Patent Document 2] PCT International Publication No. WO2012/157581 [Disclosure of the Invention] [Problems to be Solved by the Invention]
[0008]
The present invention has been made in view of the above circumstances, and an object thereof is to provide a hot-stamped part having excellent ductility and a manufacturing method thereof. Specifically, an object thereof is to provide a hot-stamped part having a low C content, excellent ductility after hot stamping, and having the minimum strength required for a general hot-stamped part, and a manufacturing method thereof. [Means for Solving the Problem]
[0009]
The present inventors had conducted intensive research on the ductility of a
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hot-stamped part, and as a result, found that the ductility of a hot-stamped part is affected by the microstructure after hot stamping. When transformation into austenite is incurred by heating in hot stamping and thereafter hot stamping is performed from the state of austenite, a large amount of acicular ferrite having poor ductility is generated due to rapid cooling by a die. Acicular ferrite may be called bainitic ferrite, massive ferrite, or ultra low carbon martensite.
[0010]
After heating in hot stamping, when cooling is performed by air cooling with a slow cooling rate instead of rapid cooling by a die and hot stamping is performed after a portion of austenite is transformed into polygonal ferrite, the microstructure after the hot stamping becomes a composite structure of polygonal ferrite and acicular ferrite. Since polygonal ferrite has excellent ductility, by forming a composite structure as described above, it is possible to manufacture a hot-stamped part having excellent ductility.
[0011]
Whether or not the transformation from austenite into polygonal ferrite has started can be determined by measuring the temperature of a steel sheet during air cooling after being taken out of a heating furnace and observing a transformation latent heat evolution. By starting hot stamping after the transformation latent heat evolution occurs, a microstructure containing polygonal ferrite can be obtained in the hot-stamped part. The present inventors found that a hot-stamped part having excellent ductility can be manufactured by performing hot stamping using a steel sheet having an optimized chemical composition by the method as described above.
[0012]
However, in a case where ultra low carbon steel is used, when the proportion
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of polygonal ferrite becomes excessive, the hot-stamped part becomes soft and has insufficient strength. Therefore, by adjusting the temperature at which hot stamping is started so that the proportion of polygonal ferrite does not become excessive, the strength after forming can be secured.
[0013]
The present invention has been obtained based on the above findings, and the gist of the present invention is as follows.
(1) A hot-stamped part according to an aspect of the present invention includes, as a chemical composition, by mass%:
C: 0.0005% to 0.0080%;
Si: 0.005% to 1.000%;
Mn: 0.01% to 2.50%;
Al: 0.010% to 0.100%;
P: 0.200% or less;
S: 0.100% or less;
N: 0.0100% or less;
Ti:0% to 0.150%;
Nb:0% to 0.100%;
V:0% to 0.100%;
Zr:0% to 0.100%;
B: 0% to 0.0050%; and
a remainder consisting of Fe and impurities,
in which the hot-stamped part has a microstructure including, by area fraction, 20% or more and less than 95% of acicular ferrite, 5% to 80% of polygonal ferrite, and 0% to 5% of a remainder in a microstructure.
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(2) The hot-stamped part according to (1) may include, as the chemical
composition, by mass%, one or two or more selected from the group consisting of:
Ti: 0.005% to 0.150%; Nb: 0.005% to 0.100%; V: 0.005% to 0.100%; and Zr: 0.005% to 0.100%.
(3) The hot-stamped part according to (1) or (2) may include, as the chemical composition, by mass%: B: 0.0002% to 0.0050%.
(4) The hot-stamped part according to any one of (1) to (3) may include: a plating layer on a surface of the hot-stamped part.
(5) A manufacturing method of the hot-stamped part according to another aspect of the present invention, includes:
heating a steel sheet having the chemical composition according to (1) to a temperature range of Ac3 to 1100°C;
performing holding in the temperature range for longer than 0 seconds to 1,200 seconds or shorter;
cooling the steel sheet to a temperature range of a maximum temperature during a transformation latent heat evolution to a start temperature of the transformation latent heat evolution - 150°C at an average cooling rate of 5 to 30 °C/s, and starting forming in the temperature range; and
performing cooling to a temperature range of 600°C to 40°C at an average cooling rate of 20 to 1,000 °C/s after the forming. [Effects of the Invention]
[0014]
According to the above aspects according to the present invention, it is
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possible to provide a hot-stamped part having excellent ductility and the minimum strength required for the hot-stamped part and a manufacturing method thereof. [Brief Description of the Drawings]
[0015]
FIG. 1A is a diagram showing the relationship between the elapsed time after a steel sheet is heated to 980°C and the furnace lid of a heating furnace is opened, and the temperature of the steel sheet.
FIG. IB is a diagram showing a first derivative of the temperature the steel sheet in FIG. 1A with respect to the elapsed time.
FIG. 1C is a diagram showing a second derivative of the temperature the steel sheet in FIG. 1A with respect to the elapsed time.
FIG. 2A is a diagram showing the relationship between the elapsed time after a steel sheet is heated to 940°C and the furnace lid of a heating furnace is opened, and the temperature of the steel sheet.
FIG. 2B is a diagram showing a first derivative of the temperature the steel sheet in FIG. 2A with respect to the elapsed time.
FIG. 2C is a diagram showing a second derivative of the temperature the steel sheet in FIG. 2A with respect to the elapsed time. [Embodiments of the Invention]
[0016]
Hereinafter, a hot-stamped part according to the present embodiment and a manufacturing method thereof will be described in detail. First, the reason for limiting the chemical composition of the hot-stamped part according to the present embodiment will be described. The numerical limit range described below includes a lower limit and an upper limit in the range. Numerical values indicated as "less than"
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or "more than" do not fall within the numerical range. In addition, all % regarding the chemical composition means mass%.
[0017]
The hot-stamped part according to the present embodiment contains, as a chemical composition, by mass%: C: 0.0005% to 0.0080%, Si: 0.005% to 1.000%, Mn: 0.01% to 2.50. %, Al: 0.010% to 0.100%, P: 0.200% or less, S: 0.100% or less, N: 0.0100% or less, and a remainder: Fe and impurities. Hereinafter, each element will be described.
[0018]
C: 0.0005% to 0.0080%
C is an element that greatly affects the strength and ductility of the hot-stamped part. When the C content is too high, low temperature transformation phases such as carbides and bainite are formed after hot stamping, and the ductility of the hot-stamped part decreases. For this reason, the C content is set to 0.0080% or less. Preferably, the C content is 0.0070% or less, 0.0050% or less, 0.0040% or less, 0.0034% or less, or 0.0030% or less. When the C content is too low, the strength of the hot-stamped part decreases, and fracture due to insufficient strength is likely to occur. Therefore, the C content is set to 0.0005% or more. Preferably, the C content is 0.0010% or more.
[0019]
Si: 0.005% to 1.000%
Si is an alloying element having a solid solution strengthening ability, and is contained in order to obtain the strength of the hot-stamped part. However, when the Si content exceeds 1.000%, a surface scale problem arises. That is, after pickling the scale generated during hot rolling, a pattern due to surface unevenness is generated,
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and the surface external appearance becomes inferior. Therefore, the Si content is set to 1.000% or less. In a case of plating the surface of a steel sheet, there are cases where a high Si content causes deterioration of plating properties. Therefore, the Si content is preferably set to 0.500% or less. When the Si content is too low, the hot-stamped part cannot achieve desired strength. Therefore, the Si content is set to 0.005% or more. Preferably, the Si content is 0.100% or more, 0.120% or more, 0.150% or more, or 0.200% or more.
[0020]
Mn: 0.01% to 2.50%
Mn is also an alloying element having a solid solution strengthening ability like Si, and is contained in order to obtain a desired strength of a hot-stamped part. However, when the Mn content exceeds 2.50%, the hardenability of the steel sheet increases, and the formation of polygonal ferrite in air cooling after heating in hot stamping is suppressed, so that the ductility of the hot-stamped part decreases. Therefore, the Mn content is set to 2.50% or less. The Mn content is preferably 2.35% or less, or 2.00% or less. When the Mn content is too low, the desired strength of the hot-stamped part cannot be obtained. Therefore, the Mn content is set to 0.01% or more. Preferably, the Mn content is 0.10% or more, 0.50% or more, 0.80% or more, or 1.00% or more.
[0021]
Al: 0.010% to 0.100%
Al is an element used for deoxidizing molten steel. The Al content is 0.010% or more in order to sufficiently deoxidize the molten steel. Preferably, the Al content is 0.015% or more, 0.020% or more, or 0.025% or more. However, when the Al content exceeds 0.100%, a large amount of non-metal inclusions are formed in the
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steel, and defects are likely to occur on the surface of the hot-stamped part. Therefore, the Al content is set to 0.100% or less. Preferably, the Al content is 0.080% or less, or 0.070% or less.
[0022]
P: 0.200% or Less
Like Si and Mn, P also has a solid solution strengthening ability and is an effective element for obtaining the strength of the hot-stamped part. However, when the P content exceeds 0.200%, the weld crackability and toughness of the hot-stamped part deteriorate. Therefore, the P content is set to 0.200% or less. Preferably, the P content is 0.100% or less, or 0.070% or less. The lower limit is not particularly specified. However, from the viewpoint of securing the strength by P, the P content may be set to 0.020% or more, or 0.030% or more.
[0023]
S: 0.100% or Less
S increases the size of the non-metal inclusions in the steel. When a large amount of S is contained, voids are generated from the non-metal inclusions having a large size as the origin and are likely to cause fracture, so that the ductility of the hot-stamped part deteriorates. Therefore, the S content is set to 0.100% or less. Preferably, the S content is 0.030% or less, or 0.020% or less. The lower limit is not particularly specified. However, since an excessive reduction in the S content causes an increase in the manufacturing cost in a desulfurization process, the S content may be set to 0.001% or more.
[0024]
N: 0.0100% or Less
N is an impurity element, and is an element that forms nitrides in the steel and
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deteriorates the ductility of the hot-stamped part. When the N content exceeds 0.0100%, nitrides in the steel becomes coarse and the ductility of the hot-stamped part deteriorates. Therefore, the N content is set to 0.0100% or less. Preferably, the N content is 0.0095% or less, 0.0070% or less, 0.0050% or less, or 0.0035% or less. The lower limit is not particularly specified. However, since an excessive reduction in the N content causes an increase in the manufacturing cost in a steelmaking process, the N content may be set to 0.0010% or more.
[0025]
The hot-stamped part according to the present embodiment may contain the above elements, and the remainder consisting of Fe and impurities. However, in order to improve various properties, the following elements (optional elements) may be contained instead of a portion of Fe. In order to reduce the alloy cost, these optional elements do not necessarily have to be contained in the steel. Therefore, the lower limits of the amounts of these optional elements are all 0%. Examples of the impurities include elements that are unavoidably incorporated from steel raw materials or scrap and/or in a steelmaking process and are allowed in a range in which the properties of the hot-stamped part according to the present embodiment are not inhibited.
[0026]
Ti: 0% to 0.150%, Nb: 0% to 0.100%, V: 0% to 0.100%, and Zr: 0% to 0.100%
Ti, Nb, V, and Zr have an effect of forming carbides in the steel and improving the strength of the hot-stamped part by precipitation strengthening and thus may be contained as necessary. In order to reliably exhibit the above effect, the amount of any one of Ti, Nb, V, and Zr is preferably set to 0.005% or more. On the
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other hand, when the Ti content exceeds 0.150%, or when the amount of any one of Nb, V, and Zr exceeds 0.100%, the strength of the hot-stamped part is excessively increased, resulting in a decrease in the ductility. Therefore, the Ti content is set to 0.150% or less, and the Nb content, V content, and Zr content are set to 0.100% or less.
[0027]
B: 0% to 0.0050%
B is an element that has an effect of suppressing grain growth and contributes to the improvement in the strength of the hot-stamped part, and thus may be contained as necessary. In order to reliably exhibit the above effect, the B content is preferably set to 0.0002% or more. More preferably, the B content is 0.005% or more, or 0.0010% or more. When the B content is too high, the ductility of the hot-stamped part decreases. Therefore, the B content is set to 0.0050% or less.
[0028]
In addition to the above-mentioned optional elements, Cr, Ni, Cu, Mo, Sn, Sb, and As may be contained. The amounts of Cr, Ni, Cu, and Mo are not particularly specified. However, since there are cases where an excessive inclusion of these elements causes a decrease in castability, the total amount of these elements may be set to 1.00% or less. In addition, there are cases where an excessive inclusion of impurity elements that are unavoidably contained, such as Sn, Sb, and As causes deterioration of the ductility of the hot-stamped part. Therefore, the total amount of these elements may be set to 0.10% or less.
[0029]
The chemical composition of the hot-stamped part described above may be measured by a general analytical method. For example, the chemical composition may be measured using inductively coupled plasma-atomic emission spectrometry
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(ICP-AES). C and S may be measured using a combustion-infrared absorption method, and N may be measured using an inert gas fusion-thermal conductivity method. In a case where the hot-stamped part is provided with a plating layer on the surface, the chemical composition may be analyzed after removing the plating layer on the surface by mechanical grinding.
[0030]
Next, the microstructure of the hot-stamped part according to the present embodiment will be described.
The hot-stamped part according to the present embodiment has a microstructure including, by area fraction, 20% or more and less than 95% of acicular ferrite, 5% to 80% of polygonal ferrite, and 0% to 5% or less of the remainder in the microstructure.
[0031]
Area Fraction of Acicular Ferrite: 20% or More and Less Than 95%
Acicular ferrite is a structure formed by transformation from austenite through rapid cooling by contact heat transfer through a die. When the area fraction of acicular ferrite is too high, the ductility of the hot-stamped part decreases. Therefore, the area fraction of acicular ferrite is set to less than 95%. The area fraction of acicular ferrite is preferably 90% or less, and more preferably 80% or less. On the other hand, when the area fraction of acicular ferrite is too low, the minimum strength required for a general hot-stamped part cannot be obtained. Therefore, the area fraction of acicular ferrite is set to 20% or more. The area fraction of acicular ferrite is preferably 40% or more, 50% or more, or 60% or more.
[0032]
Area Fraction of Polygonal Ferrite: 5% to 80%
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Polygonal ferrite is an important structure for obtaining the ductility of the hot-stamped part. Polygonal ferrite is a structure formed during air cooling from after a steel sheet is heated to a temperature of an AC3 point or higher in a heating furnace until pressing from the heating furnace is completed. The average cooling rate from 900°C to 800°C during the air cooling is approximately 10 to 30 °C/s, which is slower than that of rapid cooling through a die. Therefore, as Fe atoms and C atoms sufficiently diffuse during the air cooling, transformation from austenite into polygonal ferrite occurs. In order to obtain the ductility of the hot-stamped part, the area fraction of polygonal ferrite is set to 5% or more. The area fraction of polygonal ferrite is preferably 10% or more, and more preferably 20% or more. When the area fraction of the polygonal ferrite is too high, the area fraction of acicular ferrite decreases, and the minimum strength required for a general hot-stamped part cannot be obtained. Therefore, the area fraction of polygonal ferrite is set to 80% or less. The area fraction of polygonal ferrite is preferably 60% or less, 50% or less, or 40% or less.
[0033]
Area Fraction of Remainder in Microstructure: 0% to 5%
The remainder in the microstructure other than acicular ferrite and polygonal ferrite refers to a structure that cannot be distinguished by structure observation described later. The remainder in the microstructure includes low temperature transformation phases such as carbides and bainite, precipitates, and non-metal inclusions. When the area fraction of the remainder in the microstructure increases, the ductility of the hot-stamped part decreases. Therefore, the area fraction of the remainder in the microstructure is set to 5% or less. Preferably, the area fraction of the remainder in the microstructure is 3% or less, 2% or less, or 1% or less. It is preferable that the remainder in the microstructure is not included in order to improve
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the ductility of the hot-stamped part. Therefore, the area fraction of the remainder in the microstructure is more preferably 0%.
[0034]
Method of Measuring Area Fraction of Microstructure
The area fraction of the microstructure is measured by the following methods.
First, a sample is collected from a position 10 mm or more away from the end face of the hot-stamped part so that the cross section perpendicular to the surface (sheet thickness cross section) becomes an observed section. The size of the sample depends on a measuring apparatus, but may be set so that a size of about 10 mm can be observed in a rolling direction. The cross section of the cut-out sample is polished using #600 to #1500 silicon carbide paper and thereafter mirror-finished using a liquid obtained by dispersing a diamond powder having a particle size of 1 to 6 um in a diluted solution such as alcohol or pure water. Next, the cross section of the sample is polished at room temperature using colloidal silica containing no alkaline solution for 8 minutes to remove strain introduced into the surface layer of the sample.
In a case where a sample cannot be collected from a position 10 mm or more away from the end surface of the hot-stamped part because of the shape of the hot-stamped part, a sample may be collected from a point other than the end portion that has not been sufficiently heat-treated.
[0035]
At a thickness 1/4 position of the cross section of the sample from the surface, a region of 50 um in the rolling direction and 50 um in the sheet thickness direction is measured by an electron backscatter diffraction method at a measurement interval of 0.1 um to obtain crystal orientation information. For the measurement, an apparatus including a thermal field-emission scanning electron microscope (JSM-7001F
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manufactured by JEOL Ltd.) and an EBSD detector (DVC5 type detector manufactured by TSL) is used. At this time, the degree of vacuum in the apparatus is set to 9.6 x 10-5 Pa or less, the acceleration voltage is set to 15 kV, the irradiation current level is set to 13, and the electron beam irradiation time is set to 0.01 sec/point. From the obtained crystal orientation information, using the "Image Quality" function installed in the software "OIM Analysis (registered trademark)" attached to the EBSD analyzer, grain boundaries having an orientation difference (grain average misorientation (GAM value)) of 5° or more, grains having an average crystal orientation difference of 0.5° or less inside grains surrounded by the grain boundaries having an orientation difference of 5° or more, and grains having an average crystal orientation difference of more than 0.5° inside the grains surrounded by the grain boundaries having an orientation difference of 5° or more are specified. The above operation is performed on at least five regions. By calculating the average value of the area fractions of the grains having an average crystal orientation difference of 0.5° or less inside the grains surrounded by the grain boundaries having an orientation difference of 5° or more, the area fraction of polygonal ferrite is obtained. In addition, by calculating the average value of the area fractions of the grains having an average crystal orientation difference of more than 0.5° inside the grains surrounded by the grain boundaries having an orientation difference of 5° or more, the area fraction of acicular ferrite is obtained.
[0036]
Ferrite transformed by rapid cooling like acicular ferrite, a large amount of strain is introduced into the grains. Therefore, fluctuation in the crystal orientations inside the grains increases, and the average crystal orientation difference exceeds 0.5°. On the other hand, polygonal ferrite transformed due to a slow cooling rate and the
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diffusion of Fe atoms and C atoms has a smaller amount of strain introduced into the grains compared to acicular ferrite, so that the average crystal orientation difference becomes 0.5° or less.
[0037]
The area fraction of the remainder in the microstructure is obtained by calculating a value obtained by subtracting the sum of the area fraction of polygonal ferrite and the area fraction of acicular ferrite obtained by the above method from 100%.
[0038]
The hot-stamped part according to the present embodiment may have a plating layer on the surface. Having a plating layer on the surface is preferable because the corrosion resistance of the hot-stamped part is improved.
Examples of the plating to be applied include aluminum plating, aluminum-zinc plating, hot-dip galvanizing, electrogalvanizing, and hot-dip galvannealing.
[0039]
The hot-stamped part according to the present embodiment can be obtained by performing hot stamping a steel sheet. Therefore, the hot-stamped part according to the present embodiment may have a shape formed by hot stamping. Examples of the shape formed by hot stamping include a shape with a bent portion having a radius of 2 to 50 mm. In a case where the hot-stamped part has a shape with a bent portion having a radius of 2 to 50 mm, the impact absorbability at the time of a collision can be improved.
[0040]
Next, a manufacturing method of the hot-stamped part according to the present embodiment will be described. First, a manufacturing method of the steel
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sheet applied to the hot-stamped part according to the present embodiment will be described.
[0041]
The manufacturing method of the steel sheet applied to the hot-stamped part according to the present embodiment is not particularly limited, and a general manufacturing method may be used. For example, a steel piece manufactured by a general method such as a continuously cast slab or a thin slab caster is heated, hot-rolled, then cooled, pickled as necessary, and cold-rolled, thereby obtaining a steel sheet.
[0042]
The steel sheet may be plated as necessary. By performing hot stamping on the steel sheet having a plating on the surface, a hot-stamped part having a plating layer on the surface is obtained.
Kinds of plating include aluminum plating, aluminum-zinc plating, and galvanizing. A method of applying the plating may be a general method. For example, for the aluminum plating, a Si concentration in the bath of 5% to 12% is suitable, for the aluminum-zinc plating, a Zn concentration in the bath of 40% to 50% is suitable, and for the galvanizing. Even if Mg, Zn, and the like are mixed in an aluminum plating layer or Mg is mixed in an aluminum-zinc plating layer, a steel sheet having the same properties can be manufactured without any particular problem. Regarding the atmosphere at the time of plating, plating can be performed even in a continuous plating facility having a non-oxidizing furnace, even in a continuous plating facility without a non-oxidizing furnace, or even under general atmosphere conditions. In addition, regarding the galvanizing, the plating may be applied by a method such as hot-dip galvanizing, electrogalvanizing, or hot-dip galvannealing.

