Automotive suspension parts

Abstract

The present invention provides an automotive suspension component that, with a novel configuration, has excellent impact resistance in a bent part.　An automotive suspension component (1) according to the present invention comprises a bent part (2) which is formed from a steel sheet, said automotive suspension component (1) being characterized in that: the steel sheet has a specific chemical composition containing C, Si, Al, Ti, and Sn as essential components; the Vickers hardness of the bent part is not less than 250 HV; the peak of in-crystal-grain orientation difference of an inner surface layer of the bent part is 1.0-3.0 degrees; and the peak of in-crystal-grain orientation difference of an outer surface layer of the bent part is 2.0-4.0 degrees.

Description

[Technical Field]

[0001] The present invention relates to an automobile suspension part formed from a steel plate and having a bent portion. [Background technology]

[0002] Generally, automotive suspension parts such as upper arms, lower arms, torsion beams, and stabilizers are formed by forming steel sheets into predetermined shapes according to their respective purposes. These automotive suspension parts may crack when subjected to impacts such as when the vehicle crashes or runs over a curb. Therefore, automotive suspension parts are required to have impact resistance to prevent such cracks from occurring.

[0003] As means for improving the impact resistance of automotive suspension parts, techniques related to structures for mitigating stress concentration, techniques for applying reinforcing members such as patches, etc. Also proposed is a technique for improving the steel sheets used in the manufacture of automotive suspension parts to achieve both the bending workability of the steel sheets and the strength properties such as impact resistance and fatigue properties of the formed parts.

[0004] As an example of such technology, Patent Document 1 discloses a high-strength hot-rolled steel sheet with excellent bending workability, which is used for manufacturing automobile suspension parts, etc. Specifically, Patent Document 1 discloses a high-strength hot-rolled steel sheet containing C, Si, Mn, and sol.Al as chemical components, and having a surface region with a {211} <111> ~{111} <112> The average pole density of the orientation group consisting of {110} <001> and the pole density of the crystal orientations is 0.5 or more and 6.0 or less, and the tensile strength is 780 MPa or more and 1370 MPa or less.

[0005] Similarly, Patent Document 2 discloses a high-strength steel sheet having excellent bending workability and fatigue properties as a steel sheet used for manufacturing automobile suspension parts, etc. Specifically, Patent Document 2 discloses a high-strength steel sheet having a base and a surface layer, the average Vickers hardness of the surface layer is 50 to 80% of the average Vickers hardness at a position halfway through the thickness of the high-strength hot-rolled steel sheet, the arithmetic mean roughness Ra of the surface of the surface layer is 3.0 μm or less, the effective crystal grain size of the surface layer is 50.0 μm or less, and the Si content Si B and the Si content of the surface layer Si S The difference between ΔSi and Si B -Si s is 0.60% by mass or more, and the metal structure of the base is tempered martensite: 90% or more in area ratio.

[0006] Furthermore, Patent Document 3 discloses a high-strength steel sheet that has excellent bending properties for punched members with large clearances, as a steel sheet used in the manufacture of automobile suspension parts and the like. Specifically, Patent Document 3 discloses a high-strength steel sheet having a chemical composition containing 0.04 to 0.20% C, 0.6 to 1.5% Si, 1.0 to 3.0% Mn, 0.10% or less P, 0.030% or less S, 0.10% or less Al, 0.010% or less N, 0.01 to 1.0% each of one or more of Ti, Nb, and V, with the balance being iron and unavoidable impurities, a structure in which bainite accounts for more than 50% by area, an average grain size at a position 50 μm from the steel sheet surface in the sheet thickness depth direction is 2500 × [tensile strength TS (MPa)] − 0.85 μm or less, precipitates having a particle size of less than 20 nm precipitated in the steel have a C content of 0.005% by mass or more, and an arithmetic mean roughness Ra is 3.0 μm or less.

[0007] Furthermore, Patent Document 4 discloses a high-strength structural steel material with excellent cold bending properties as a steel sheet used for manufacturing automobile suspension parts, etc. Specifically, Patent Document 4 discloses a high-strength structural steel material containing, by weight, 0.02 to 0.1% C, 0.01 to 0.6% Si, 1.7 to 2.5% Mn, 0.005 to 0.5% or less Al, 0.02% or less P, 0.01% or less S, 0.0015 to 0.015% N, with the remainder being Fe and other unavoidable impurities, the steel material being microstructurally divided along the thickness direction into an outer surface layer portion and an inner center portion, the surface layer portion containing tempered bainite as a matrix structure, and the center portion containing bainitic ferrite as a matrix structure. [Prior art documents] [Patent documents]

[0008] [Patent Document 1] International Publication No. 2020 / 110843 [Patent Document 2] International Publication No. 2020 / 203934 [Patent Document 3] Japanese Patent Application Laid-Open No. 2017-150051 [Patent Document 4] Special Publication No. 2022-514018 Summary of the Invention [Problem to be solved by the invention]

[0009] Cracks that can occur in automotive suspension parts when an impact is applied are likely to occur in bent portions formed on the edges of the parts. These bent portions function as ribs to ensure the strength of the automotive suspension part, and are formed in stress-generating areas such as the edges where stress transmitted from tires and the like is likely to concentrate. Current automotive suspension parts are less likely to crack in such bent portions by adopting various improved steel plates such as those described above. However, if cracks do occur in such bent portions, they can cause brittle fracture during a vehicle collision. Therefore, in order to respond to future increases in the weight and complexity of automotive parts, parts that are less likely to develop such cracks and have excellent impact resistance are required.

[0010] SUMMARY OF THE INVENTION It is therefore an object of the present invention to provide an automobile suspension part having a bent portion with excellent impact resistance, using a novel configuration. [Means for solving the problem]

[0011] The present invention includes the following aspects.

