JP4044434B2 - High-strength tailored blank material with excellent deformation characteristics - Google Patents

Description

【００２８】
得られた各接合材（テーラードブランク材）について、引張試験を行なって機械的特性を測定すると共に、張出し試験を行って成形性について評価した。このときの引張試験条件および張り出し試験条件は、下記の通りである。
【００２９】
（引張試験条件）
図１に示すＪＩＳ１３号Ｂ材試験片を用い（標点距離：５０ｍｍ、幅：１２．５ｍｍ）、溶接線（ビード部）に対して垂直な方向で引張試験を行い、機械的特性（引張強度ＴＳ、伸びＥＬ）を測定した。このときの試験機は、インスロン型万能試験機を用い、クロスヘッド速度：１ｍｍ／ｍｉｎとした。
【００３０】
（張出し試験条件）
張り出し試験の状態を図２［図２（ａ）は試験装置の断面図、同（ｂ）は試験片の平面図］に示す。２種類の鋼板を接合（ビード部）した試験片を用い、張り出し試験装置によって、室温（２５℃）にてパンチで変形（変形速度１ｍｍ／ｍｉｎ）したときに、破断するまでの高さ（最大張出し高さ：Ｈｍａｘ）によって評価した。
【００３１】
その結果を、組織因子（被接合材の平均Ｖγ、平均Ｃγ、平均Ｃｅｑ）と共に下記表２に示す。尚、表２のＮｏ．１〜６のものは、素材鋼板をそのまま（接合せず）、引張試験および張出し試験を行った結果（前記表１と同じ）を示したものである。
【００３２】
【表２】

【００３３】
この結果から明らかなように、ＴＤＰ１〜３とＭＤＰ０，４を接合したテーラードブランク材料（Ｎｏ．８〜１２）では、ＴＤＰ（ＴＲＩＰ鋼板）同士のものに比べて、成形性（Ｈｍａｘ）および延性（伸びＥＬ）のいいずれも良好な値を示していることが分かる。特に、残留オーステナイト分率Ｖγ、残留オーステナイト中の炭素濃度Ｃγおよび炭素当量Ｃｅｑが本発明の好ましい範囲を満足するもの（Ｎｏ．８，９，１１）では、一方の被接合材（ＭＤＰ０、ＭＤＰ４）に比べて、延性および成形性のいずれかが更に向上していることが分かる。
【００３４】
実施例２
上記表２に示したＮｏ．８のサンプルを用い、試験実施温度（変形温度）を変化させて張り出し試験を行った。その結果を、下記表３に示す。
【００３５】
【表３】

