Molded body

Abstract

The present invention provides a molded body that is less susceptible to fracture of the burring structure, starting from surface damage at the base of the burring structure. The molded body (1) of the present invention is a steel molded body having a burring structure (2) and a plate-like portion (3) extending therearound thereof, wherein the burring structure (2) has a hole (21) and a wall portion (22) provided around the hole (21) and having a tip portion (22a) and a base portion (22b), The above-mentioned root portion (22b) has a plurality of band-shaped Mn segregation portions that are spaced apart in the thickness direction of the root portion (22b) and extend in the plane direction of the root portion (22b), The present invention is characterized in that, at the 1t / 4 position of the thickness t of the root portion (22b), the average spacing of the band-shaped Mn segregation portions in the thickness direction is 4.0 μm or more and 20.0 μm or less, and the Mn segregation strength of the band-shaped Mn segregation portions is 2.00 mass% or more and 3.50 mass% or less.

Description

Technical Field

[0001] The present invention relates to a molded body formed of a steel plate and having a flanging structure.

Background Art

[0002] 2]In order to achieve weight reduction, the use of higher-strength materials and the strengthening of component structures are being studied for automotive chassis components. For example, components called lower arms and trailing arms usually have a flanging structure. The flanging structure can reinforce the attachment part by increasing the area of the joint part when connecting to other components via a ball joint or the like.

[0003] As a member having such a flanging structure, for example, Patent Document 1 discloses a flanging structure member in which the Charpy impact value vE(0) at 0°C of a plate-like part is 50 J / cm 2 or more, and the curvature radius R of the curved wall part based on the Charpy impact value vE(0) and the maximum length Lc of the bending internal crack of the curved wall part of the flanging structure part is within a specific range. According to the flanging structure member disclosed in this Patent Document 1, the flanging structure part is said to be difficult to break even when the flanging structure part is bent back or compressed.

[0004] Further, Patent Document 2 discloses a flanging processed member having a plate-like part and a flanging processed part, wherein the flanging end face has a specific arithmetic mean roughness, the flanging wall part has a specific height, and the tensile strength of the curved wall part and the plate-like part and the surface roughness of the flanging end face satisfy a specific relational expression. According to the flanging processed member disclosed in this Patent Document 2, it is said that fatigue cracks are difficult to occur in the flanging processed part.

[0005] Patent Document 3 discloses a steel sheet having a specific chemical composition, a cleanliness of 0.08% or less in the metal structure, a segregation degree of Mn, α, of 1.6 or less, and a difference in average hardness ΔHv after hot forming between a low-strain formed section subjected to a plastic strain of 5% or less and a high-strain formed section subjected to a plastic strain of 20% or more, of 40 or less.

[0006] Patent Document 4 discloses a hot-stamped molded article having a first region having a specific metallic structure and a second region having a specific metallic structure, wherein when the average grain size of prior austenite grains in the first region is G1 (μm) and the average grain size of prior austenite grains in the second region is G2 (μm), G1 is 15.0 μm or less and G2 is 25.0 μm or less, and (i) equation: G2 - G1 ≥ 3.0 is satisfied, and the tensile strength measured by a tensile test using a test piece taken from the first region is 1250 to 2540 MPa. [Prior art documents] [Patent Documents]

[0007] [Patent Document 1] Patent No. 7436944 [Patent Document 2] Patent No. 7036298 [Patent Document 3] International Publication No. 2014 / 034714 [Patent Document 4] International Publication No. 2024 / 190645 [Overview of the Initiative] [Problems that the invention aims to solve]

[0008] Molded bodies with burring structures formed using high-strength steel plates are susceptible to surface damage at the base of the burring structure due to the burring process. If such surface damage occurs at the base of the burring structure, there is a concern that fracture may occur starting from the base of the burring structure when the molded body is subjected to a strong impact. Surface damage at the base of the burring structure becomes more likely with increasing material strength, and is also induced by mold deterioration, which is accelerated by increased strength. Therefore, this presents challenges such as increased production management costs and constraints on component structure (and consequently, a decrease in component performance). Consequently, there is a need to develop a technology that can suppress fracture of the burring structure even if there is minor surface damage at the base of the burring structure.

[0009] This invention has been made in view of these circumstances, and aims to provide a molded body that is less prone to fracture of the burring structure, which is caused by surface damage at the base of the burring structure. [Means for solving the problem]

[0010] The present invention includes the following embodiments.

[0011] (Aspect 1) A steel molded body having a burring structure and a plate-like portion extending around the burring structure, The above-mentioned burring structure has a hole and a wall portion provided around the hole, having a tip portion and a base portion. The above-mentioned root portion has a plurality of band-shaped Mn segregation portions that are spaced apart in the thickness direction of the root portion and extend in the plane direction of the root portion. A molded article characterized in that, at a position 1t / 4 of the thickness t of the root portion, the average spacing of the band-shaped Mn segregation portions in the thickness direction is 4.0 μm or more and 20.0 μm or less, and the Mn segregation strength of the band-shaped Mn segregation portions is 2.00 mass% or more and 3.50 mass% or less.

[0012] (Aspect 2) The molded body according to the above aspect 1, wherein the Vickers hardness of the plate-like portion is 250 Hv or more.

[0013] (Aspect 3) The molded body according to the above aspect 1, wherein the Vickers hardness of the plate-like portion is 300 Hv or more.

[0014] (Aspect 4) The molded body according to the above aspect 1, wherein the Vickers hardness of the plate-like portion is 350 Hv or more.

Advantages of the Invention

[0015] According to the present invention, it is possible to provide a molded body in which breakage of the burring structure portion hardly occurs starting from surface damage at the root portion of the burring structure portion.

Brief Description of the Drawings

[0016] [Figure 1] FIG. 1 is a perspective view schematically showing a molded body 1 which is an embodiment of the present invention. [Figure 2] FIG. 2 is a cross-sectional view schematically showing a cross-section taken along line II-II of FIG. 1. [Figure 3] FIG. 3 is an elemental mapping image of Mn and a Fourier transform image obtained therefrom.

Modes for Carrying Out the Invention

[0017] Hereinafter, preferred embodiments of the molded body having a burring structure portion of the present invention will be described in detail with reference to the drawings.

[0018] In order to achieve the above object, the present inventors first analyzed the causes and mechanisms of breakage occurring in the burring structure portion of the molded body. As a result, it was found that breakage of the burring structure portion starts from surface damage at the root portion and cracks progress in the plate thickness direction, and then propagate along the circumferential direction of the hole portion of the burring structure portion.

[0019] The Charpy test is widely used as a method for evaluating the toughness of parts. The fracture mode in the Charpy test is that the crack typically originates near the center of the plate thickness where the stress triaxiality is high, propagates perpendicular to the plate thickness direction, and then propagates in the plate thickness direction. To suppress component failure, it is generally effective to focus on the initial stages of failure. Once a crack becomes long enough, the stress intensity factor increases, leading to catastrophic failure with little to no energy absorption. Thus, the initial failure modes that should be considered differ between the Charpy test and the failure of the burring structure described above. In other words, a microstructure design that increases absorbed energy based on the evaluation results of conventional Charpy tests may not be suitable for designs that can suppress the failure of the burring structure.

[0020] Therefore, the inventors considered the crack propagation behavior in the burring structure and investigated means of suppressing the failure of the burring structure. They found that suppressing crack propagation in the thickness direction is effective in suppressing the failure of the burring structure. Furthermore, as a specific means, the inventors conceived of forming a metallic structure at the root of the burring structure that causes cracks propagating in the thickness direction to meander.

[0021] The present invention has been completed based on these findings and encompasses the following embodiments.