[0012] (Aspect 1) An automobile suspension part having a bent portion formed from a steel plate, The chemical composition of the steel plate is, in mass%, C: 0.02 to 0.30%, Si: 0.01 to 2.00%, Mn: 0.50 to 3.00% Al: 0.010 to 1.000%, Ti: 0.06 to 0.20% P: 0.100% or less, S: 0.0150% or less, N: 0.0100% or less, Nb: 0 to 0.10% Ca: 0 to 0.006%, Mo: 0-1.00%, Cr: 0~1.00%, V: 0~0.40%, Ni: 0 to 0.40% B: 0 to 0.0020%, Cu: 0-1.00% Sn: 0 to 0.50% Zr: 0 to 0.050%, and The balance is Fe and impurities. The Vickers hardness of the bent portion is 250 HV or more, The peak misorientation within the crystal grains in the inner surface layer of the bent portion is 1.0 to 3.0 degrees, The automobile suspension part is characterized in that the peak misorientation within crystal grains in the outer surface layer of the bent portion is 2.0 to 4.0 degrees.

[0013] (Aspect 2) The chemical composition of the steel plate is, in mass%, Nb: 0.001 to 0.10%, Ca: 0.0001 to 0.006%, Mo: 0.001 to 1.00%, Cr: 0.001 to 1.00%, V: 0.001~0.40%, Ni: 0.001 to 0.40% B: 0.0001 to 0.0020%, Cu: 0.001 to 1.00%, Sn: 0.001 to 0.50%, and Zr: 0.0001 to 0.050%, The automobile suspension part according to the above-mentioned aspect 1, characterized in that it contains one or more selected from the group consisting of: [Effects of the Invention]

[0014] According to the present invention, it is possible to provide an automobile suspension part having excellent impact resistance at a bent portion. [Brief explanation of the drawings]

[0015] [Figure 1] FIG. 1 is a plan view of an automobile suspension component 1 according to one embodiment of the present invention. [Figure 2]FIG. 2 is a cross-sectional end view of the bent portion 2 of the automobile suspension part 1 shown in FIG. 1 taken along line II-II. [Figure 3] FIG. 3 is a schematic diagram for explaining the relationship between the bending axis AB and the rolling direction DR when a blank p is punched out from a hot-rolled steel sheet SP and subjected to bending. DETAILED DESCRIPTION OF THE INVENTION

[0016] In order to achieve the above object, the inventors conducted detailed studies focusing on the metallographic structure and crystalline structure of the bent portion of an automotive suspension part. As a result, the inventors discovered that the impact resistance of the bent portion can be improved by controlling the crystalline structure of the bent portion to be appropriate on the inside and outside of the bend. In controlling the crystalline structure of the bent portion, it is important to control the crystal orientation and the crystalline strain within the steel sheet structure, because the structural fraction of the steel sheet does not change due to bending. The present invention was completed based on these findings and includes the following aspects of the embodiments.

[0017] Hereinafter, a preferred embodiment of the automotive suspension component of the present invention will be described in detail using a lower arm as an example. In this specification, various numerical ranges mean ranges that include the upper and lower limits unless otherwise specified.

[0018] <Automotive suspension parts> Fig. 1 is a plan view of an automobile suspension part 1 according to one embodiment of the present invention. Fig. 2 is an end view of a cross section taken along line II-II of a bent portion 2 in the automobile suspension part 1 shown in Fig. 1.

[0019] The automotive suspension part 1 of this embodiment shown in Fig. 1 is a lower arm for connecting the suspension and tire of an automobile, and is formed by forming a steel plate into a predetermined shape. As shown in Fig. 1, the automotive suspension part 1 has an overall elongated outer shape having a longitudinal direction DL and a width direction DW in a plan view. The automotive suspension part 1 has joint portions J1 and J2 that connect to the suspension and tire, respectively, at both ends in the longitudinal direction DL of the outer shape, and an arm portion AP formed between the joint portions J1 and J2.

[0020] 1, the joint part J1 located at one end in the longitudinal direction DL is the joint part located on the suspension side S1 within the vehicle, while the joint part J2 located at the other end in the longitudinal direction DL is the joint part located on the tire side S2 within the vehicle.

[0021] As shown in Fig. 1 and Fig. 2, the automobile suspension part 1 has a bent part 2 formed by bending the edge part so that it stands up in the direction along the plate thickness TP (i.e., plate thickness direction DT) over substantially the entire outer peripheral edge part except for the vicinity of the joint part J1 located on the suspension side S1. This bent part 2 functions as a rib for ensuring the strength of the automobile suspension part 1. In Fig. 2, the part indicated by the symbol A B indicates the bending axis of the bent portion 2. Bending axis A B is the virtual axis when bending the steel plate. In Figure 2, the bending axis A B is shown as a virtual axis extending in a direction perpendicular to the plane including the end face of the bent portion 2, and no member or the like that serves as the axis is actually disposed inside the steel plate.

[0022] Note that the bent portion 2 does not need to be formed over substantially the entire outer peripheral edge portion except for the vicinity of the joint portion J1 located on the suspension side S1 as described above, as long as it is formed in a stress generation portion, such as an edge portion, where stress transmitted from a tire or the like is likely to concentrate. In the automotive suspension component 1 shown in top view in FIG. 1, the maximum stress generation portion, where stress is most likely to concentrate, is the edge portion of the most curved portion located on the tire side S2 in the longitudinal direction DL, and an end view of the cross section of that edge is shown in FIG. 2. The bent portion 2 is preferably formed at a position that includes at least such a maximum stress generation portion. In this embodiment, the bent portion 2 may be formed only in the edge portion of the most curved portion located on the tire side S2 in the longitudinal direction DL of the automotive suspension component 1, i.e., the maximum stress generation portion, or may be formed on at least a portion of the outer peripheral edge portion of the automotive suspension component 1 that includes at least the maximum stress generation portion.