[0043]
Furthermore, there is no particular problem even if Ni pre-plating, Fe pre-plating, or other metal pre-plating for improving the plating properties is performed. There is no particular problem even if different kinds of metal plating, films of inorganic compounds or organic compounds, or the like are applied to the surface of the plating layer.
[0044]
Next, the manufacturing method of the hot-stamped part according to the present embodiment using the steel sheet obtained by the above method will be described.
The manufacturing method of the hot-stamped part according to the present embodiment includes:
a heating step of heating a steel sheet having to a temperature range of Ac3 to 1,100°C;
a holding step of performing holding in the temperature range for longer than 0 seconds to 1,200 seconds or shorter;
a hot forming step of cooling the steel sheet to a temperature range of a maximum temperature during a transformation latent heat evolution to a start temperature of the transformation latent heat evolution - 150°C at an average cooling rate of 5 to 30 °C/s, and starting forming in the temperature range; and
a rapid cooling step of performing cooling to a temperature range of 600°C to 40°C at an average cooling rate of 20 to 1,000 °C/s after the forming.
Hereinafter, each step will be described.
[0045]
[Heating Step]
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In a case of performing hot stamping on a tailored blank in which a low strength material (a material according to the embodiment of the present invention) and a high strength material (a material that is not related to the embodiment of the present invention) are joined, the microstructure of the high strength material needs to be sufficiently austenitized. Therefore, the heating temperature in the heating step is set to the AC3 point or higher. In general, since the AC3 point temperature of the high strength material is lower than the AC3 point temperature of the low strength material, the microstructure of the high strength material can also be austenitized when heated to the AC3 point temperature of the low strength material or higher, which is preferable. The Ac3 point of the low strength material according to the embodiment of the present invention can be obtained by the following expression. When the steel sheet is heated to the Ac3 point or higher, ferrite is transformed into austenite, and strain introduced by shearing or welding is released. The heating temperature is set to 1,100°C or lower in order to prevent a large amount of scale from being generated in a case where the surface does not have a plating layer and to prevent excessive alloying in a case where the surface has a plating layer. Preferably, the heating temperature is 1,000°C or lower.
[0046]
Ac3 (°C) = exp(X) - 28
X = 6.8165 - 0.47132 x C - 0.057321 x Mn + 0.0660261 x Si + 0.10593 x Ti + 2.0272 x N + 1.0536 x S - 0.12024 x Si x C + 0.29225 x C2 +0.015660 x Mn2
Here, the element symbol in the above expression indicates the amount of the corresponding element in the steel by mass%, and 0 is substituted in a case where the corresponding element is not contained.
[0047]
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[Holding Step]
After heating the steel sheet to the above heating temperature, the retention time is set to longer than 0 seconds and 1,200 seconds or shorter in order to cause ferrite to be transformed into austenite. The retention time is preferably 10 seconds or longer, 20 seconds or longer, or 30 seconds or longer.
When the retention time is longer than 1,200 seconds, a large amount of scale is generated in a case where the surface does not have a plating layer, and when the surface has a plating layer, it is excessively alloyed. Therefore, the retention time is set to 1,200 seconds or shorter. Preferably, it is 500 seconds or less, or 200 seconds or less.
[0048]
[Hot Forming Step]
Next, the steel sheet after the holding is cooled to a temperature range of a maximum temperature during a transformation latent heat evolution to a start temperature of the transformation latent heat evolution - 150°C at an average cooling rate of 5 to 30 °C/s, and forming is started in the temperature range (the temperature range of the maximum temperature during the transformation latent heat evolution to the start temperature of the transformation latent heat evolution - 150°C). In order to allow the average cooling rate to be slower than 5 °C/s, special heat insulation facility is required. In addition, in order to allow the average cooling rate to exceed 30 °C/s, equipment for forcible cooling is required. Cooling with an average cooling rate of 5 to 30 °C/s may be performed by air cooling.
[0049]
The reason that the forming is started in the temperature range of the maximum temperature during the transformation latent heat evolution or lower is to
- 21 -