[0022] One embodiment of the molded body having a burring structure of the present invention is a molded body having a burring structure formed from a steel plate. That is, it is a steel molded body having a burring structure and a plate-like portion extending around it. Furthermore, in this embodiment, the molded body has a burring structure having a hole and a wall portion provided around the hole and having a tip portion and a root portion. The root portion has a plurality of band-shaped Mn segregation portions that are spaced apart in the thickness direction of the steel plate (i.e., the thickness direction of the root portion) and extend in the plane direction of the steel plate (i.e., the plane direction of the root portion). Furthermore, at the base, the average spacing of the band-shaped Mn segregation portions in the thickness direction (i.e., the thickness direction) at a position 1 t / 4 of the steel plate thickness (i.e., 1 t / 4 of the thickness t of the base) is 4.0 μm or more and 20.0 μm or less, and the Mn segregation strength of the band-shaped Mn segregation portions is 2.00 mass% or more and 3.50 mass% or less.

[0023] Hereinafter, preferred embodiments of the molded article having the burring structure of the present invention will be described in detail with reference to the drawings. Here, Figure 1 is a schematic perspective view showing a molded article 1 which is one embodiment of the present invention. Figure 2 is a schematic cross-sectional view showing a cross section along the line II-II in Figure 1. In this specification, unless otherwise specified, various numerical ranges mean a range including their upper and lower limits.

[0024] <Molded body> One embodiment of the present invention, the molded body 1, is a molded body made of steel sheet and is applied to automobile chassis parts and the like. The molded body 1 of this embodiment is a molded body obtained, for example, by cold press forming. As shown in Figures 1 and 2, the molded body 1 of this embodiment has a burring structure 2 formed by ironing burring, which will be described later, and a flat plate-like portion 3 extending around the burring structure 2.

[0025] [Plate-like part] In the molded body 1, the plate-like portion 3 has a flat, plate-like structure, as shown in Figures 1 and 2. The outer edge shape of the plate-like portion 3, that is, the overall planar shape of the molded body 1, is not particularly limited and can be appropriately determined according to the intended use of the molded body 1. The plate-like portion 3 does not need to be perfectly flat; for example, it may have an uneven structure, a bent structure, a notched structure, etc.

[0026] The thickness of the plate-like portion 3, i.e., the thickness of the plate-like portion 3, can be, for example, 0.5 mm or more. The thickness of the plate-like portion 3 may be 0.8 mm or more, 1.0 mm or more, 1.5 mm or more, 2.0 mm or more, or 2.2 mm or more. There is no particular upper limit to the thickness of the plate-like portion 3, but in order to ensure good workability during burring, the thickness of the plate-like portion 3 may be 6.0 mm or less, 5.5 mm or less, 5.0 mm or less, 4.5 mm or less, or 4.0 mm or less.

[0027] The thickness of the plate-like portion 3 is measured using a micrometer from a relatively smooth position on the plate-like portion of the part (molded body). The thickness of the plate-like portion 3 is basically the same as the thickness of the steel sheet forming the molded body 1 before forming. However, if there are areas where the thickness has decreased or increased locally due to thinning during press working, these areas are not included in the measurement.

[0028] [Burring structure] In the molded body 1, the burring structure 2 has a hole 21 formed by applying a burring process to a steel plate, which will be described later, and a wall portion 22 provided around the hole 21.

[0029] (hole) The hole 21 is a hole that penetrates the steel plate forming the molded body 1 in the thickness direction. The planar shape of the hole 21, i.e., the opening shape, is circular. Here, "circular" does not have to mean a perfect circle, and may have an error that is acceptable in industrial production. For example, the opening shape of the hole 21 can be considered "circular" if the ratio of the diameter of the inscribed circle to the diameter of the circumscribed circle is 1.00 or more and 1.10 or less.

[0030] The size of the hole 21 is not particularly limited and can be appropriately determined according to the application of the molded body 1. For example, the diameter of the hole 21 may be 20.0 mm or more, 30.0 mm or more, 40.0 mm or more, or 50.0 mm or more. Furthermore, the diameter of the hole 21 may be 100.0 mm or less, 90.0 mm or less, 80.0 mm or less, 70.0 mm or less, or 60.0 mm or less. In addition, the diameter of the hole 21 may be 5 times or more, 10 times or more, 100 times or less, or 50 times or less the thickness t of the steel plate constituting the molded body 1. The diameter of the hole 21 refers to the inner circumferential surface of the wall portion 22 provided around the hole 21, that is, the inner diameter of the cylindrical wall portion 22.

[0031] (wall) The wall portion 22 is formed when a part of the steel plate is raised in a substantially cylindrical shape during the ironing and burring process, and as shown in Figures 1 and 2, it has a structure that protrudes from one surface of the plate-like portion 3. The direction in which the wall portion 22 protrudes is a direction that intersects with the surface direction of the plate-like portion 3 (specifically, the surface direction of the steel plate in the plate-like portion 3), and may also be a direction perpendicular to the surface direction of the plate-like portion 3, for example.

[0032] As shown in Figures 1 and 2, the wall portion 22 has a tip portion 22a located at the front end and a base portion 22b located on the opposite side and connected to the plate-like portion 3 described above. The portion of the wall portion 22 between the tip portion 22a and the base portion 22b is referred to as the intermediate portion 22c. The wall portion 22 has a substantially cylindrical inner surface S1 along the punching direction during the ironing burring process. The surface of the wall portion 22 facing the hole portion 21 is referred to as the "inner surface," and the surface on the opposite side is referred to as the "outer surface." In Figure 2, the surface indicated by the symbol "S1" is the inner surface, and the surface indicated by the symbol "S2" is the outer surface.

[0033] As shown in Figure 2, in the cross-sectional shape of the wall portion 22 along the central axis of the hole portion 21, the inner surfaces S1 of the intermediate portions 22C of the opposing wall portions 22 may be parallel to each other, or the inner surfaces S1 of the opposing wall portions 22 may be inclined so that the tip portion 22a side tapers. That is, the outer surfaces S2 of the intermediate portions 22C of the wall portion 22 may be perpendicular to each other with respect to the surface direction of the plate-like portion 3, or they may intersect each other in a form other than perpendicular.

[0034] The height of the wall portion 22, that is, the distance from the surface of the plate-like portion 3 to the tip portion 22a of the wall portion 22, is not particularly limited and can be set appropriately according to the intended use of the molded body 1.

[0035] As shown in Figure 2, the base portion 22b of the wall portion 22 is the part connected to the plate-like portion 3 while having a predetermined radius of curvature. That is, the base portion 22b can be clearly distinguished from the other parts of the wall portion 22 and the plate-like portion 3 connected to it as a curved wall portion having a predetermined radius of curvature. The radius of curvature of the base portion 22b, specifically the radius of curvature on the outer surface S2 side of the base portion 22b in a cross section along the central axis of the hole portion 21 (the so-called internal bending radius in the bent portion), is not particularly limited and can be set appropriately according to the application of the molded body 1. For example, the radius of curvature on the outer surface S2 side of the base portion 22b may be 8.0 mm or less, 5.0 mm or less, or 3.0 mm or less. Also, in order to suppress surface damage due to wrinkles and cracks that may occur on the outer surface S2 of the base portion 22b, the radius of curvature may be 0.8 mm or more, 1.2 mm or more, 1.6 mm or more, or 2.0 mm or more.

[0036] The holes 21 and walls 22 of the burring structure 2 are formed by applying a scrubbing burring process to the steel plate. Compared to conventional burring, scrubbing burring is a method of forming the holes and walls while reducing the thickness of the steel plate by providing a smaller plate thickness clearance.

[0037] Therefore, the wall portion 22 including the root portion 22b has a thickness smaller than the thickness of the steel plate before forming, i.e., the thickness of the plate-like portion 3. For example, the thickness of the wall portion 22 may be 0.3 mm or more, 0.6 mm or more, 0.8 mm or more, 1.2 mm or more, 1.8 mm or more, or 2.0 mm or more. Alternatively, the thickness of the wall portion 22 may be 5.8 mm or less, 5.2 mm or less, 4.8 mm or less, 4.2 mm or less, or 3.8 mm or less.

[0038] The thickness of the wall portion 22 may be uniform throughout the entire wall portion 22, or it may vary from part to part of the wall portion 22. For example, the thickness of the base portion 22b may be smaller than that of the other parts of the wall portion 22.