[0023] The maximum stress occurring portion of an automobile suspension part is determined by the type, overall structure, shape, application, etc. An example of the maximum stress occurring portion of an automobile suspension part is a portion with the smallest radius of curvature.

[0024] The means for forming the bent portion 2 is not particularly limited, and the bent portion 2 can be formed using general processing means and processing conditions such as press forming (particularly cold press forming).

[0025] As described above, the automobile suspension part 1 of this embodiment is an automobile suspension part formed of a steel plate and having a bent part 2. The automobile suspension part 1 is formed of a steel plate whose chemical composition is, in mass %, C: 0.02 to 0.30%, Si: 0.01 to 2.00%, Mn: 0.50 to 3.00% Al: 0.010 to 1.000%, Ti: 0.06 to 0.20% P: 0.100% or less, S: 0.0150% or less, N: 0.0100% or less, Nb: 0 to 0.10% Ca: 0 to 0.006%, Mo: 0-1.00%, Cr: 0~1.00%, V: 0~0.40%, Ni: 0 to 0.40% B: 0~0.0020%, Cu: 0-1.00% Sn: 0 to 0.50% Zr: 0 to 0.050%, and It has a specific chemical composition in which the balance is Fe and impurities. Furthermore, the bent portion 2 of the automobile suspension component 1 has a Vickers hardness of 250 HV or more. The automobile suspension part 1 has a characteristic crystal structure in which the peak of the misorientation within the crystal grains of the inner surface layer 21 of the bent portion 2 is 1.0 to 3.0 degrees, and the peak of the misorientation within the crystal grains of the outer surface layer 22 of the bent portion 2 is 2.0 to 4.0 degrees.

[0026] Typically, in steel sheets that have been bent, the crystal grains rotate due to slip deformation, and there is an orientation distribution within the crystal grains. The higher the misorientation within the crystal grains, the higher the strain applied, resulting in poorer deformability and impact resistance of the part after bending. Furthermore, when deformation is applied to the bent part, stress concentrates toward the inside of the bent part, so the crystal orientation misorientation tends to be high, especially on the inside of the bent part, and this tends to result in poor deformability and impact resistance. However, in the automotive suspension part 1 of this embodiment, in addition to the steel plate having the above-mentioned specific chemical composition and the bent portion 2 having the above-mentioned specific hardness, the peak of the crystal grain intra-orientation difference of the inner surface layer 21 of the bent portion 2 is 1.0 to 3.0 degrees, and the peak of the crystal grain intra-orientation difference of the outer surface layer 22 of the bent portion 2 is 2.0 to 4.0 degrees, i.e., the crystal grain intra-orientation difference of the bent portion 2 is controlled to be low, so that the bent portion 2 is less likely to crack and has excellent impact resistance.

[0027] The means for controlling the crystal structures of the inner surface layer 21 and the outer surface layer 22 of the bent portion 2 within the above-mentioned specific range of intra-grain orientation difference will be described in detail later in the manufacturing method.

[0028] The steel sheet used in the automotive suspension component 1 of this embodiment will be described in more detail below. In the following description, "%", which is the unit of content of each element, means "mass%" unless otherwise specified.

[0029] (chemical composition) In this embodiment, the steel plate used for the automobile suspension part 1 is, as described above, C: 0.02 to 0.30%, Si: 0.01 to 2.00%, Mn: 0.50 to 3.00% Al: 0.010 to 1.000%, Ti: 0.06 to 0.20% P: 0.100% or less, S: 0.0150% or less, N: 0.0100% or less, Nb: 0 to 0.10% Ca: 0 to 0.006%, Mo: 0-1.00%, Cr: 0~1.00%, V: 0~0.40%, Ni: 0 to 0.40% B: 0 to 0.0020%, Cu: 0-1.00% Sn: 0 to 0.50% Zr: 0 to 0.050%, and It has a specific chemical composition, with the balance being Fe and impurities. The chemical composition of the steel sheet will be described in more detail below.

[0030] [C: 0.02~0.30%] C is an element that increases the strength of steel sheet. To fully obtain this effect, the C content is set to 0.02% or more. The C content may be 0.03% or more, 0.04% or more, or 0.05% or more. On the other hand, from the viewpoint of suppressing cracking during forming, particularly during cold press forming, or from the viewpoint of ensuring good bending workability, the C content is set to 0.30% or less. The C content may be 0.25% or less, 0.20% or less, or 0.15% or less.

[0031] [Si: 0.01 to 2.00%] Si is a deoxidizing element for steel and a solid-solution strengthening element that is effective in increasing the strength of steel sheet without impairing its ductility. To fully obtain this effect, the Si content is set to 0.01% or more. The Si content may be 0.05% or more, 0.10% or more, or 0.15% or more. On the other hand, from the viewpoint of suppressing ferrite formation and ensuring a predetermined strength or ensuring good bending workability, the Si content is set to 2.00% or less. The Si content may be 1.80% or less, 1.60% or less, or 1.40% or less.

[0032] [Mn: 0.50~3.00%] Mn is an element that improves the hardenability of steel and contributes to improving strength. To fully obtain this effect, the Mn content is set to 0.50% or more. The Mn content may be 0.60% or more, 0.80% or more, or 1.00% or more. On the other hand, from the viewpoint of suppressing the inhomogeneity of the structure due to microsegregation of Mn and ensuring good bending workability, the Mn content is set to 3.00% or less. The Mn content may be 2.80% or less, 2.60% or less, or 2.40% or less.

[0033] [Al: 0.010~1.000%] Al is an element that functions as a deoxidizer and is a solid-solution strengthening element that is effective in 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 set to 0.010% or more. The Al content may be 0.050% or more, 0.100% or more, or 0.150% or more. On the other hand, from the viewpoint of suppressing the formation of coarse precipitates at the grain boundaries of prior austenite grains and ensuring good bending workability, the Al content is set to 1.000% or less. The Al content may be 0.800% or less, 0.600% or less, or 0.400% or less.