obtain a desired amount of polygonal ferrite in the hot-stamped part. Transformation latent heat evolution occurs during the transformation from austenite into polygonal ferrite. Therefore, when the transformation latent heat evolution is maximized, a change from a gradual temperature decrease to a sudden temperature decrease occurs, or a transition from a temperature increase to a temperature decrease occurs. A temperature at which a temperature decrease during air cooling becomes slow due to the transformation latent heat evolution and the temperature drop becomes steep again, or a temperature at which a temperature increase occurs due to the transformation latent heat evolution and a transition from the temperature increase to a temperature decrease is the maximum temperature during the transformation latent heat evolution. A temperature at which a temperature decrease during air cooling becomes slow due to the transformation latent heat evolution, or a temperature at which a temperature increase occurs due to the transformation latent heat evolution is the start temperature of the transformation latent heat evolution.
[0050]
At a temperature of the maximum temperature during the transformation latent heat evolution or lower, transformation from austenite into polygonal ferrite is sufficiently completed. Therefore, by starting forming in this temperature range, a desired amount of polygonal ferrite can be obtained in the hot-stamped part. The lower limit of the temperature at which forming is started is set to the start temperature of the transformation latent heat evolution of -150°C because forming becomes difficult when the temperature is excessively decreased. Preferably, the start temperature of the transformation latent heat evolution is -100°C, or the start temperature of the transformation latent heat evolution is -50°C.
[0051]
- 22 -