[0039] [Band-shaped Mn segregation] Furthermore, the base portion 22b of the wall portion 22 has multiple band-shaped Mn segregation portions that are spaced apart in the plate thickness direction, that is, in the thickness direction of the base portion 22b, and that extend in the plane direction of the steel plate, that is, in the plane direction of the base portion.

[0040] In this specification, "band-shaped Mn segregation portion" means a band-shaped Mn segregation portion having a thickness of 3 μm or more and a length of 200 μm or more, and extending continuously or intermittently. Here, "thickness" of the Mn segregation portion means the length of the Mn segregation portion in the thickness direction of the plate, and "length" of the Mn segregation portion means the length of the Mn segregation portion in the plane direction of the steel plate. The plane direction of the steel plate means any one direction parallel to the surface of the steel plate.

[0041] The multiple band-shaped Mn segregation portions of the root portion 22b are formed such that band-shaped Mn segregation portions extending in the plane direction of the steel plate are stacked at constant or irregular intervals in the thickness direction. In other words, the root portion 22b has a structure in which layers with high and low Mn concentrations are stacked alternately at equal or uneven intervals along the thickness direction.

[0042] As described later, the multiple band-shaped Mn segregation regions are arranged at specific average intervals in the thickness direction and have specific Mn segregation strengths. This causes cracks that propagate in the thickness direction starting from surface damage at the root portion 22b to change course each time they reach a band-shaped Mn segregation region, resulting in a meandering crack propagation (hereinafter, this effect may be referred to as the "crack meandering effect"). This makes it less likely for the burring structure 2 to break, starting from surface damage at the root portion 22b of the burring structure 2 (hereinafter, this effect may be referred to as the "fracture suppression effect").

[0043] [Average spacing in the thickness direction of multiple band-like Mn segregation regions at the 1t / 4 position of the thickness t at the base: 4.0 μm or more and 20.0 μm or less] At the base portion 22b, the average spacing of multiple band-shaped Mn segregation portions in the thickness direction (i.e., the thickness direction) at the position 1 t / 4 of the steel plate thickness, i.e., 1 t / 4 of the thickness t of the base portion 22b, is between 4.0 μm and 20.0 μm. The average spacing of multiple band-shaped Mn segregation portions in the thickness direction is the arithmetic mean of the spacing between band-shaped Mn segregation portions aligned in the thickness direction, and can be measured by the measurement method described later.

[0044] The 1t / 4 position of the steel plate thickness refers to the depth position in the thickness direction starting from the surface of the steel plate, and means the depth position where the thickness of the steel plate is 1t / 4 of the thickness t.

[0045] If the average spacing in the thickness direction of the multiple band-shaped Mn segregation portions at the 1t / 4 position of the steel plate thickness in the root portion 22b is too short, cracks that propagate in the thickness direction starting from surface damage in the root portion 22b may not meander, or even if they do meander, the cracks may quickly propagate in the thickness direction, and the intended fracture suppression effect in the burring structure portion 2 may not be sufficiently obtained. Therefore, the average spacing in the thickness direction of the multiple band-shaped Mn segregation portions should be 4.0 μm or more.

[0046] On the other hand, if the average spacing in the thickness direction of the multiple band-shaped Mn segregation portions is too long, cracks that propagate in the thickness direction starting from surface damage at the root portion 22b will meander at long intervals, and the intended fracture suppression effect in the burring structure portion 2 may not be sufficiently obtained. Therefore, the average spacing in the thickness direction of the multiple band-shaped Mn segregation portions should be 20.0 μm or less.

[0047] The average spacing of the multiple band-shaped Mn segregation portions in the thickness direction is preferably 6.0 μm to 17.0 μm, and more preferably 10.0 μm to 15.0 μm, in order to more reliably obtain the fracture suppression effect in the burring structure portion 2.

[0048] (Method for measuring the average spacing in the thickness direction of band-shaped Mn segregation areas) The average spacing of the band-shaped Mn segregation regions in the thickness direction can be obtained by the following method. First, a sample with a cross-section exposing the entire plate thickness is taken from the wall portion 22 of the burring structure. The cross-section of the sample is finished to a mirror surface. An electron probe microanalyzer (SEM-EPMA) combined with a scanning electron microscope (SEM) is used to measure the elemental mapping of the Mn concentration in the cross-section of the sample. More specifically, the JEOL JXA-8500F field emission electron probe microanalyzer is used, with an acceleration voltage of 15kV, an irradiation current of 100nA, a WD of 11mm, and a measurement time of 50msec per point.

[0049] Regarding the measurement area for the elemental mapping described above, at the base portion 22b of the wall portion 22, the shape has a radius of curvature, making it difficult to measure the average spacing of Mn segregation, which will be described later. Therefore, a portion as close as possible to the base portion 22b and without a bent shape (i.e., the boundary portion between the base portion 22b and the portion without a bent shape) is selected. In this portion, a rectangular area of ​​100 μm in the direction perpendicular to the plate thickness direction (X direction) and 100 μm in the plate thickness direction (Y direction) is selected, centered at the plate thickness 1t / 4 position, and elemental mapping of Mn is measured in three fields with a measurement interval of 0.5 μm. Note that the plate thickness of the portion selected as described above (measurement area) and the base portion 22b have approximately the same plate thickness reduction rate.

[0050] Next, the obtained elemental mapping is treated as a two-dimensional array of elemental concentrations totaling 40,000 points, and this is subjected to a Fourier transform as shown in Figure 3 to obtain a Fourier transform image. Figure 3 shows the elemental mapping image of Mn (two-dimensional array of Mn concentrations) and the Fourier transform image obtained from it.

[0051] A specific method for obtaining the Fourier transform image is to calculate the Fourier transform using the numpy.fft.fft2 function available in the Python® language. The normalization mode is set to "ortho". Then, the result of this calculation is rearranged using the numpy.fft.shift function so that the low-frequency components are located at the center (origin) of the transformed image. The return value of this function is also a two-dimensional array data (where the dimension of each coordinate is μm). -1 This is the result. For convenience, we will call this X'-Y'. The natural logarithm of the absolute value of all the returned values ​​is referred to as luminance in this specification. By mapping this luminance onto the X'-Y' coordinate system, we obtain the Fourier transform image shown in Figure 3.

[0052] Finally, the average spacing of the band-like Mn segregation in the thickness direction is obtained from the Fourier transform image as follows. The Fourier transform image shows the periodicity of the elemental mapping. The band-like Mn segregation we are focusing on here appears as a periodic modulation of the Mn concentration in the thickness direction (Y' direction), and the brightness on the Y' axis of the Fourier transform image corresponds to this. The higher the brightness, the clearer the periodicity. Also, the further the brightness is from the origin, the shorter the period. The relationship between the brightness on the Y' axis and the Y' coordinate is plotted, and as shown in Figure 3, the distance K[μm] between the Y coordinate where the brightness is below 0 and the origin is obtained. -1 The distance in frequency space after the Fourier transform is obtained. Then, the average spacing of the band-shaped Mn segregation in the thickness direction [μm] is calculated using the formula: [μm] = 50 ÷ K. The same measurement is performed in three fields of view for each sample, and the average value is used as the representative value.

[0053] Here, we will provide some supplementary information about the formula for calculating the average spacing of the band-shaped Mn segregation regions in the plate thickness direction, namely, the average spacing [μm] = 50 ÷ K. When the concentration modulation due to the band-shaped Mn segregation regions is considered as a periodic function of wavelength λ, the reciprocal of λ (1 / λ) is the distance "K" (μm) between the Y coordinate where the brightness in the Fourier transform image is below 0 and the origin. -1 The following equation is used, where ) (i.e., the distance in frequency space after the Fourier transform), "B" is the number of pixels in the original image B×B, and "L" is the length of one side (μm). 1 / λ = B × K / 2L Since the average interval is half the period λ / 2, rearranging the above equation gives us average interval λ / 2 = L / B ÷ K. Substituting the above measurement conditions, namely L = 100 μm and B = 200 (one side of the measurement area is 100 μm / measurement interval is 0.5 μm), we obtain average interval λ / 2 = 50 ÷ K.