[0034] [Ti: 0.06~0.20%] Ti is an element that controls the morphology of carbides and increases the strength of ferrite. To achieve this effect, the Ti content is set to 0.06% or more. The Ti content may be set to 0.08% or more, or 0.10% or more. On the other hand, from the viewpoint of suppressing the formation of coarse Ti oxides or TiN and ensuring good bending workability, the Ti content is set to 0.20% or less. The Ti content may be set to 0.18% or less, 0.16% or less, or 0.14% or less.

[0035] [P:0.100% or less] P is an element that is mixed in during the manufacturing process and is an impurity. Furthermore, P segregates at prior austenite grain boundaries, which can reduce the formability of the steel sheet due to grain boundary embrittlement. Therefore, the lower the P content, the better. The P content may be 0%. However, from the viewpoint of shortening the refining time and ensuring good productivity, the P content may be 0.0001% or more, 0.0005% or more, or 0.0010% or more. On the other hand, from the viewpoint of suppressing a decrease in the toughness of the steel sheet and ensuring good bending workability, 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.

[0036] [S:0.0150% or less] S is an element that is mixed in during the manufacturing process and is an impurity. Furthermore, S may generate non-metallic inclusions such as MnS in steel, which may increase the hardness and reduce the ductility of the steel sheet. Therefore, the lower the S content, the better. The S content may be 0%. However, from the viewpoint of shortening the refining time and ensuring good productivity, the S content may be 0.0001% or more, 0.0005% or more, or 0.0010% or more. On the other hand, from the viewpoint of ensuring good bending workability, the S content may be 0.0150% or less. The S content may be 0.0130% or less, 0.0120% or less, or 0.0110% or less.

[0037] [N:0.0100% or less] N is an element that is mixed in during the manufacturing process. Like C, N is an element that is effective in increasing the strength of steel, but it also affects the occurrence of dislocation cross-slip during forming, particularly cold press forming. If the N content is high, strain concentration cannot be suppressed during steel sheet forming, causing voids to occur, thereby reducing formability. From the perspective of ensuring good formability, the lower the N content, the better. The N content may be 0%. However, from the perspective of shortening 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 perspective of ensuring good bending workability, the N content may be 0.0100% or less. The N content may be 0.0080% or less, 0.0060% or less, or 0.0050% or less.

[0038] In this embodiment, the basic chemical composition of the steel sheet used in the automotive suspension component 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 part of the remaining Fe, as necessary. These optional elements will be described in detail below.

[0039] [Nb: 0~0.10%] Nb is an element effective in controlling the morphology of carbides, and is also effective in refining crystal grains to improve the toughness and bending 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, from the viewpoint of suppressing a decrease in formability of steel sheets due to an increase in non-recrystallized ferrite, the Nb content may be 0.10% or less. The Nb content may be 0.08% or less, 0.06% or less, or 0.04% or less.

[0040] [Ca: 0~0.006%] Ca is an element that contributes to finely dispersing inclusions and enhances toughness. That is, Ca is an element that contributes to improving the formability of steel sheet. The Ca content may be 0%, but to fully obtain this effect, 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, from the viewpoint of ensuring good bending workability, the Ca content is preferably 0.006% or less. The Ca content may be 0.005% or less, 0.004% or less, or 0.003% or less.

[0041] [Mo: 0-1.00%] 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 fully obtain this effect, the Mo content may be 0.001% or more. The Mo content may be 0.01% or more, 0.05% or more, or 0.10% or more. On the other hand, from the viewpoint of suppressing deterioration of bending workability due to the formation of coarse Mo carbides, the Mo content may be 1.00% or less. The Mo content may be 0.80% or less, 0.60% or less, or 0.40% or less.

[0042] [Cr: 0~1.00%] Cr is an element that improves the hardenability of steel and contributes to improving the strength of steel sheet. 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, from the viewpoint of suppressing deterioration of bending 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.

[0043] [V:0~0.40%] V is an element effective in controlling the morphology of carbides, and is also effective in refining crystal grains and improving the toughness and bending workability of steel sheets. The V content may be 0%, but to fully obtain these effects, 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, from the viewpoint of suppressing a decrease in formability of steel sheets due to the precipitation of a large amount 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.

[0044] [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 fully obtain this effect, 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, from the viewpoint of suppressing a decrease in ductility of the steel sheet and ensuring good bending workability, 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.

[0045] [B: 0~0.0020%] B is an element that suppresses phase transformation at high temperatures and contributes to improving the strength of the steel sheet. The B content may be 0%, but to fully obtain this effect, 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, from the viewpoint of suppressing deterioration in bending workability caused by the generation of coarse B oxides that become the origin of voids during steel sheet forming, the B content may be 0.0020% or less. The B content may be 0.0018% or less, 0.0016% or less, or 0.0015% or less.

[0046] [Cu: 0-1.00%] Cu is present in steel in the form of fine particles and is an element that contributes to improving the strength of steel sheet. The Cu content may be 0%, but to fully obtain this effect, the Cu content may be 0.001% or more. The Cu content may be 0.01% or more, 0.03% or more, or 0.05% or more. On the other hand, from the viewpoint of suppressing deterioration of the bending workability of the steel sheet due to the precipitation of a large amount of coarse precipitates or inclusions, the Cu content may be 1.00% or less. The Cu content may be 0.80% or less, 0.60% or less, or 0.40% or less.

[0047] [Sn: 0~0.50%] Sn is an element that suppresses coarsening of crystal grains and contributes to improving the strength of steel sheet. The Sn content may be 0%, but to fully obtain this effect, the Sn content may be 0.001% or more. The Sn content may be 0.01% or more, 0.05% or more, or 0.08% or more. On the other hand, from the viewpoint of suppressing a decrease in the bending workability of the steel sheet due to an increase in coarse precipitates and inclusions, the Sn content may be 0.50% or less. The Sn content may be 0.40% or less, 0.30% or less, or 0.20% or less.