The start temperature of the transformation latent heat evolution and the maximum temperature during the transformation latent heat evolution can be determined by measuring a change in the temperature of the steel sheet taken out of the heating furnace over time, and obtaining a first derivative and a second derivative of the temperature of the steel sheet with respect to the elapsed time.
FIG. 1A shows the relationship between the elapsed time after a steel sheet having the chemical composition shown in Table 1 (unit: mass%, the remainder consists of Fe and impurities) is heated to 980°C and the furnace lid of the heating furnace is opened, and the temperature of the steel sheet, FIG. IB shows the first derivative of the temperature of the steel sheet with respect to the elapsed time, and FIG. IC shows the second derivative of the temperature of the steel sheet with respect to the elapsed time. FIG. 2A shows the relationship between the elapsed time after the steel sheet having the chemical composition shown in Table 1 is heated to 940°C and the furnace lid of the heating furnace is opened, and the temperature of the steel sheet, FIG. 2B shows the first derivative of the temperature of the steel sheet with respect to the elapsed time, and FIG. 2C shows the second derivative of the temperature of the steel sheet with respect to the elapsed time. The temperatures of the steel sheet shown in FIGS. 1A and 2A are numerical values obtained by attaching a thermocouple to the surface of the steel sheet and measuring the temperature at a sampling interval of 0.5 seconds.
[0052] [Table 1]