【００３６】
まず、変形温度が−５０℃および３００℃のものでは（Ｎｏ．１３および２０）、ＴＳ×Ｈｍａｘの値が３６００未満となっているが、それ以外の変形温度では３６００以上の優れた値が得られていた。
【００３７】
この結果から、本発明のテーラードブランク材の加工に際しては、−５０℃超〜３００℃未満、好ましくは−２５℃〜２７５℃、更に好ましくは０℃〜２５０℃の範囲を採用するのが良いことが分かる。
【００３８】
【発明の効果】
本発明は以上のように構成されており、溶接継ぎ手部を含む材料全体の変形特性の向上を図ったテーラードブランク材料が実現できた。
【図面の簡単な説明】
【図１】引張試験片の形状を示す概略説明図である。
【図２】張り出し試験の状態を示す概略説明図である。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a tailored blank material in which two or more kinds of metal materials having different mechanical properties are welded and joined, and more particularly to a tailored blank material for improving the deformation characteristics of the entire material including a weld joint. is there.
[0002]
[Prior art]
TRIP type composite structure that effectively uses transformation induced plasticity (TRIP) of retained austenite (metastable austenite) among high strength steel sheets developed for the purpose of reducing the weight of automobile bodies and improving collision safety in recent years. It is known that steel plates (hereinafter sometimes referred to as “TDP steel plates”) have particularly excellent press formability.
[0003]
Also, in a frame such as an automobile, the strength required for each part may differ, and it is necessary to join and integrate metal materials having different mechanical properties by welding under such a background. Such a dissimilar material joining member is called a tailored blank material. Such a tailored blank material is press-molded after being integrated, but there is a problem that the press formability is deteriorated more than that of the raw steel plate, and the base material or the welded portion is broken.
[0004]
In order to solve such problems, for example, by controlling the ratio (n1 / n2) of the respective work hardening indices (n1, n2) of dissimilar metals to be joined within an appropriate range, the formability of the blank material due to welding is deteriorated. A technique for suppressing the above has also been proposed (for example, Patent Document 1).
[0005]
However, in a situation where the strength of the metal material on the high strength side exceeds 650 MPa and the weldability of the material steel plate deteriorates (for example, the weldability deteriorates when the carbon equivalent is increased to increase the strength), The weldability of the entire tailored blank material will deteriorate, and it has been difficult to achieve high ductility and high formability.
[0006]
[Patent Document 1]
[Patent Document 1] Japanese Patent Laid-Open No. 11-222643 Patent Claims
[Problems to be solved by the invention]
The present invention has been made under such circumstances, and an object of the present invention is to provide a tailored blank material in which the deformation characteristics of the entire material including the weld joint portion are improved.
[0008]
[Means for Solving the Problems]
The tailored blank material of the present invention capable of achieving the above object is a high strength steel sheet having a retained austenite fraction Vγ of less than 2% by volume and a strength of 650 MPa or more, and a transformation having a retained austenite fraction Vγ of 2% by volume or more. It is a tailored blank material in which different materials with the induced plastic steel sheet are joined, and has a gist in that the ductility or formability is better than the tailored blank material in which the transformation induced plastic steel sheets are joined together. .
[0009]
In the tailored blank material, the carbon equivalent Ceq represented by the following formula (1) in the high-strength steel plate and the transformation-induced plastic mold steel plate is 0.55% by mass or less, and the carbon equivalent Ceq of both The average value is in the range of 0.42 to 0.50 mass%, and the average value of the residual austenite fraction Vγ of both: 1 to 8% by volume, the average value of the carbon concentration in the residual austenite: 0.6 to It is preferable that it is 1.2 mass%.
[0010]
Ceq = [C] +1/6 [Mn] +1/24 [Si] (1)
However, [C], [Mn] and [Si] indicate the contents (mass%) of C, Mn and Si, respectively.
[0011]
DETAILED DESCRIPTION OF THE INVENTION
The inventors of the present invention have studied the configuration of tailored blank materials that can solve the above problems from various angles. As a result, if blanking TRIP steel sheet that is high strength and high ductility as a single material and high-strength steel sheet that is widely used (for example, general-purpose high-tensile steel sheet), the strain-induced transformation in the TRIP steel sheet can be effectively utilized, The present invention has been completed by finding that the ductility and formability of the tailored blanking material as a whole can be improved.
[0012]
In particular, according to the tailored blank material of the present invention, it is completely expected that ductility or formability will be better than that of a tailored blank material in which one high-strength and high-ductility steel sheet is joined. The characteristics that did not occur were able to be exhibited.
[0013]
The reason why such an effect is obtained by the present invention has not been clarified, but could be considered as follows. That is, while taking advantage of the high ductility that is the advantage of TRIP steel, the weldability that is the disadvantage is multiplied by the low carbon equivalent Ceq material of dual-phase steel with a retained austenite fraction Vγ of less than 2% by volume. Thus, since the welded portion (bead) strength is not excessively hardened, an optimum structure as a tailored blank material is possible, and it can be inferred that good formability has been achieved.
[0014]
The TRIP steel sheet used in the present invention needs to secure a residual austenite fraction Vγ of 2% by volume or more in order to ensure its good formability. On the other hand, for a high-strength steel plate blanked to a TRIP steel plate, it is necessary to set the strength to 650 MPa or more and the residual austenite fraction Vγ to less than 2% by volume in consideration of the required characteristics. . When the retained austenite fraction Vγ of this high-strength steel sheet is 2% by volume or more, the carbon equivalent Ceq is inevitably increased in order to increase the retained austenite fraction Vγ (a certain amount of C, Si, Mn is required). The ductility and formability as a tailored blank material deteriorate.
[0015]
Specific requirements for realizing the tailored blank material of the present invention capable of exhibiting the above characteristics include carbon equivalent Ceq of each steel plate, carbon equivalent of both (high strength steel plate and TRIP steel plate) It is preferable to appropriately control the average value of Ceq, the average value of the residual austenite fraction Vγ, the average value of the carbon concentration in the residual austenite, and the like. Preferred ranges and reasons for these requirements are as follows.
[0016]
It is known that the carbon equivalent Ceq of each steel plate is 0.55% by mass or less. If this carbon equivalent Ceq is too high, it is known that the weldability of the steel plate itself deteriorates. In the material, the carbon equivalent Ceq is preferably controlled to 0.55% by mass or less from the viewpoint of ensuring the weldability. That is, if this carbon equivalent Ceq exceeds 0.55% by mass in any of the materials to be joined, the weldability of the tailored blank material will deteriorate. A more preferable range of the carbon equivalent Ceq is 0.45% by mass or less. The lower limit of the carbon equivalent Ceq is not particularly limited, but is naturally determined by controlling the average value of both of the carbon equivalents Ceq to be described later to an appropriate range.
[0017]
The average value of both carbon equivalents Ceq is 0.42 to 0.50 mass%.
When the carbon equivalent Ceq is increased as a whole, the retained austenite fraction necessary for expressing ductility is sufficiently high, but the weldability of the joint portion (bead) is deteriorated, and as a result, the ductility as a blank material is reduced. It will deteriorate. Further, when the overall carbon equivalent Ceq is lowered, weldability is improved, but sufficient austenite necessary for ductility development cannot be ensured, and ductility as a blank material is deteriorated. From such a viewpoint, it is preferable to control the average value of the carbon equivalent Ceq of both (for example, the high-tensile steel plate and the TRIP steel plate as the materials to be joined) within an appropriate range. That is, in order to obtain good ductility and formability while ensuring weldability in the tailored blank material, the average value of the carbon equivalent Ceq of both is controlled to be in the range of 0.42 to 0.50 mass%. It is preferable.
[0018]
Average value of residual austenite fraction Vγ of both: 1 to 8% by volume
As described above, in order to ensure the formability of the blank material due to the TRIP effect, it is necessary to control the overall carbon equivalent Ceq to an appropriate range so that an appropriate amount of retained austenite is in the range. From such a viewpoint, the average value of the retained austenite fraction Vγ is preferably in the range of 1 to 8% by volume.
[0019]
Average value of carbon concentration Cγ in retained austenite: 0.6 to 1.2% by mass
When the carbon concentration Cγ in the retained austenite increases as a whole, the strength of the welded portion becomes too high, and ductility and formability as a tailored blank material deteriorate. If it is too low, the retained austenite will be insufficient, and ductility and formability other than the welded portion will deteriorate, and even in this case, the ductility and formability of the tailored blank material will deteriorate as a result. For these reasons, the average value of the carbon concentration Cγ of both is preferably in the range of 0.6 to 1.2% by mass.
[0020]
Joined materials in the tailored blank material of the present invention are TRIP steel plates and high-strength steel plates. Among these, the high-strength steel sheet having a strength of 650 MPa or more is basically assumed to be a high-tensile steel (high-tensile) widely used as a welded structure. It exhibits excellent properties in terms of heat resistance, low temperature toughness and weather resistance. In addition, the composition of the chemical components is Nb in addition to normal basic components (C: 0.06 to 0.25%, Mn: 0.5 to 2.5%, Si: about 0.2 to 2.0%). , V, Ti, Ni, and the like may be included.
[0021]
On the other hand, the most representative transformation induced plastic steel sheet (TRIP steel sheet) is the TRIP type composite structure steel sheet (TDP). If this TRIP steel sheet can utilize the transformation induced plasticity phenomenon, for example, a bainite mother plate can be used. The present invention can also be applied to a TRIP steel sheet of a material type, a tempered martensite type, or a composite type of them and a polygonal ferrite type. The chemical component composition of this TRIP steel sheet can be applied as long as it is generally known so far. Typical examples include C: 0.01 to 0.25%, Mn: 0.5. In addition to -3.0%, Si: 0.5-2.5%, those containing a predetermined amount (for example, about 1% each) of Ni, Cr, Mo and the like can be mentioned.
[0022]
The welding method for joining both the above-mentioned joining materials is not particularly limited as long as the materials to be joined can be joined to each other, and the laser welding method (for example, YAG laser method) shown in the examples described later. In addition, any of seam welding, arc welding, and electron beam welding can be employed.
[0023]
Hereinafter, the present invention will be described in more detail by way of examples. However, the following examples are not intended to limit the present invention, and any design changes in accordance with the gist of the preceding and following descriptions are technical aspects of the present invention. It is included in the range.
[0024]
【Example】
Example 1
Using test materials having the chemical composition shown in Table 1 below, various combinations (Table 2 below) were butt welded, and the mechanical properties were investigated. In Table 1 below, MS is normal mild steel, and TDP1 to 3 are subjected to (two-phase annealing + austempering) on the steel sheet as cold-rolled, and the three-phase structure of ferrite + bainite + retained austenite (γ _R ) TRIP steel sheet. Further, MDP0 is a ferritic / composite structure steel plate not containing retained austenite (γ _R ), and MDP4 is a steel plate obtained by tempering the composite structure steel plate at 400 ° C. The retained austenite fraction Vγ and the carbon concentration Cγ in the retained austenite were determined by X-ray diffraction.
[0025]
The butt welding at this time cut | disconnected each raw material after heat processing with the fine cutter, and welded it with the pulse YAG laser processing machine. The processing conditions at this time are as follows.
[0026]
(Laser welding process conditions)
Pulse energy: 6J / P
Pulse width: 3.8ms
Pulse repetition rate: 45Hz
Average output: 270W (maximum average output 350W, maximum peak output 4.5kW)
Welding method: One side welding (I type groove)
Welding speed: 100 to 900 mm / min
Shielding gas: N ₂
[0027]
[Table 1]