[0048] [Zr: 0~0.050%] Zr is an element that contributes to improving the formability of steel sheet. 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, from the viewpoint of suppressing a decrease in ductility of the steel sheet and ensuring good bending workability, 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.

[0049] Regarding the above optional elements, in this embodiment, the chemical composition of the steel sheet is, in mass%, Nb: 0.001 to 0.10%, Ca: 0.0001 to 0.006%, Mo: 0.001 to 1.00%, Cr: 0.001 to 1.00%, V: 0.001~0.40%, Ni: 0.001 to 0.40% B: 0.0001 to 0.0020%, Cu: 0.001 to 1.00%, Sn: 0.001 to 0.50%, and Zr: 0.0001 to 0.050%, The resin composition may contain one or more selected from the group consisting of:

[0050] [balance: Fe and impurities] In this embodiment, the remainder of the steel sheet other than the above elements consists of Fe and impurities. Here, the impurities refer to components that are mixed in during the industrial production of steel sheet due to various factors in the manufacturing process, including raw materials such as ore and scrap. Examples of impurities include H, O, 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, Sb, and Po. The impurities may be contained in a total amount of 0.100% or less.

[0051] Here, the chemical composition of the steel sheet can be measured by a common analytical method. For example, the chemical composition of the steel sheet can be measured using inductively coupled plasma atomic emission spectrometry (ICP-AES). C and S can be measured using a combustion-infrared absorption method, N can be measured using an inert gas fusion-thermal conductivity method, and O can be measured using an inert gas fusion-non-dispersive infrared absorption method.

[0052] The bent portion 2 of the automobile suspension component 1 of this embodiment will be described below.

[0053] [Vickers hardness of bent part is 250HV or more] In the automobile suspension part 1 of this embodiment, the Vickers hardness of the bent part 2 is 250 HV or more. From the viewpoint of the impact resistance of the automobile suspension part, the Vickers hardness of the bent part may be 252 HV or more, 255 HV or more, 258 HV or more, or 260 HV or more. The upper limit of the Vickers hardness of the bent part is not particularly limited. The Vickers hardness of the bent part may be 450 HV or less, 400 HV or less, 350 HV or less, or 300 HV or less.

[0054] (Method for measuring Vickers hardness of bent parts) The Vickers hardness of the bent portion can be measured in accordance with JIS Z 2244:2009. Specifically, the Vickers hardness of the bent portion can be measured by dividing the area between 100 μm from the inner surface of the bent portion (inner bend surface) and 100 μm from the outer surface of the bent portion (outer bend surface) into five equal intervals on the cross section in the thickness direction of the plate, measuring at a load of 500 gf (approximately 4.90 N), and calculating the average value of the five measured values. In this case, the distance between the measurement positions must be at least three times the distance of the indentation.

[0055] [The peak misorientation within the grains on the inner surface of the bent part is 1.0 to 3.0 degrees] [The peak misorientation within the grains on the outer surface of the bent part is 2.0 to 4.0 degrees] The automotive suspension part 1 of this embodiment has a characteristic crystal structure in which the peak of the crystal grain misorientation in the inner surface layer 21 of the bent portion 2 is 1.0 to 3.0 degrees, and the peak of the crystal grain misorientation in the outer surface layer 22 is 2.0 to 4.0 degrees. In addition to the steel sheet having the specific chemical composition and the bent portion 2 having the specific Vickers hardness, the automotive suspension part 1 has the peaks of the crystal grain misorientation in the inner surface layer 21 and the outer surface layer 22 of the bent portion 2 each falling within the specific range, i.e., the crystal grain misorientation in the bent portion 2 is controlled to be low, thereby enabling the automotive suspension part 1 to exhibit excellent impact resistance properties at least in the bent portion 2. This makes the bent portion 2 of the automotive suspension part 1 less susceptible to cracking.

[0056] The inner surface layer of the bent portion refers to the region within the range from the steel sheet surface inside the bent portion to a depth of 1 / 8 of the sheet thickness in the sheet thickness direction. Similarly, the outer surface layer of the bent portion refers to the region within the range from the steel sheet surface outside the bent portion to a depth of 1 / 8 of the sheet thickness in the sheet thickness direction.

[0057] In this embodiment, the histogram (i.e., peak) of the misorientation within the crystal grains in an area of ​​150 μm in the thickness direction × 400 μm in the direction parallel to the steel sheet surface of the inner surface layer 21 of the thickness direction cross section of the bent portion 2 may be 1.2 degrees or more or 1.4 degrees or more. Furthermore, from the viewpoint of obtaining better impact resistance properties, the peak of the misorientation within the crystal grains in the inner surface layer 21 of the bent portion 2 may be 2.6 degrees or less or 2.0 degrees or less.

[0058] In this embodiment, the histogram (i.e., peak) of the misorientation within the crystal grains in an area of ​​150 μm in the thickness direction × 400 μm in the direction parallel to the steel sheet surface of the outer surface layer 22 of the thickness direction cross section of the bent portion 2 may be 2.2 degrees or more or 2.5 degrees or more. Furthermore, the peak of the misorientation within the crystal grains in the outer surface layer 22 of the bent portion 2 may be 3.8 degrees or less or 3.6 degrees or less in order to obtain better impact resistance properties.

[0059] (Method for measuring peak misorientation within crystal grains in the inner and outer surface layers of a bent part) The peak of the intra-grain misorientation in the inner and outer surface layers of the thickness-direction cross section of the bent portion can be measured as follows. First, a cross section perpendicular to the bending axis of the bent portion is observed using a scanning electron microscope (SEM). A region 1 / 8 of the thickness from the inner or outer surface of the bent portion is measured by electron backscatter diffraction (EBSD) in a range of 150 μm in the thickness direction and 400 μm in the direction parallel to the steel sheet surface. The surface of the sample, including the measurement region, is polished to a mirror finish and then electropolished. An EBSD analyzer consisting of a thermal field emission scanning electron microscope (e.g., JEOL JSM-7200F) and an EBSD detector (e.g., EDAX Velocity® ultra-high-speed EBSD detector) is used for the measurement. The degree of vacuum in the apparatus is 9.6 × 10 -5 The conditions were: 1. The scanning electron microscope (SEM) was used ... The peak of misorientation within a crystal grain means the maximum value of the number frequency of the average misorientation within a crystal grain.