[0054] Furthermore, the average spacing of the Mn segregation regions is equal to the thickness of the Mn segregation regions as described above. For example, when the average spacing of the Mn segregation regions is 4 μm or more, the thickness of the Mn segregation regions will also be 4 μm or more.

[0055] [Mn segregation strength of band-shaped Mn segregation area: 2.00% by mass or more and 3.50% by mass or less] Furthermore, at the base portion 22b, the Mn segregation strength of the band-shaped Mn segregation portion at the position 1 t / 4 of the steel plate thickness is 2.00 mass% or more and 3.50 mass% or less. The Mn segregation strength of the band-shaped Mn segregation portion is the arithmetic mean of the Mn segregation strength of each band-shaped Mn segregation portion, and can be measured by the measurement method described later.

[0056] As the Mn segregation strength increases, the resulting structure tends to become harder. Therefore, when a crack propagating in the thickness direction reaches such a hard structure, the crack's path can be altered. Furthermore, if multiple band-shaped Mn segregation regions with a certain level of Mn segregation strength are formed aligned in the thickness direction, the crack propagating in the thickness direction starting from surface damage at the root portion 22b can be altered each time it reaches a band-shaped Mn segregation region, thus causing the crack to meander. If the Mn segregation strength is too low, this crack meandering effect cannot be sufficiently obtained. Therefore, the Mn segregation strength of the band-shaped Mn segregation regions should be 2.00 mass% or higher.

[0057] On the other hand, if the Mn segregation strength is too high, that part will become excessively hard, which may actually become a source of crack formation. Therefore, the Mn segregation strength of the band-shaped Mn segregation area should be 3.50 mass% or less.

[0058] The Mn segregation strength of the band-shaped Mn segregation portion is preferably 2.40% by mass or more and 3.10% by mass or less, and more preferably 2.60% by mass or more and 2.90% by mass or less, in order to more reliably cause cracks propagating in the thickness direction of the plate to meander.

[0059] (Method for measuring the Mn segregation intensity in band-shaped Mn segregation areas) The Mn segregation strength of the band-shaped Mn segregation region can be obtained by the following method. Elemental mapping of Mn concentration is measured using the same method as for measuring the average spacing in the thickness direction of the band-shaped Mn segregation zone. When the Mn concentrations at each measurement point are arranged in descending order, the Mn concentration corresponding to the top 10% is defined as the Mn segregation intensity. The average value of the Mn segregation intensities determined in three fields of view is used as the representative value.

[0060] (Method for forming multiple band-shaped Mn segregation regions having the above-mentioned specific average interval and Mn segregation intensity) Multiple band-shaped Mn segregation regions having the above-mentioned specific average interval and Mn segregation intensity can be formed by combining (1) concentration control of the band-shaped Mn segregation regions and (2) spacing control of the band-shaped Mn segregation regions.

[0061] (1) Control of the concentration of band-shaped Mn segregation In the manufacturing process of steel sheets that form molded bodies, Mn solidifies during casting, resulting in segregation. It should be noted that the steel sheet must have a composition that allows for such Mn segregation to occur. The composition of the steel sheet will be described later. Next, the slab before hot rolling is heated under specific temperature and time conditions described later, causing the Mn segregation to diffuse widely within the slab. Then, as this diffused Mn segregation is compressed in the thickness direction and stretched in the plane direction by rolling, multiple band-shaped Mn segregation regions are formed, possessing the specific Mn segregation strength described above, and arranged at intervals in the thickness direction of the steel sheet, extending in the plane direction of the steel sheet. These band-shaped Mn segregation regions are formed as areas with a high concentration of Mn (positive Mn segregation regions), and their structure becomes hard.

[0062] When a crack propagating in the thickness direction reaches such a hard Mn segregation zone, its path is altered, and the crack propagates further. As the crack propagates in this way, it changes its path each time it reaches multiple Mn segregation zones aligned in the thickness direction, causing the crack to meander. By setting the steel plate composition and the heating conditions of the slag before hot rolling (i.e., heating temperature and time) within appropriate ranges, the crack meandering effect caused by the band-shaped Mn segregation zones can be maximized.

[0063] (2) Control of the spacing of band-shaped Mn segregation regions A burring structure 2 is formed by applying ironing burring to a steel sheet manufactured with the concentration control of the band-shaped Mn segregation portion described in (1) above. When the steel sheet undergoes ironing burring, a process that applies strain in a direction that reduces the thickness of the steel sheet, the spacing in the thickness direction of the multiple band-shaped Mn segregation portions, which are spaced apart in the thickness direction of the steel sheet and extend in the surface direction of the steel sheet, becomes shorter. At this time, by setting the clearance of the ironing burring process to an appropriate range and controlling the rate of thickness reduction due to ironing, as described later, the average spacing in the thickness direction of the multiple band-shaped Mn segregation portions can be controlled to be within the specified range.

[0064] As described above, in the molded body 1 of this embodiment, at least at the root portion 22b of the burring structure portion 2, a plurality of band-shaped Mn segregation portions are formed, which are arranged at specific average intervals in the thickness direction and have a specific Mn segregation strength. As a result, cracks that propagate in the thickness direction starting from surface damage at the root portion 22b are forced to change course each time they reach a band-shaped Mn segregation portion, and consequently the propagation of the crack is made to meander (crack meandering effect). This makes it difficult for the molded body 1 to break the burring structure portion 2, which starts from surface damage at the root portion 22b of the burring structure portion 2 (breakage suppression effect).

[0065] Furthermore, surface damage to the root portion 22b of the burring structure 2 can occur on either the inner surface S1 facing the hole 21 or the outer surface S2 on the opposite side. Surface damage occurring on the inner surface S1 (outer surface of the bend) of the root portion 22b is caused by tensile stress applied to the surface of the steel plate during the burring process. On the other hand, surface damage occurring on the outer surface S2 (inner surface of the bend) of the root portion 22b is caused by compressive stress applied to the surface of the steel plate during the burring process. Of these surface damages, surface damage occurring on the outer surface S2 of the root portion 22b has a greater impact on the fracture of the burring structure described above. The molded body 1 of this embodiment can cause the crack propagation to meander not only for cracks that propagate in the thickness direction starting from surface damage to the inner surface S1 of the root portion 22b, but also for cracks that propagate in the thickness direction starting from surface damage to the outer surface S2. Therefore, compared to conventional methods for improving impact resistance, the molded body 1 of this embodiment is less prone to significant fracture of the burring structure 2.

[0066] In this embodiment, the molded body 1 is not particularly limited except for the requirements relating to the multiple band-shaped Mn segregation portions at the root portion 22b of the burring structure portion 2. For example, the steel sheet forming the molded body 1 may have any strength appropriate to the intended use of the molded body 1.

[0067] [Vickers hardness of steel plate] The strength of the steel sheet forming the molded body 1 may be, for example, a Vickers hardness of 250 Hv or more. In this specification, the Vickers hardness of the steel sheet means the Vickers hardness at a position of 1 t / 4 of the sheet thickness.

[0068] Normally, as the strength of such steel plates increases, cracks tend to occur on the outer surface S2 of the root portion 22b of the burring structure 2. However, in the molded body 1 of this embodiment, even when using a steel plate with a Vickers hardness of 250 Hv or higher, the crack meandering effect described above makes it difficult for the burring structure 2 to break, starting from surface damage to the root portion 22b of the burring structure 2.

[0069] The Vickers hardness of the steel plate may be 280 Hv or higher, 300 Hv or higher, 320 Hv or higher, or 350 Hv or higher. On the other hand, there is no particular upper limit to the Vickers hardness of the steel plate, but for example, the Vickers hardness of the steel plate may be 500 Hv or lower, 480 Hv or lower, 450 Hv or lower, 420 Hv or lower, or 400 Hv or lower.

[0070] Furthermore, the present invention can also be applied to steel plates with a Vickers hardness of less than 250 Hv. Because such steel plates have lower strength, they have excellent workability during burring and are less prone to cracking at the base of the burring structure. Therefore, by applying the present invention, it is possible to more reliably prevent the failure of the burring structure.