c Si Mn P S Al Ti Nb B N Cr Cu Ni
0.0028 0.030 1.17 0.068 0.005 0.041 0.081 0.005 0.0033 0.0023 0.03 0.008 0.011
[0053]
- 23 -

The time at which the transformation latent heat evolution is maximized can be determined by obtaining the first derivative of the temperature of the steel sheet with respect to the elapsed time. As shown in FIG. IB, in a case where the first derivative of the temperature with respect to time changes from a negative value to a positive value after being taken out of the furnace, the transformation latent heat evolution decreases after the change from the negative value to the positive value, and at the time at which a change from a positive value to a negative value occurs, the transformation latent heat evolution is maximized. In addition, as shown in FIG. 2B, in a case where the first derivative of the temperature with respect to time remains at a negative value after being taken out of the furnace and does not become a positive value, at the time at which the value of the first derivative of the temperature with respect to time is maximized (in FIG. 2C, the time at which the second derivative of the temperature with respect to time changes from a positive value to a negative value), the transformation latent heat evolution is maximized. In addition, the time at which the transformation latent heat evolution is started can be determined by obtaining the second derivative of the temperature of the steel sheet with respect to the elapsed time. In FIGS. 1C and 2C, the inflection point immediately before the time at which the transformation latent heat evolution is maximized is the time at which the transformation latent heat evolution is started.
[0054]
As described above, the time at which the transformation latent heat evolution is started and the time at which the transformation latent heat evolution is maximized are determined from the figures of the first derivative and the second derivative of the temperature of the steel sheet with respect to the elapsed time, and the temperatures of the steel sheet at the times are obtained, thereby obtaining the start temperature of the
- 24 -

transformation latent heat evolution and the maximum temperature during the transformation latent heat evolution. That is, the time at which the transformation latent heat evolution is maximized is obtained from FIGS. IB and 2B (and FIG. 2C), the time at which the transformation latent heat evolution is started is obtained from FIGS. IC and 2C, and the temperatures of the steel sheet at the times are obtained from FIGS. 1A and 2A, thereby obtaining the start temperature of the transformation latent heat evolution and the maximum temperature during the transformation latent heat evolution.
[0055]
[Rapid Cooling Step]
After the forming, the steel sheet is held in the die and cooled to a temperature range of 600°C to 40°C at an average cooling rate of 20 to 1,000 °C/s. Accordingly, a hot-stamped part having a microstructure having excellent ductility can be manufactured. In a case where the average cooling rate is slower than 20 °C/s, a desired amount of acicular ferrite cannot be obtained. In a case where the average cooling rate exceeds 1,000 °C/s, large-scale cooling equipment is necessary, and the equipment cost increases. In addition, when the temperature at which cooling is stopped is outside the above temperature range, large-scale cooling equipment is necessary, and the equipment cost increases. During the cooling, the cooling rate may be adjusted by lowering the temperature of a cooling medium, using a die having high thermal conductivity, increasing thermal conductivity by increasing welding pressure, or using a method of spraying water onto a hot-stamped part after hot stamping. The average cooling rate is preferably 40 °C/s or faster, or 50 °C/s or faster. The average cooling rate is preferably 500 °C/s or slower, 200 °C/s or slower, or 150 °C/s or slower.
[0056]
- 25 -