[0060] (Minimum radius of curvature r of the bent part) In the automotive suspension component 1 of this embodiment, the minimum radius of curvature r on the inside of the bent portion 2 is not particularly limited. From the viewpoint of effectively utilizing the limited design space, the minimum radius of curvature r may be 8.0 mm or less, 5.0 mm or less, or 3.0 mm or less. Furthermore, from the viewpoint of suppressing wrinkles and cracks that may occur on the surface during bending, the minimum radius of curvature r may be 1.2 mm or more, 1.6 mm or more, or 2.0 mm or more.

[0061] In the automotive suspension part 1 of this embodiment, the requirements for the steel plate other than that it has the above-mentioned specific chemical composition, that at least the bent portion 2 has the above-mentioned specific Vickers hardness, and that the peaks of the crystal grain orientation difference in the inner surface layer and the outer surface layer of the bent portion 2 are each within the above-mentioned specific ranges, are not particularly limited, and any steel plate having a predetermined strength, plate thickness, etc. that can be used for automotive suspension parts can be used.

[0062] Examples of the strength of such steel sheet include a tensile strength of 750 MPa or more. The tensile strength of the steel sheet may be 800 MPa or more, 850 MPa or more, 900 MPa or more, or 980 MPa or more. There is no particular upper limit to the tensile strength of the steel sheet, but from the viewpoint of ensuring good bending workability, the tensile strength of the steel sheet may be 2000 MPa or less, 1500 MPa or less, or 1200 MPa or less.

[0063] The thickness of the steel plate may be, for example, 1.5 mm or more. The thickness of the steel plate may be 1.8 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 steel plate, but from the viewpoint of ensuring good bending workability, the thickness of the steel plate 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. The thickness of the steel plate can be measured using a micrometer.

[0064] The automotive suspension part 1 of this embodiment can exhibit excellent impact resistance at least in the bent portion 2, and cracks are less likely to occur in the bent portion 2. Therefore, the automotive suspension part 1 of this embodiment can be applied to various suspension parts that require high impact resistance in an automobile, such as upper arms, lower arms, torsion beams, stabilizers, etc.

[0065] <Manufacturing method for automobile suspension parts> Next, a description will be given of a preferred method for manufacturing the automotive suspension part 1, which is one embodiment of the present invention. Note that the following description is intended to exemplify a characteristic method for manufacturing the automotive suspension part 1, which is one embodiment of the present invention, and is not intended to limit the automotive suspension part 1 to those manufactured by the manufacturing method described below.

[0066] In manufacturing the automobile suspension component 1 of this embodiment, first, a steel plate having the above-mentioned specific chemical composition and having the above-mentioned specific Vickers hardness at the bent portion 2 is manufactured.

[0067] (Steel plate manufacturing method) In the present embodiment, the method for producing the steel sheet is not particularly limited as long as it can produce a steel sheet having the above-mentioned specific chemical composition and having the above-mentioned specific Vickers hardness at the bent portion. An example of a method for producing such a steel sheet 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.

[0068] Preferred conditions for these steps will be described below.

[0069] (Casting process) In the method for producing a steel sheet according to this embodiment, the casting step is a step of casting a slab having the specific chemical composition described above. The casting step uses a continuous casting machine having a plurality of reduction rolls adjacent to each other in the conveying direction of the slab, and the roll pitch between the adjacent reduction rolls is 290 mm or less.

[0070] (Hot rolling process) The hot rolling step is a step of hot rolling the cast slab. In the hot rolling step, it is preferable to heat the slab to 1200°C or higher prior to hot rolling. By setting the heating temperature to 1200°C or higher, the rolling reaction force during hot rolling does not become excessively large, making it easier to obtain the desired thickness. There is no particular upper limit to the heating temperature, but from an economical viewpoint, it is preferable that the heating temperature be 1300°C or lower.

[0071] In the hot rolling process, the heated slab is subjected to rough rolling and finish rolling. Here, by appropriately selecting various conditions for rough rolling, the steel sheet can be made to have the specific Vickers hardness as described above, and the peaks of the crystal grain misorientation in the inner surface layer and the outer surface layer of the bent portion 2 formed after bending can be easily controlled to fall within the specific ranges.

[0072] In the hot rolling process, the starting temperature of rough rolling is, for example, 1150°C or lower. When the starting temperature of rough rolling is 1150°C or lower, the influence of heat removal by the rolling rolls is reduced, and the steel sheet can be rolled uniformly on both sides. On the other hand, the starting temperature of rough rolling is, for example, 1050°C or higher. When the starting temperature of rough rolling is 1050°C or higher, the rolling reaction force can be controlled so as not to become excessively large.

[0073] In the hot rolling process, the end temperature of finish rolling is, for example, 800°C or higher. If the end temperature of finish rolling is 800°C or higher, the average grain size of the hot-rolled steel sheet and the final product can be reduced, thereby ensuring sufficient yield strength. On the other hand, although there is no particular upper limit for the end temperature of finish rolling, from an economical viewpoint, the end temperature of finish rolling is, for example, 980°C or lower.

[0074] The hot-rolled steel sheet obtained in the hot rolling process is coiled at a coiling temperature of, for example, 450 to 600°C. By setting the coiling temperature to 450°C or higher, the strength of the hot-rolled steel sheet does not become excessively high, and deterioration of bending workability can be suppressed. On the other hand, by setting the coiling temperature to 600°C or lower, coarse ferrite and pearlite are less likely to be formed in the structure of the hot-rolled steel sheet, and bainite is more likely to be formed in the structure of the hot-rolled steel sheet, thereby improving the strength of the steel sheet.