[0071] (Method for measuring the Vickers hardness of steel plates) The Vickers hardness of steel plates can be measured according to JIS Z 2244:2009. The Vickers hardness of a steel plate is obtained by taking 10 measurements at a point 1 t / 4 of the plate thickness with a load of 1 kgf (approximately 9.80 N), and taking the arithmetic mean of these 10 measurements. At this time, the distance between measurement points should be at least three times the distance of the indentation. The Vickers hardness of the steel plate is measured on the plate-like portion of the molded body.

[0072] (chemical composition) The steel plate used in the molded body 1 is chosen because it more reliably generates the aforementioned Mn segregation portion. C: 0.030~0.300%, Si: 0.03~2.00%, Mn: 1.20~2.70%, Al: 0.010~1.000% Ti: 0.060~0.200%, P: 0.100% or less, S: 0.0100% or less, N: 0.0150% or less, Nb: 0~0.100%, Ca: 0 ~ 0.0060%, Mo: 0~1.000%, Cr: 0~1.00%, V: 0~0.40%, Ni: 0~0.40%, B: 0~0.0200%, Cu: 0~1.000%, W: 0~1.000%, Sn: 0~0.500%, Zr: 0~0.050%, Sb: 0~0.080%, and, The remainder preferably has a specific chemical composition consisting of Fe and impurities.

[0073] The chemical composition of steel plates will be explained in more detail below. In the following explanation, the unit "%" for the content of each element refers to "mass%" unless otherwise specified.

[0074] [C:0.030~0.300%] Carbon (C) is an element that increases the strength of steel sheets. To obtain this effect fully, the C content is preferably 0.030% or more. The C content may also be 0.035% or more, 0.040% or more, or 0.045% or more. On the other hand, in order to ensure good workability during forming, especially during cold press forming and burring, the C content is preferably 0.300% or less. The C content may also be 0.250% or less, 0.200% or less, or 0.150% or less.

[0075] [Si: 0.03~2.00%] Si is a deoxidizing element for steel and an effective solid solution strengthening element for increasing the strength of steel sheets without impairing their ductility. To fully obtain this effect, the Si content is preferably 0.03% or more. The Si content may be 0.05% or more, 0.08% or more, or 0.10% or more. On the other hand, from the standpoint of suppressing ferrite formation and ensuring a predetermined strength, or ensuring good workability during burring, the Si content is preferably 2.00% or less. The Si content may be 1.80% or less, 1.60% or less, or 1.40% or less.

[0076] [Mn: 1.20~2.70%] Mn is an element that enhances the hardenability of steel and contributes to improving its strength. Furthermore, Mn is also an essential element for forming the specific Mn segregation zones mentioned above. To fully obtain these effects, the Mn content is preferably 1.20% or more. The Mn content may be 1.40% or more, 1.60% or more, 1.80% or more, or 2.00% or more. On the other hand, in order to suppress the non-uniformity of the structure due to microsegregation of Mn and ensure good workability during burring, the Mn content is preferably 2.70% or less. The Mn content may be 2.60% or less, 2.50% or less, or 2.40% or less.

[0077] [Al:0.010~1.000%] Al is an element that functions as a deoxidizing agent and is an effective solid solution strengthening element for increasing the strength of steel. Al also suppresses the formation of carbides and facilitates the formation of retained austenite. To fully obtain these effects, the Al content is preferably 0.010% or higher. The Al content may be 0.050% or higher, 0.100% or higher, or 0.150% or higher. On the other hand, to suppress the formation of coarse precipitates at the grain boundaries of prior austenite grains and ensure good workability during burring, the Al content is preferably 1.000% or lower. The Al content may be 0.800% or lower, 0.600% or lower, or 0.400% or lower.

[0078] [Ti: 0.060~0.200%] Ti is an element that controls the morphology of carbides and increases the strength of ferrite. To fully obtain these effects, the Ti content is preferably 0.060% or more. The Ti content may be 0.080% or more or 0.100% or more. On the other hand, in order to suppress the formation of coarse Ti oxide or TiN and ensure good workability during burring, the Ti content is preferably 0.200% or less. The Ti content may be 0.180% or less, 0.160% or less or 0.140% or less.

[0079] [P:0.100% or less] P is an element that is introduced during the manufacturing process and is an impurity. Furthermore, P can segregate at the prior austenite grain boundaries, potentially reducing the formability of the steel sheet due to grain boundary embrittlement. For this reason, a lower P content is preferable. The P content may be 0%. However, in order to shorten the refining time and ensure good productivity, the P content may be 0.0001% or more, 0.0005% or more, or 0.001% or more. On the other hand, in order to suppress the reduction in toughness of the steel sheet and ensure good workability during burring, the P content may be 0.100% or less. The P content may be 0.080% or less, 0.060% or less, or 0.040% or less.

[0080] [S:0.0100% or less] S is an element that is introduced during the manufacturing process and is an impurity. Furthermore, S can form nonmetallic inclusions such as MnS in steel, which can lead to an increase in the hardness and a decrease in the ductility of the steel sheet. For this reason, the lower the S content, the better. The S content may be 0%. However, in order to shorten the refining time and ensure good productivity, the S content may be 0.0001% or more, 0.0005% or more, or 0.0010% or more. On the other hand, in order to ensure good workability during burring, the S content may be 0.0100% or less. The S content may be 0.0080% or less, 0.0060% or less, or 0.0040% or less.

[0081] [N:0.0150% or less] N is an element that is introduced during the manufacturing process. Like C, N is an element that is effective in increasing the strength of steel, but it is also an element that affects the occurrence of cross-slip of dislocations during forming. If the N content is high, strain concentration cannot be suppressed during the forming of the steel sheet, causing void formation and thus reducing formability. From the standpoint of ensuring good formability, the lower the N content, the better. The N content may be 0%. However, from the standpoint of shortening the refining time and ensuring good productivity, the N content may be 0.0001% or more, 0.0005% or more, or 0.0010% or more. On the other hand, from the standpoint of ensuring good workability during burring, the N content may be 0.0150% or less. The N content may be 0.0100% or less, 0.0080% or less, 0.0060% or less, or 0.0050% or less.

[0082] In this embodiment, the preferred basic chemical composition of the steel sheet used in the molded body 1 is as described above. Furthermore, in this embodiment, the steel sheet may contain one or more of the following optional elements in place of a portion of the remaining Fe, if necessary. These optional elements will be described in detail below.

[0083] [Nb: 0~0.100%] Nb is an effective element for controlling the morphology of carbides and is also effective in refining crystal grains, thereby improving the toughness and workability of steel sheets. The Nb content may be 0%, but to fully obtain these effects, the Nb content may be 0.001% or more. The Nb content may be 0.005% or more, 0.007% or more, or 0.010% or more. On the other hand, to suppress excessive strengthening, the Nb content may be 0.100% or less. From the standpoint of economic rationality, the Nb content may be 0.080% or less, 0.060% or less, or 0.040% or less.

[0084] [Ca: 0~0.0060%] Ca is an element that contributes to the fine dispersion of inclusions and enhances toughness. In other words, Ca is an element that contributes to improving the formability of steel sheets. The Ca content may be 0%, but to obtain such effects sufficiently, the Ca content may be 0.0001% or more. The Ca content may be 0.0005% or more, 0.0010% or more, or 0.0015% or more. On the other hand, in order to ensure good workability during burring, the Ca content is preferably 0.0060% or less. The Ca content may be 0.0050% or less, 0.0040% or less, or 0.0030% or less.

[0085] [Mo: 0~1.000%] Mo is an element that suppresses phase transformation at high temperatures and contributes to improving the strength of steel sheets. The Mo content may be 0%, but to obtain this effect fully, the Mo content may be 0.001% or more. The Mo content may be 0.010% or more, 0.050% or more, or 0.100% or more. On the other hand, in order to suppress the decrease in workability due to the formation of coarse Mo carbides, the Mo content may be 1.000% or less. The Mo content may be 0.800% or less, 0.600% or less, or 0.400% or less.