[0028]
About each obtained joining material (tailored blank material), while performing the tensile test and measuring a mechanical characteristic, the overhang test was done and the moldability was evaluated. The tensile test conditions and the overhang test conditions at this time are as follows.
[0029]
(Tensile test conditions)
Using a JIS No. 13 B material specimen shown in FIG. 1 (marking distance: 50 mm, width: 12.5 mm), a tensile test was performed in a direction perpendicular to the weld line (bead part), and mechanical properties (tensile strength) TS, elongation EL) were measured. The testing machine at this time was an Instron type universal testing machine, with a crosshead speed of 1 mm / min.
[0030]
(Overhang test conditions)
The state of the overhang test is shown in FIG. 2 [FIG. 2 (a) is a cross-sectional view of the test apparatus, and (b) is a plan view of the test piece]. Using a test piece in which two types of steel plates are joined (bead part), the height until fracture (maximum) when deformed with a punch at a room temperature (25 ° C.) (deformation speed: 1 mm / min) using a bulging test device The overhang height was evaluated by Hmax).
[0031]
The results are shown in Table 2 below together with the tissue factors (average Vγ, average Cγ, average Ceq of the materials to be joined). In Table 2, No. Samples 1 to 6 show the results of the tensile test and the bulging test (same as in Table 1) as they are (without joining) the raw steel plates.
[0032]
[Table 2]