According to the above-described method, the hot-stamped part according to the present embodiment can be obtained. Since the steel sheet applied to the hot-stamped part according to the present embodiment has a low C content and low strength, the steel sheet is joined to a steel sheet that has high strength after hot stamping to obtain a tailored blank, and the tailored blank is then subjected to hot stamping to be formed into a vehicle body component. This vehicle body component is manufactured by performing hot stamping on the tailored blank including the low strength material and the high strength material and thus has a low strength part and a high strength part.
[0057]
As a welding method for manufacturing the tailored blank, various methods such as laser welding, seam welding, arc welding, and plasma welding can be considered, but the welding method is not particularly limited. In addition, the high strength material (the steel sheet that has high strength after hot stamping) used together with the low strength material applied to the hot-stamped part according to the present embodiment is also not particularly limited. These may be selected as appropriate for each component to be manufactured.
[0058]
Even if the steel sheet having the chemical composition of the hot-stamped part according to the present embodiment is not applied to the tailored blank and a vehicle body component or the like is manufactured using only the steel sheet, there is no problem. Even if a blank is produced by joining and overlapping steel sheets such as patchwork by spot welding and the blank is subjected to hot stamping, there is no problem. [Examples]
- 26 -

[0059]
Next, examples of the present invention will be described. The conditions in the examples are one example of conditions adopted to confirm the feasibility and effects of the present invention, and the present invention is not limited to this one example of conditions. The present invention can adopt various conditions as long as the object of the present invention is achieved without departing from the gist of the present invention.
[0060]
Cold-rolled steel sheets and plated steel sheets having the chemical compositions shown in Table 2 were subjected to hot stamping under the conditions shown in Tables 3 A and 3B to obtain hot-stamped parts shown in Tables 4A and 4B. Prior to the hot stamping, a thermocouple is attached to the center of the steel sheet, a change in the temperature during natural cooling in the air after being taken out of the heating furnace with respect to time was measured, and the start temperature of the transformation latent heat evolution and the maximum temperature during the transformation latent heat evolution were obtained by the above-described method. In the columns "Plating" in Tables 4A and 4B, CR indicates plating absent, GA indicates hot-dip galvannealing, and AL indicates Al plating.
[0061]
Hot stamping was performed by sandwiching and pressurizing the steel sheet between flat sheet-shaped water cooling dies so that a tensile test piece and a test piece for observing the microstructure could be easily produced. In this example, a flat sheet-shaped hot-stamped part was manufactured. However, the shape of the hot-stamped part is not limited thereto, and a hot-stamped part having a shape with a bent portion having a radius of 2 to 50 mm may be manufactured.
- 27 -

A JIS No. 5 test piece was collected from the steel sheet after hot stamping (hot-stamped part) and was subjected to a tensile test in accordance with JIS Z 2241:2011 to obtain a tensile (maximum) strength (MPa) and a total elongation (%). In addition, a sample for microstructure observation was collected to obtain the area fraction of the microstructure by the above-described method.
[0062]
The results of the above tests are shown in Tables 4A and 4B.
A case where the total elongation was 8% or more was determined to be excellent in ductility and thus acceptable, and a case where the total elongation was less than 8% was determined to be unacceptable. In addition, a case where the total elongation was 12% or more was determined to be superior in ductility.
A case where the tensile strength was 360 MPa or more was determined to have the minimum strength required for the hot-stamped part and thus be acceptable, and a case where the tensile strength was less than 360 MPa was determined to be unacceptable.

[0063]
[Table 2]

Steel sheet
No. Chemical composition (mass%), remainder consisting of Fe and impurities Ac3 Note

C Si Mn Al P S N Ti Nb V Zr B

1 0.0023 0.310 1.23 0.042 0.065 0.012 0.0026 0.092 0.0025 885 Invention Example
2 0.0067 0.290 1.22 0.054 0.032 0.012 0.0039 0.082 0.0021 883 Invention Example
3 0.0048 0.310 1.11 0.059 0.072 0.005 0.0037 0.034 0.031 0.0021 876 Invention Example
4 0.0014 0.290 1.13 0.035 0.068 0.006 0.0021 0.035 0.032 0.0024 874 Invention Example
5 0.0045 0.120 1.49 0.072 0.088 0.008 0.0021 0.019 0.023 858 Invention Example
6 0.0023 0.110 1.58 0.043 0.091 0.013 0.0032 0.021 0.021 0.0012 864 Invention Example
7 0.0025 0.007 1.82 0.044 0.041 0.009 0.0024 0.019 0.025 0.0009 852 Invention Example
8 0.0013 0.350 1.57 0.032 0.038 0.005 0.0029 0.019 0.023 0.0010 870 Invention Example
9 0.0032 0.830 1.68 0.038 0.038 0.012 0.0035 0.021 0.033 0.0011 906 Invention Example
10 0.0023 0.330 1.82 0.043 0.081 0.007 0.0032 0.011 0.023 0.0007 870 Invention Example
11 0.0023 0.340 2.32 0.093 0.073 0.004 0.0045 0.013 0.018 873 Invention Example
12 0.0018 0.130 0.83 0.031 0.182 0.009 0.0032 0.019 0.018 0.0013 875 Invention Example
13 0.0020 0.330 1.34 0.042 0.069 0.009 0.0034 0.123 0.0023 886 Invention Example
14 0.0032 0.290 1.63 0.018 0.045 0.011 0.0031 0.021 0.082 0.0025 872 Invention Example
15 0.0023 0.290 1.65 0.043 0.042 0.010 0.0041 0.021 0.034 0.0012 873 Invention Example
16 0.0032 0.330 1.59 0.024 0.038 0.010 0.0041 0.019 0.085 0.0012 875 Invention Example
17 0.0035 0.280 1.61 0.012 0.042 0.009 0.0031 0.019 0.034 869 Invention Example
18 0.0037 0.310 1.59 0.045 0.041 0.006 0.0093 0.023 0.071 880 Invention Example
19 0.0073 0.890 1.67 0.034 0.047 0.003 0.0013 892 Invention Example
20 0.0002 0.280 1.19 0.034 0.071 0.006 0.0013 0.082 0.0018 876 Comparative Example
21 0.0120 0.280 1.19 0.023 0.071 0.006 0.0013 0.082 0.0018 870 Comparative Example
22 0.0023 0.003 1.61 0.043 0.035 0.011 0.0031 0.021 0.022 0.0012 856 Comparative Example
23 0.0042 1.210 1.43 0.033 0.032 0.011 0.0034 0.023 0.019 930 Comparative Example
24 0.0031 0.490 0.003 0.032 0.092 0.004 0.0033 0.023 0.025 0.0010 926 Comparative Example
25 0.0043 0.012 2.63 0.032 0.042 0.009 0.0038 0.025 0.021 863 Comparative Example
26 0.0031 0.039 1.61 0.044 0.021 0.011 0.0031 0.023 0.031 0.0009 858 Invention Example
27 0.0021 0.310 1.18 0.052 0.071 0.093 0.0039 0.041 0.029 965 Invention Example
28 0.0034 0.290 1.19 0.032 0.068 0.015 0.0132 0.082 906 Comparative Example
29 0.0019 0.290 1.31 0.083 0.073 0.010 0.0031 0.163 0.0025 888 Comparative Example
30 0.0043 0.310 1.61 0.044 0.041 0.008 0.0023 0.019 0.120 0.0025 868 Comparative Example
31 0.0041 0.280 1.58 0.013 0.039 0.011 0.0034 0.021 0.113 0.0012 872 Comparative Example
32 0.0038 0.330 1.63 0.034 0.047 0.007 0.0042 0.190 0.142 0.0012 888 Comparative Example
33 0.0033 0.240 1.57 0.032 0.043 0.008 0.0033 0.019 0.032 867 Invention Example
34 0.0018 0.140 1.53 0.035 0.043 0.011 0.0028 0.023 0.023 0.0011 864 Invention Example
35 0.0021 0.150 1.52 0.031 0.045 0.009 0.0031 0.022 0.024 0.0012 863 Invention Example
36 0.0022 0.293 1.66 0.044 0.043 0.010 0.0051 0.024 0.023 0.0011 875 Invention Example
37 0.0023 0.291 1.65 0.043 0.044 0.009 0.0048 0.021 0.022 0.0011 873 Invention Example
Underline indicates outside the range of the present invention.
- 29 -