[0086] [Cr: 0~1.00%] Cr is an element that enhances the hardenability of steel and contributes to improving the strength of steel sheets. The Cr content may be 0%, but to fully obtain this effect, the Cr content may be 0.001% or more. The Cr content may be 0.01% or more, 0.10% or more, 0.20% or more, or 0.30% or more. On the other hand, in order to suppress the decrease in workability due to the formation of coarse Cr carbides, the Cr content may be 1.00% or less. The Cr content may be 0.80% or less, 0.60% or less, or 0.50% or less.

[0087] [V: 0~0.40%] V is an element effective in controlling the morphology of carbides and is also effective in refining crystal grains, thereby improving the toughness and workability of steel sheets. The V content may be 0%, but to obtain these effects sufficiently, the V content may be 0.001% or more. The V content may be 0.005% or more, 0.01% or more, or 0.02% or more. On the other hand, in order to suppress the decrease in formability of steel sheets due to the precipitation of large amounts of carbonitrides, the V content may be 0.40% or less. The V content may be 0.30% or less, 0.20% or less, or 0.10% or less.

[0088] [Ni: 0~0.40%] Ni is an element that suppresses phase transformation at high temperatures and contributes to improving the strength of steel sheets. The Ni content may be 0%, but to obtain this effect sufficiently, the Ni content may be 0.001% or more. The Ni content may be 0.01% or more, 0.03% or more, or 0.05% or more. On the other hand, in order to suppress the decrease in ductility of the steel sheet and ensure good workability during burring, the Ni content may be 0.40% or less. The Ni content may be 0.30% or less, 0.25% or less, or 0.20% or less.

[0089] [B: 0~0.0200%] B is an element that suppresses phase transformation at high temperatures and contributes to improving the strength of steel sheets. The B content may be 0%, but to obtain this effect sufficiently, the B content may be 0.0001% or more. The B content may be 0.0005% or more, 0.0010% or more, or 0.0015% or more. On the other hand, in order to suppress the generation of coarse B carbides, etc., which would cause voids to form during the forming of steel sheets and reduce workability, the B content may be 0.0200% or less. The B content may be 0.0180% or less, 0.0160% or less, 0.0140% or less, 0.0120% or less, or 0.0100% or less.

[0090] [Cu: 0~1.000%] Cu exists in steel in the form of fine particles and is an element that contributes to improving the strength of steel sheets. The Cu content may be 0%, but to obtain this effect sufficiently, the Cu content may be 0.001% or more. The Cu content may be 0.010% or more, 0.030% or more, or 0.050% or more. On the other hand, in order to suppress the decrease in workability of the steel sheet due to the precipitation of large amounts of coarse precipitates and inclusions, the Cu content may be 1.000% or less. The Cu content may be 0.800% or less, 0.600% or less, or 0.400% or less.

[0091] [W: 0~1.000%] W is an element that suppresses phase transformation at high temperatures and contributes to improving the strength of steel sheets. The W content may be 0%, but to obtain this effect sufficiently, the W content may be 0.001% or more, or 0.005% or more. On the other hand, in order to prevent a decrease in productivity due to a decrease in workability, the W content should be 1.000% or less. The W content may also be 0.080% or less.

[0092] [Sn: 0~0.500%] Sn is an element that suppresses grain coarsening and contributes to improving the strength of steel sheets. The Sn content may be 0%, but to obtain this effect sufficiently, the Sn content may be 0.001% or more. The Sn content may be 0.010% or more, 0.050% or more, or 0.080% or more. On the other hand, in order to suppress the decrease in workability of the steel sheet due to the increase in coarse precipitates and inclusions, the Sn content may be 0.500% or less. The Sn content may be 0.400% or less, 0.300% or less, or 0.200% or less.

[0093] [Zr:0~0.050%] Zr is an element that contributes to improving the formability of steel sheets. The Zr content may be 0%, but to fully obtain this effect, the Zr content may be 0.0001% or more. The Zr content may be 0.0005% or more, 0.0010% or more, or 0.0015% or more. On the other hand, in order to suppress the decrease in ductility of the steel sheet and ensure good workability during burring, the Zr content may be 0.050% or less. The Zr content may be 0.040% or less, 0.030% or less, or 0.020% or less.

[0094] [Sb: 0~0.080%] Sb is an element that contributes to improving the formability of steel sheets. The Sb content may be 0%, but to fully obtain this effect, the Sb content may be 0.001% or more. The Sb content may be 0.005% or more, 0.010% or more, or 0.015% or more. On the other hand, considering cost performance, the Sb content may be 0.080% or less. The Sb content may be 0.060% or less, 0.040% or less, or 0.020% or less.

[0095] [Remaining components: Fe and impurities] In this embodiment, the remainder of the steel sheet, other than the elements mentioned above, consists of Fe and impurities. Here, impurities are components that are mixed in during the industrial production of steel sheets due to various factors in the manufacturing process, including raw materials such as ore and scrap. Examples of impurities include O, H, Na, Cl, Co, Zn, Ga, Ge, As, Se, Y, Tc, Ru, Rh, Pd, Ag, Cd, In, Te, Cs, Ta, Re, Os, Ir, Pt, Au, Pb, Bi, and Po. The impurities may be present in an amount of 0.100% or less in total.

[0096] Here, the chemical composition of steel sheets can be measured by general analytical methods. For example, the chemical composition of steel sheets can be measured using inductively coupled plasma-atomic emission spectrometry (ICP-AES). C and S can be measured using the combustion-infrared absorption method, N using the inert gas fusion-thermal conductivity method, and O using the inert gas fusion-nondispersive infrared absorption method.

[0097] As described above, the molded body 1 of this embodiment is less susceptible to fracture of the burring structure 2, which is caused by surface damage to the root portion 22b of the burring structure 2. For this reason, the molded body 1 can be suitably used, for example, in automobile chassis parts where high impact resistance is required.

[0098] <Method for manufacturing molded articles> Next, a preferred method for manufacturing the molded article 1, which is one embodiment of the present invention, will be described. The following description is intended to illustrate a characteristic method for manufacturing the molded article 1, which is one embodiment of the present invention, and is not intended to limit the molded article 1 to those manufactured by the manufacturing method described below.

[0099] In manufacturing the molded body 1 of this embodiment, first, a steel plate is manufactured.

[0100] (Method of manufacturing steel plates) The steel sheet manufacturing method of this embodiment includes a casting step of casting a slab having the above-mentioned specific chemical composition, and a hot rolling step of hot rolling the cast slab. Preferred conditions for these steps will be described below.

[0101] (Casting process) The casting process is a process of casting a slab having the above-mentioned specific chemical composition. The casting process uses a continuous casting machine equipped with multiple adjacent reduction rolls in the direction of slab transport, with a roll pitch of 290 mm or less between adjacent reduction rolls. By casting a slab having the above-mentioned specific chemical composition in this manner, segregation of Mn can be induced during solidification.

[0102] (Hot rolling process) The hot rolling process involves hot rolling the cast slab. Prior to hot rolling, the slab undergoes a heating process at a temperature of 1180°C to 1360°C for 120 to 360 minutes. By heating the slab to a temperature of 1180°C to 1360°C and for 120 to 360 minutes, the Mn segregation formed during casting can be widely diffused within the slab. Furthermore, heating the slab under these conditions has the advantage of preventing excessive rolling forces during hot rolling, making it easier to achieve the desired thickness.

[0103] The heating temperature of the slab should be 1180°C or higher, preferably 1230°C or higher. On the other hand, if the heating temperature of the slab exceeds 1360°C, the diffusion distance of Mn becomes excessively long, making it impossible to obtain the desired Mn segregation strength in the burring structure. Therefore, the heating temperature should be 1360°C or lower.

[0104] The heating time for the slab should be 120 minutes or more, preferably 180 minutes or more. On the other hand, if the heating time for the slab exceeds 360 minutes, the diffusion distance of Mn becomes excessively long, making it impossible to obtain the desired Mn segregation strength in the burring structure. Therefore, the heating time should be 360 ​​minutes or less.