[0075] The coiling temperature of the hot-rolled steel sheet may be, for example, 450°C or lower. By setting the coiling temperature to 450°C or lower, a large amount of structures such as bainite and martensite are formed, making it easier to ensure the desired strength even with the addition of a small amount of alloying elements. The coiling temperature may be, for example, 200°C or lower. By setting the coiling temperature to 200°C or lower, a large amount of martensite is formed, making it possible to improve the strength of the steel sheet even with a small amount of alloying elements added.

[0076] (Skin pass rolling process) The hot-rolled steel sheet obtained by the hot rolling step may be subjected to skin-pass rolling for the purpose of correcting the shape. Note that, since skin-pass rolling is not included in cold rolling, the steel sheet after skin-pass rolling is also a hot-rolled steel sheet.

[0077] Furthermore, a steel sheet obtained by hot rolling or a steel sheet obtained by skin-pass rolling (hereinafter, these are collectively referred to as "hot-rolled steel sheet") may be subjected to any treatment step such as plating, if necessary. That is, the steel sheet used for the automotive suspension part of this embodiment is preferably a hot-rolled steel sheet, since the effects of the present invention can be more reliably obtained from this.

[0078] The hot-rolled steel sheet obtained by the above manufacturing method is then subjected to the next forming step.

[0079] (molding process) The forming process includes a punching process in which a steel plate is punched into a predetermined shape, and a bending process in which the blank material punched into the predetermined shape is bent.

[0080] The punching process is a process of punching out a blank material having a shape in plan view corresponding to the shape of the automobile suspension part 1 before bending from a hot-rolled steel sheet having the above-mentioned specific chemical composition.

[0081] Here, FIG. 3 shows the bending axis A when a blank p is punched out from a hot-rolled steel sheet SP and subjected to bending. B 3 is a schematic diagram for explaining the relationship between the rolling direction DR and the bending axis A when punching a blank material p from the hot-rolled steel sheet SP after the rolling process. B The punching direction of the blank material p (i.e., the cutting direction) is adjusted so that the bending axis A intersects with the rolling direction DR of the hot-rolled steel sheet SP. B The sheet cutting direction is adjusted so that the bending axis A of the portion to be bent 2 in the hot-rolled steel sheet SP intersects with the rolling direction DR at an angle of 45 degrees to 135 degrees. B The hot-rolled steel sheet SP is subjected to a punching process and a bending process so that the direction of the rolling direction DR of the hot-rolled steel sheet SP intersects with the rolling direction DR of the hot-rolled steel sheet SP at an angle of 45 degrees to 135 degrees.

[0082] In FIG. 3, the blank p is shown in a rectangular shape in plan view for the sake of convenience, but in reality, it has a shape in plan view that corresponds to the automobile suspension part 1 before bending.

[0083] Typically, when steel sheet is subjected to bending deformation, strain is introduced into the crystal grains, causing crystal rotation. This crystal rotation is determined by the crystal orientation formed by hot rolling of the steel sheet and the crystal rotation added by the direction of bending. Furthermore, during bending, the inside of the bent portion is compressed, while the outside of the bent portion is tensile. This results in different amounts of crystal rotation on the inside and outside of the bent portion, which changes the misorientation within the crystal grains after bending. As mentioned above, the performance of automotive suspension parts depends on the misorientation after bending. Therefore, by controlling the misorientation on the inside and outside of the bent portion to an appropriate level, excellent impact resistance can be achieved in the bent portion. In this embodiment, as described above, the bending axis A of the portion to be bent 2 in the hot-rolled steel sheet SP is B By performing the punching process and bending process of the hot-rolled steel sheet SP so that the direction of the rolling DR of the hot-rolled steel sheet SP intersects with the rolling direction DR of the hot-rolled steel sheet SP, the intra-grain orientation difference of the inner surface layer 21 and the outer surface layer 22 of the bent processed portion 2 of the automobile suspension part 1 can be controlled within appropriate ranges, and excellent impact resistance can be obtained.

[0084] In this embodiment, the blank p is cut in the direction of the bending axis A. B and the rolling direction DR is not particularly limited as long as it is an angle of 45 degrees to 135 degrees, but from the viewpoint of impact resistance, it may be in the range of 60 degrees or more and 120 degrees or less, 85 degrees or more and 95 degrees or less, or 90 degrees.

[0085] Depending on the type of automotive suspension part, the direction of the bending axis may not be constant. Specifically, the bending axis may be curved, or there may be multiple bending axes. In such cases, the punching direction of the blank (i.e., the blank cutting direction) can be set to match the maximum stress generation location where stress is most likely to concentrate, taking into account the expected stress direction in the environment in which the automotive suspension part will be used and the dimensions of the bent part.

[0086] The bending process is a process of bending a blank material that has been punched into a predetermined shape. In the bending process, the means for bending the blank material is not particularly limited, and examples thereof include press forming. In addition, in the bending process, such a bending means as press forming may be performed only once, or may be performed in stages multiple times.

[0087] In the above-described embodiment, the bending process is performed on the blank material after it has been punched into a predetermined shape in the punching process, but this is not limited to this. The bending process may also be performed by bending predetermined locations of the blank material while punching out the blank material in the punching process. That is, the punching process and the bending process may be performed in parallel, or the bending process may be performed after the punching process as described above.

[0088] The automobile suspension component 1 obtained by the above molding process may be subjected to a painting process or other surface treatment process for the purpose of improving the appearance and design.

[0089] The present invention is not limited to the above-described embodiments or the following examples, and appropriate combinations, substitutions, modifications, etc. are possible within the scope that does not deviate from the object and intent of the present invention.