[0064]
[Table 3A]

Manufacturing
No. Steel sheet
No. Heating
temperature °C Retention
time s Average
cooling rate °C/s Maximum
temperature during
transformation
latent heat
evolution
°C Start temperature of
transformation
latent heat
evolution
°C Start temperature of
transformation
latent heat
evolution - 150°C Hot forming
start
temperature
°C Average coo rate in rap cooling °C/s
Al 1 920 90 16 743 744 594 722 70
A2 2 940 81 21 722 723 573 703 83
A3 3 910 72 23 753 755 605 741 73
A4 4 910 63 27 741 742 592 733 66
A5 5 960 73 7 742 739 589 728 73
A6 6 900 45 21 704 706 556 691 89
A7 7 900 131 27 673 674 524 653 103
A8 8 932 83 8 702 704 554 691 83
A9 9 922 142 22 712 713 563 680 76
A10 10 904 93 15 733 734 584 710 66
All 11 893 41 12 704 706 556 689 78
A12 12 930 83 12 783 784 634 743 89
A13 13 910 93 19 731 732 582 711 87
A14 14 915 103 20 703 705 555 688 67
A15 15 930 92 23 711 713 563 693 93
A16 16 930 24 22 714 706 556 693 104
A17 17 890 30 20 698 699 549 683 83
A18 18 910 19 23 705 706 556 690 92
A19 19 910 140 16 705 707 557 693 77
A20 20 905 230 22 763 765 615 731 65
A21 21 895 320 12 733 734 584 703 58
A22 22 895 280 11 693 694 544 683 83
A23 23 951 30 8 744 743 593 711 84
A24 24 930 83 7 825 824 674 780 92
A25 25 930 92 29 452 452 302 452 83
A26 26 920 95 12 683 683 533 651 83
A27 27 980 122 11 741 740 590 732 70
A28 28 945 103 22 733 733 583 726 83
A29 29 930 83 22 704 703 553 699 82
A30 30 934 94 22 703 703 553 688 68
A31 31 942 17 14 703 701 551 697 77
A32 32 922 19 13 704 707 557 699 83
- 30 -

A33 33 890 30 20 702 703 553 675 81
A34 34 930 80 12 744 743 593 723 73
A3 5 35 930 80 12 743 744 594 722 72
A3 6 36 930 90 23 712 712 562 695 95
A37 37 930 90 23 711 713 563 694 93
Underline indicates outside the range of the present invention.
[0065]
[Table 3B]

Manufacturing
No. Steel sheet
No. Heating
temperature °C Retention time
s Average
cooling rate
°C/s Maximum
temperature
during
transformation
latent heat
evolution
°C Start temperature
of transformation
latent heat
evolution
°C Start temperature of transformation
latent heat evolution - 150°C Hot forming
stail
temperature
°C Average c rate in ra coolin °C/s
Bl 1 920 90 17 743 744 594 762 83
B2 920 90 17 743 744 594 743 88
B3 920 90 17 743 744 594 733 90
B4 1082 3 17 704 701 551 689 88
B5 965 132 17 723 720 570 703 76
B6 920 90 17 743 744 594 723 45
B7 920 90 17 743 745 595 713 92
B8 920 1184 17 704 703 553 682 77
B9 920 90 17 743 744 594 713 32
BIO 920 90 17 743 744 594 700 13
Bll 6 900 122 21 704 706 556 714 63
B12 6 900 122 21 704 706 556 704 67
B13 6 900 122 21 704 706 556 683 63
B14 6 900 122 21 704 706 556 673 61
B15 6 900 122 21 704 706 556 677 42
B16 6 900 122 21 704 706 556 681 15
B17 8 932 83 8 702 704 554 732 73
B18 8 932 83 8 702 704 554 702 73
B19 8 932 83 8 702 704 554 683 83
B20 8 932 83 8 702 704 554 674 34
- 31 -

B21 8 932 83 8 702 704 554 663 13
Underline indicates outside the range of the present invention.
- 32 -

[0066]
[Table 4A]