[0105] The preferred range of heating temperature and heating time for a slab to obtain a desirable Mn segregation strength also changes depending on the concentration of Mn contained in the slab. That is, as the Mn concentration increases, these preferred ranges shift towards higher temperatures or longer heating times. Conversely, as the Mn concentration decreases, these preferred ranges shift towards lower temperatures or shorter heating times. Therefore, even if the heating temperature and heating time of the slab are within the above preferred ranges, if the Mn concentration is high, it becomes easier to obtain a desirable Mn segregation strength by increasing the heating temperature of the slab, increasing the heating time of the slab, or a combination of both. On the other hand, if the Mn concentration is low, it becomes easier to obtain a desirable Mn segregation strength by decreasing the heating temperature of the slab, shortening the heating time of the slab, or a combination of both.

[0106] In the hot rolling process, rough rolling and finish rolling are performed on the slab heated in the specific heating process described above. The diffused Mn segregation portions described above are compressed in the thickness direction and stretched in the plane direction by rolling, thereby forming multiple band-shaped Mn segregation portions that have the specific Mn segregation strength described above, are spaced apart in the thickness direction of the steel sheet, and extend in the plane direction of the steel sheet. At this time, by appropriately selecting various conditions for rough rolling and finish rolling as described below, the Mn segregation strength of the multiple band-shaped Mn segregation portions, the average spacing in the thickness direction, and the Vickers hardness of the steel sheet can be controlled.

[0107] In the hot rolling process, the starting temperature for rough rolling is, for example, 1150°C or lower. A starting temperature of 1150°C or lower reduces the effect of heat dissipation by the rolling rolls, allowing the steel sheet to be rolled uniformly on both sides. On the other hand, a starting temperature of 1050°C or higher is also acceptable. A starting temperature of 1050°C or higher allows for control to prevent the rolling reaction force from becoming excessively large.

[0108] In the hot rolling process, the time from the completion of rough rolling to the start of finish rolling is preferably 20 seconds or less, and more preferably 10 seconds or less, or 5 seconds or less. After the completion of rough rolling, the diffusion of Mn segregation can proceed in a shorter time than in the heating process due to the strain introduced in rough rolling. If this time is 20 seconds or less, the state of Mn segregation formed in the heating process is less likely to change, and it becomes easier to obtain the desired Mn segregation strength in the burring structure. Furthermore, there is no particular lower limit, but from the point of suppressing damage to the equipment that conveys the steel sheet, for example, it should be 2 seconds or more. Note that both the completion of rough rolling and the start of finish rolling are defined as the time when the leading edge of the steel sheet is rolled.

[0109] In the hot rolling process, the finishing rolling temperature is, for example, 800°C or higher. A finishing rolling temperature of 800°C or higher allows for a reduction in the average grain size of the hot-rolled steel sheet and the final product, thereby ensuring sufficient yield strength. On the other hand, there is no particular upper limit to the finishing rolling temperature, but from a productivity standpoint, the finishing rolling temperature is, for example, 980°C or lower.

[0110] Furthermore, the diameter of the rolling rolls used in the hot rolling process is, for example, 100 mm or more. When the diameter of the rolling rolls is 100 mm or more, strain is less likely to concentrate on the surface in contact with the rolling rolls, and the steel sheet can be rolled uniformly on both sides. On the other hand, there is no particular upper limit to the diameter of the rolling rolls, but from an economic standpoint, it is, for example, 700 mm or less. In addition, the rolling rolls used in the hot rolling process may be heated beforehand. If the rolling rolls are preheated, heat removal from the steel sheet by the rolling rolls is suppressed, and unevenness in heat removal from the steel sheet by the rolling rolls can be reduced.

[0111] Hot-rolled steel sheets obtained in the hot-rolling process are wound at a winding temperature of, for example, 450 to 600°C. By setting the winding temperature to 450°C or higher, the strength of the hot-rolled steel sheet does not become excessively high, and the decrease in workability during burring can be suppressed. On the other hand, by setting the winding temperature to 600°C or lower, coarse ferrite and pearlite are less likely to form in the structure of the hot-rolled steel sheet, and bainite is more likely to be obtained in the structure of the hot-rolled steel sheet, thereby improving the strength of the steel sheet.

[0112] Furthermore, the winding temperature of the hot-rolled steel sheet may be, for example, 450°C or lower. By setting the winding temperature to 450°C or lower, a large amount of microstructure such as bainite and martensite is formed, making it easier to secure the desired strength even with the addition of a small amount of alloying elements. The winding temperature may also be, for example, 200°C or lower. By setting it to 200°C or lower, a large amount of martensite is generated, making it possible to improve the strength of the steel sheet even with a small amount of alloying elements.

[0113] The hot-rolled steel sheet obtained by the hot-rolling process may be subjected to skin-pass rolling for the purpose of correcting its shape.

[0114] Steel sheets obtained by hot rolling or by skin pass rolling may be subjected to any processing step, such as plating, as needed.

[0115] The steel sheets obtained by the above manufacturing method are then subjected to the next forming process.

[0116] (molding process) The forming process includes, for example, a process of processing a steel sheet into a predetermined shape using forming means such as cold press forming, and a burring process of applying ironing burring to the blank material that has been processed into a predetermined shape.

[0117] The process of processing steel sheets into a predetermined shape involves processing rolled steel sheets into blank materials having a planar shape corresponding to the shape of the part of the molded body 1 before processing. The processing means in this process are not particularly limited, and any punching or cutting means can be used.

[0118] The burring process involves applying ironing burring to a blank material that has been processed into a predetermined shape, using punches and dies. In this process, ironing burring is performed with a smaller clearance than in normal burring, thereby reducing the thickness of the steel plate while forming a burring structure 2 having holes 21 and walls 22. When using a conventional burring process where the clearance is set to be the same as or greater than the plate thickness, microscopic fracture may occur within or at the interface of the hard structure formed in the band-shaped Mn segregation during the burring process. Therefore, ironing burring, in which the clearance is set to be smaller than the plate thickness, is used. The clearance for the ironing burring process is set so that the rate of reduction in plate thickness (i.e., the amount of ironing) in the wall portion 22 is between 5% and 35%. When ironing burring is performed on a steel plate in which multiple band-shaped Mn segregation portions are formed, spaced apart in the plate thickness direction and extending in the direction of the surface of the steel plate, under clearance conditions that yield such a specific amount of ironing, the steel plate receives appropriate processing stress in the direction of plate thickness reduction, thereby controlling the average spacing of the multiple band-shaped Mn segregation portions in the plate thickness direction to within the specified range.

[0119] The amount of friction may be set to 30% or less, 28% or less, or 26% or less. Alternatively, the amount of friction may be set to 7% or more, 9% or more, or 10% or more.

[0120] Note that the clearance for ironing burring refers to the distance (spacing) between the punch or die and the pilot hole when performing ironing burring. The clearance for ironing burring does not necessarily correspond to the rate of plate thickness reduction (amount of ironing) in the wall portion 22 due to the effects of die deflection, steel plate strength, etc. Therefore, when setting the clearance, it is sufficient to perform one to several prototypes to ensure that the rate of plate thickness reduction in the wall portion 22 falls within the above-mentioned specific range.

[0121] (Method for measuring the amount of ironing in molded body 1) The amount of ironing in the molded body 1 is determined by the rate of reduction in the thickness of the wall portion 22 relative to the plate-like portion 3. Specifically, the thickness of the plate-like portion 3 and the wall portion 22 is measured, and the reduction rate of the wall portion 22 thickness (%) is calculated as follows: [(thickness of plate-like portion 3 - thickness of wall portion 22) / thickness of plate-like portion 3] × 100.