[0033]
As is clear from this result, in the tailored blank material (Nos. 8 to 12) in which TDP 1 to 3 and MDP 0 and 4 are joined, formability (Hmax) and ductility (in comparison with those of TDP (TRIP steel plates)) ( It can be seen that all of the elongations EL) show good values. In particular, when the retained austenite fraction Vγ, the carbon concentration Cγ and the carbon equivalent Ceq in the retained austenite satisfy the preferred ranges of the present invention (No. 8, 9, 11), one of the materials to be joined (MDP0, MDP4) It can be seen that either ductility or formability is further improved as compared with.
[0034]
Example 2
No. shown in Table 2 above. Using the 8 samples, the overhang test was performed while changing the test execution temperature (deformation temperature). The results are shown in Table 3 below.
[0035]
[Table 3]

[0036]
First, when the deformation temperatures are −50 ° C. and 300 ° C. (Nos. 13 and 20), the value of TS × Hmax is less than 3600, but at other deformation temperatures, an excellent value of 3600 or more is obtained. It was done.
[0037]
From this result, when processing the tailored blank material of the present invention, it is preferable to adopt a range of more than −50 ° C. to less than 300 ° C., preferably −25 ° C. to 275 ° C., more preferably 0 ° C. to 250 ° C. I understand.
[0038]
【The invention's effect】
The present invention is configured as described above, and a tailored blank material in which the deformation characteristics of the entire material including the weld joint portion are improved can be realized.
[Brief description of the drawings]
FIG. 1 is a schematic explanatory view showing the shape of a tensile test piece.
FIG. 2 is a schematic explanatory view showing a state of an overhang test.

Claims (2)

A tailored blank material in which dissimilar materials of a high-strength steel plate having a retained austenite fraction Vγ of less than 2% by volume and a strength of 650 MPa or more and a transformation-induced plastic mold steel plate having a retained austenite fraction Vγ of 2% by volume or more are welded. There, said the welded joint portion ductility or formability of the whole tailored blank material containing the said those good for good than ductile or molding of the entire tailored blank material obtained by bonding the transformation induced plasticity type steel plates High-strength tailored blank material with excellent deformation characteristics. The carbon equivalent Ceq represented by the following formula (1) in the high-strength steel plate and the transformation-induced plastic steel plate is 0.55% by mass or less, and the average value of the carbon equivalent Ceq of both is 0.42 to 0. Within the range of 50% by mass, the average value of the residual austenite fraction Vγ of both: 1 to 8% by volume, the average value of the carbon concentration in the residual austenite: 0.6 to 1.2% by mass Item 2. The high-strength tailored blank material according to Item 1.
Ceq = [C] +1/6 [Mn] +1/24 [Si] (1)
However, [C], [Mn] and [Si] indicate the contents (mass%) of C, Mn and Si, respectively.

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