Manufacturing
No. Steel sheet
No. Acicular ferrite area% Polygonal ferrite area% Remainder area% Plating Tensile
strength
MPa Total elongation
% Note
Al 1 86 13 1 AL 588 10 Invention Example
A2 2 85 14 1 AL 603 11 Invention Example
A3 3 87 13 0 CR 591 13 Invention Example
A4 4 90 9 1 AL 603 11 Invention Example
A5 5 91 7 2 AL 611 9 Invention Example
A6 6 86 13 1 GA 573 15 Invention Example
A7 7 83 16 1 AL 582 12 Invention Example
A8 8 89 10 1 CR 581 12 Invention Example
A9 9 63 35 2 GA 583 14 Invention Example
A10 10 76 24 0 CR 561 15 Invention Example
All 11 81 18 1 CR 604 13 Invention Example
A12 12 63 36 1 CR 531 18 Invention Example
A13 13 80 18 2 AL 554 16 Invention Example
A14 14 85 14 1 AL 563 14 Invention Example
A15 15 77 21 2 CR 543 17 Invention Example
A16 16 63 36 1 CR 489 23 Invention Example
A17 17 68 32 0 GA 493 23 Invention Example
A18 18 70 28 2 CR 473 22 Invention Example
A19 19 83 16 1 GA 561 15 Invention Example
A20 20 64 36 0 AL 342 32 Comparative Example
A21 21 53 39 8 GA 642 5 Comparative Example
A22 22 89 11 0 GA 321 28 Comparative Example
A23 23 - - - GA - - Comparative Example
A24 24 57 43 0 CR 352 29 Comparative Example
A25 25 97 2 1 CR 723 5 Comparative Example
A26 26 60 39 1 AL 371 27 Invention Example
A27 27 88 11 1 CR 603 10 Invention Example
A28 28 90 8 2 CR 604 5 Comparative Example
A29 29 92 8 0 CR 603 6 Comparative Example
A30 30 93 7 0 CR 611 6 Comparative Example
A31 31 92 7 1 CR 603 5 Comparative Example
A32 32 92 8 0 CR 604 6 Comparative Example
A33 33 63 37 0 GA 475 26 Invention Example
A34 34 76 23 1 AL 523 18 Invention Example
A35 35 75 24 1 AL 543 16 Invention Example
A36 36 76 22 2 CR 553 16 Invention Example
A37 37 77 21 2 CR 543 18 Invention Example
Underline indicates outside the range of the present invention or properties that are not preferable.
[0067]
- 33 -

[Table 4B]

Manufacturing No. Steel sheet
No. Acicular ferrite area% Polygonal ferrite area% Remainder area% Plating Tensile
strength MPa Total elongation
% Note
Bl 99 0 1 AL 689 7 Comparative Example
B2 94 5 1 AL 601 10 Invention Example
B3 87 12 1 AL 581 13 Invention Example
B4 88 11 1 AL 583 12 Invention Example
B5 84 15 1 AL 573 13 Invention Example
B6 85 14 1 AL 575 14 Invention Example
B7 68 32 0 AL 483 23 Invention Example
B8 81 18 1 AL 543 19 Invention Example
B9 25 73 2 AL 390 28 Invention Example
BIO 13 86 1 AL 353 31 Comparative Example
Bll 6 99 0 1 GA 682 6 Comparative Example
B12 6 94 5 1 GA 593 10 Invention Example
B13 6 84 15 1 GA 573 14 Invention Example
B14 6 78 21 1 GA 503 19 Invention Example
B15 6 38 61 1 GA 431 29 Invention Example
B16 6 12 88 0 GA 352 32 Comparative Example
B17 8 99 0 1 CR 677 6 Comparative Example
B18 8 94 6 0 CR 603 9 Invention Example
B19 8 85 15 0 CR 559 15 Invention Example
B20 8 28 71 1 CR 395 30 Invention Example
B21 8 10 88 2 CR 358 35 Comparative Example
Underline indicates outside the range of the present invention or properties that are not preferable, s
[0068]
According to Tables 2 to 4B, invention examples in which the chemical composition and the microstructure were within the ranges of the present invention had excellent ductility, a tensile strength of 360 MPa or more, and the minimum strength required for a hot-stamped part.
On the other hand, comparative examples in which the chemical composition and/or the microstructure was outside the range of the present invention were inferior in tensile strength or elongation. In addition, Manufacturing No. A23 in Table 4A had deteriorated surface external appearance due to a high Si content and was determined not to be used for a vehicle body component. Therefore, the
- 34 -

microstructure observation and property evaluation thereof were not performed. [Industrial Applicability]
[0069]
According to the above aspects according to the present invention, it is possible to provide a hot-stamped part having excellent ductility and the minimum strength required for the hot-stamped part and a manufacturing method thereof.

WE CLAIMS

A hot-stamped part comprising, as a chemical composition, by mass%:
C: 0.0005% to 0.0080%;
Si: 0.005% to 1.000%;
Mn: 0.01% to 2.50%;
Al: 0.010% to 0.100%;
P: 0.200% or less;
S: 0.100% or less;
N: 0.0100% or less;
Ti:0% to 0.150%;
Nb:0% to 0.100%;
V:0% to 0.100%;
Zr:0% to 0.100%;
B: 0% to 0.0050%; and
a remainder consisting of Fe and impurities,
wherein the hot-stamped part has a microstructure including, by area fraction, 20% or more and less than 95% of acicular ferrite, 5% to 80% of polygonal ferrite, and 0% to 5% of a remainder in a microstructure.
2. The hot-stamped part as claimed in claim 1, comprising, as the chemical
composition, by mass%, one or two or more selected from the group consisting of:
Ti: 0.005% to 0.150%; Nb: 0.005% to 0.100%; V: 0.005% to 0.100%; and
- 36 -

Zr: 0.005% to 0.100%.
3. The hot-stamped part as claimed in claim 1 or 2, comprising, as the
chemical composition, by mass%:
B: 0.0002% to 0.0050%.
4. The hot-stamped part as claimed in any one of claims 1 to 3, comprising:
a plating layer on a surface of the hot-stamped part.
5. A manufacturing method of the hot-stamped part as claimed in any one of
claims 1 to 3, comprising:
heating a steel sheet having the chemical composition as claimed in claim 1 to a temperature range of Ac3 to 1100°C;
performing holding in the temperature range for longer than 0 seconds to 1,200 seconds or shorter;
cooling the steel sheet to a temperature range of a maximum temperature during a transformation latent heat evolution to a start temperature of the transformation latent heat evolution - 150°C at an average cooling rate of 5 to 30 °C/s, and starting forming in the temperature range; and
performing cooling to a temperature range of 600°C to 40°C at an average cooling rate of 20 to 1,000 °C/s after the forming.