[0122] The thickness of the plate-like portion 3 and the wall portion 22 is measured by the following method. First, samples are obtained from the plate-like portion 3 and the wall portion 22 of the burring structure portion 2, with cross-sections exposed parallel to the plate thickness direction. The cross-sections of the samples are magnified and observed using a microscope, and the shortest distance from one surface to the other surface of the steel plate in each part of the plate-like portion 3 and the wall portion 22 is measured. These lengths are then defined as the plate thicknesses of the plate-like portion 3 and the wall portion 22. At the base portion 22b of the wall portion 22, it is difficult to measure the plate thickness due to its shape having a radius of curvature. Therefore, a portion as close as possible to the base portion 22b and that does not have a bent shape (i.e., the boundary portion between the base portion 22b and the portion without a bent shape) is selected, and the plate thickness of this portion is measured in the same way as the plate thicknesses of the plate-like portion 3 and the wall portion 22, and this is taken as the plate thickness of the wall portion 22.

[0123] In this embodiment, depending on the structure and application of the part to which the molded body 1 is applied, there may be any processing steps other than the burring process described above. Examples of such processing steps include bending.

[0124] The molded body obtained through the above molding process may be subjected to painting or other surface treatment processes for the purpose of improving its appearance and design.

[0125] Furthermore, the present invention is not limited to the embodiments described above or the following examples, and can be appropriately combined, substituted, or modified without departing from the purpose and spirit of the present invention.

[0126] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to these examples. [Examples]

[0127] In the following embodiments, a molded body according to one embodiment of the present invention was manufactured under various conditions, and the impact resistance properties of the obtained molded body were investigated.

[0128] (Manufacturing of steel plates) Slabs with the chemical composition shown in Table 1 below were cast using a continuous casting method with a continuous casting machine equipped with multiple reduction rolls arranged with a roll pitch of 290 mm or less.

[0129] Next, a hot rolling process was carried out on the obtained slabs. Specifically, the slabs were heated at temperatures of 1165°C to 1380°C for 110 to 365 minutes as shown in Table 2 below, followed by rough rolling and finish rolling. The starting temperature for rough rolling was 1050°C, and the ending temperature for finish rolling was 880°C to 980°C. The coiling temperature of the resulting rolled steel sheets was set to 200°C or lower, or 350 to 550°C. In this way, rolled steel sheets with a thickness of 3 mm were obtained.

[0130] Furthermore, analysis of the chemical composition of samples taken from the obtained rolled steel sheets confirmed that there was no change in the chemical composition of the slab. The underlines next to the various values ​​in Table 1 indicate that the conditions are not favorable for manufacturing the molded articles of the present invention.

[0131] [Table 1]

[0132] (Manufacturing of molded products) The obtained rolled steel sheet was subjected to a bending process that mimicked ironing burring under the conditions of ironing amount 3 to 36% and radius of curvature (internal bending radius) of 1.0 to 3.5 mm as shown in Table 2 below. This process produced L-shaped test specimens as molded bodies having a wall portion with a tip and a base, and a plate-like portion.

[0133] Two test specimens were prepared for each type of rolled steel sheet. One specimen was used to measure the average spacing of Mn segregation in the thickness direction at the base, the Mn segregation strength, and the Vickers hardness of the steel sheet. The other specimen was used to evaluate the impact resistance, which will be described later.

[0134] For molded bodies No. 1 to 21 obtained as described above, the average spacing of the band-shaped Mn segregation portions in the thickness direction was measured according to the "Method for measuring the average spacing of band-shaped Mn segregation portions in the thickness direction" described above. Furthermore, the Mn segregation strength of the band-shaped Mn segregation portions was measured according to the "Method for measuring the Mn segregation strength of band-shaped Mn segregation portions" described above. In addition, the Vickers hardness was measured according to the "Method for measuring the Vickers hardness of steel plates" described above. Then, the following impact resistance tests were conducted on molded bodies No. 1 to 21 to evaluate the impact resistance at the base of these molded bodies. The measurement results and evaluation results are shown in Table 2 below.

[0135] (Impact resistance test) First, on the bent portion of the L-shaped test specimen, i.e., the base (the inner surface of the bend), scratches simulating surface damage occurring on the outer surface of the base of the burring structure are applied using #80 grit sandpaper. Next, the scratched test specimen is cooled to -40°C. The cooled test specimen is placed on a horizontal test stand with the outer bend of the bent portion (base) facing upwards. Then, a cone weighing approximately 120 kg is dropped from a height of 0.18 m above the surface of the test stand, impacting the outer bend of the bent portion (base) of the test specimen. Before free-falling, the cone is positioned with its base facing downwards. The cone used has a base area large enough to cover the entire bent portion (base) of the test specimen.

[0136] Then, the condition of the test piece after impacting it with a cone is observed. Those that do not show fracture in the bent processed portion (base) are judged to have "excellent impact resistance," while those that show fracture in the bent processed portion (base) are judged to have "poor impact resistance."

[0137] In Table 2, a "○" indicates a judgment of "excellent impact resistance," and a "×" indicates a judgment of "poor impact resistance." In Table 2, underlines next to steel grades indicate that the conditions are not favorable for manufacturing the molded articles of the present invention. Furthermore, underlines next to various numerical values ​​for the heating and molding processes in Table 2 indicate that the manufacturing conditions are such that the molded articles of the present invention cannot be obtained. In addition, underlines next to various numerical values ​​for the average spacing and segregation strength of Mn segregation areas in Table 2 indicate that these conditions are outside the scope of the present invention.

[0138] [Table 2]

[0139] As shown in Table 2, all molded articles of the present invention examples were found to have excellent impact resistance at the base, as the average spacing of the multiple band-shaped Mn segregation portions at the base was 4.0 μm or more and 20.0 μm or less, and the Mn segregation strength was 2.00 mass% or more and 3.50 mass% or less. Although the concentration of Mn in the chemical composition of the steel plate of molded articles No. 14 and 15 was outside the preferred range, the average spacing of the band-shaped Mn segregation portions and the Mn segregation strength were satisfied within the preferred range by adjusting the heating temperature and heating time of the slab (i.e., the temperature and time of the heating process) according to the Mn concentration.

[0140] On the other hand, the comparative molded articles were found to have inferior impact resistance at the base because the average spacing of the multiple band-shaped Mn segregation areas at the base was outside the range of 4.0 μm to 20.0 μm, or the Mn segregation strength was outside the range of 2.00 mass% to 3.50 mass%. In particular, molded articles No. 16 and 17 failed to obtain the desired Mn segregation strength because the heating temperature of the slab was too low or too high. Also, molded articles No. 18 and 19 failed to obtain the desired Mn segregation strength because the heating time of the slab was too short or too long. Furthermore, molded articles No. 20 and 21 failed to obtain the desired average spacing of the Mn segregation areas because the amount of ironing during the molding process was too small or too large. In molded articles No. 22 and 23 failed to obtain the desired Mn segregation strength because, despite the Mn concentration in the chemical composition of the steel plate being outside the preferred range, the heating temperature and heating time of the slab were not adjusted according to the Mn concentration. [Explanation of Symbols]

[0141] 1 Molded body 2. Burring structure 21 Hole 22 Wall 22a Tip 22b Root 22c middle part 3 Plate-shaped part

Claims

1. A steel molded body having a burring structure and a plate-like portion extending around the burring structure, The burring structure comprises a hole and a wall portion provided around the hole, having a tip portion and a base portion. The root portion has a plurality of band-shaped Mn segregation portions that are spaced apart in the thickness direction of the root portion and extend in the plane direction of the root portion. A molded article characterized in that, at a position 1 t / 4 of the thickness t of the root portion, the average spacing of the band-shaped Mn segregation portions in the thickness direction is 4.0 μm or more and 20.0 μm or less, and the Mn segregation strength of the band-shaped Mn segregation portions is 2.00 mass% or more and 3.50 mass% or less.

2. The molded article according to claim 1, characterized in that the Vickers hardness of the plate-like portion is 250 Hv or more.

3. The molded article according to claim 1, characterized in that the Vickers hardness of the plate-like portion is 300 Hv or more.

4. The molded article according to claim 1, characterized in that the Vickers hardness of the plate-like portion is 350 Hv or more.

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