[0090] The present invention will be explained in more detail below by way of examples, but the present invention is not limited to these examples. [Example]

[0091] In the following examples, automobile suspension parts according to one embodiment of the present invention were manufactured under various conditions, and the impact resistance of the bent portions of the obtained automobile suspension parts was examined.

[0092] (Steel plate manufacturing) Slabs having the chemical compositions shown in Table 1 below were cast by continuous casting using a continuous casting machine equipped with a plurality of reduction rolls arranged at a roll pitch of 290 mm or less.

[0093] Next, the obtained slab was subjected to a hot rolling process. Specifically, the slab was heated to a temperature of 1200°C to 1300°C, and rough rolling and finish rolling were performed. The starting temperature of rough rolling was 1050°C to 1150°C, and the finishing temperature of finish rolling was 800°C to 980°C. The coiling temperature of the obtained hot-rolled steel sheet was 450°C to 600°C. In this way, several types of hot-rolled steel sheets (sheet thickness 3 mm) with different chemical compositions were obtained, as shown in Table 1 below.

[0094] The chemical composition of the samples taken from the obtained hot-rolled steel sheets was analyzed, and it was confirmed that there was no change from the chemical composition of the slab. The chemical composition of the obtained hot-rolled steel sheets is shown in Table 1 below.

[0095] (Manufacturing of automotive suspension parts) From the obtained hot-rolled steel sheet, the sheet cutting direction, i.e., the bending axis A of the planned bending processing portion B The blank was punched out so that the angle θ between the rolling direction DR and the sheet metal was 40 degrees, 50 degrees, 60 degrees, 80 degrees, 90 degrees, 95 degrees, 100 degrees, or 150 degrees. The blank was in the form of a flat plate that had not been subjected to bending, and had an elongated planar shape having a longitudinal direction and a width direction that corresponded to the planar shape of the lower arm.

[0096] Furthermore, the following steps were carried out: a step of bending both widthwise edges of the blank material extending in the longitudinal direction upright by press forming using a die and a punch to obtain an intermediate formed body; and a step of bending one longitudinal edge of the intermediate formed body by press forming using a die and a punch so that the minimum radius of curvature r was 2.0 mm or 3.0 mm to form a bent portion that would become a joint portion (mounting portion). In this way, lower arms Nos. 1 to 12 shown in Table 1, i.e., automotive suspension parts, were formed.

[0097] For the automobile suspension parts Nos. 1 to 12 obtained as described above, the peak misorientation of the crystal grains in the inner and outer surface layers of the bent part was measured according to the above-mentioned "Method for measuring the peak misorientation of the crystal grains in the inner and outer surface layers of the bent part," and the Vickers hardness of the bent part was measured according to the above-mentioned "Method for measuring Vickers hardness." Then, the automobile suspension parts Nos. 1 to 12 were subjected to the following impact resistance test to evaluate the impact resistance of the bent part of these automobile suspension parts. The measurement and evaluation results are shown in Table 1 below.

[0098] (Impact resistance test) First, the part to be tested is cooled to -40°C. Next, the cooled part is placed on a horizontal test bench with the outer surface of the bent part facing up. A cone weighing approximately 120 kg is then allowed to fall freely from a height of 0.18 m above the surface of the test bench, colliding with the outer surface of the bent part of the part. Note that before the free fall, the cone is placed with its base facing downwards. The cone used has a base area large enough to cover the entire bent part of the part. The condition of the part after the cone is collided with the bent portion of the part is observed, and parts that do not break in the bent portion are judged to have "excellent impact resistance," while parts that do break in the bent portion are judged to have "poor impact resistance."

[0099] In Table 1, a judgment of "excellent impact resistance" is indicated by "◯" and a judgment result of "poor impact resistance" is indicated by "X." In addition, underlines next to various values ​​in Table 1 indicate that the values ​​are outside the range of the present invention or that the manufacturing conditions do not allow the automotive suspension part of the present invention to be obtained.

[0100] [Table 1]

[0101] As shown in Table 1, it was found that the automobile suspension parts of the examples of the present invention had excellent impact resistance at the bent portions, while the automobile suspension parts of the comparative examples had poor impact resistance at the bent portions. [Explanation of symbols]

[0102] 1 Automotive suspension parts 2 Bending section 21 Inner surface layer 22 Outer surface layer

Claims

1. An automobile suspension part having a bent portion formed from a steel plate, The chemical composition of the steel plate is, in mass%, C: 0.02-0.30%, Si: 0.01-2.00%, Mn: 0.50-3.00%, Al: 0.010-1.000%, Ti: 0.06-0.20%, P: 0.100% or less, S: 0.0150% or less, N: 0.0100% or less, Nb: 0 to 0.10%, Ca: 0-0.006%, Mo: 0-1.00%, Cr: 0-1.00%, V: 0 to 0.40%, Ni: 0 to 0.40%, B: 0 to 0.0020%, Cu: 0 to 1.00%, Sn: 0 to 0.50%, Zr: 0 to 0.050%, and The balance is Fe and impurities. The Vickers hardness of the bent portion is 250 HV or more, the peak misorientation within the crystal grains of the inner surface layer of the bent portion is 1.0 to 3.0 degrees; An automobile suspension part, characterized in that the peak misorientation within crystal grains in the outer surface layer of the bent portion is 2.0 to 4.0 degrees.

2. The chemical composition of the steel plate is, in mass%, Nb: 0.001 to 0.10%, Ca: 0.0001-0.006%, Mo: 0.001 to 1.00%, Cr: 0.001-1.00%, V: 0.001 to 0.40%, Ni: 0.001 to 0.40%, B: 0.0001 to 0.0020%, Cu: 0.001 to 1.00%, Sn: 0.001 to 0.50%, and Zr: 0.0001 to 0.050%, The automobile suspension part according to claim 1, comprising one or more selected from the group consisting of:

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