JP3562979B2

JP3562979B2 - Automotive frame structural material for bending and arc welding made of extruded Al-Mg-Si alloy

Info

Publication number: JP3562979B2
Application number: JP31167098A
Authority: JP
Inventors: 貴志岡; 正和平野; 光弘江間
Original assignee: Kobe Steel Ltd
Current assignee: Kobe Steel Ltd
Priority date: 1998-11-02
Filing date: 1998-11-02
Publication date: 2004-09-08
Anticipated expiration: 2018-11-02
Also published as: JP2000144293A

Description

【０００１】
【発明の属する技術分野】
本発明は、Ａｌ−Ｍｇ−Ｓｉ系合金押出形材からなり、曲げ加工による成形を受け、かつアーク溶接により組み立てられるのに特に適する自動車フレーム構造材に関する。
【０００２】
【従来の技術】
Ａｌ−Ｍｇ−Ｓｉ系アルミニウム合金は、高強度が得られる合金の中では比較的耐食性に優れ、サッシ材料などとして市場に多く出回っており、リサイクルの面でも他の系のアルミニウム合金より優れていることから、押出形材として種々の構造部材への適用が注目されている。
【０００３】
【発明が解決しようとする課題】
自動車フレーム構造材は、必要に応じて所望の形状に曲げ加工を行ったり、溶接等による接合を必要とする場合があり、高強度（引張強度、耐力）を備えるとともに延性に優れ、さらにアーク溶接性が優れていることが望ましい。一方、Ａｌ−Ｍｇ−Ｓｉ系合金において強度及び延性を向上させるために、合金元素としてＣｕを添加することが知られているが、ＣｕはＡｌ−Ｍｇ−Ｓｉ系合金のアーク溶接性を阻害するという共通認識があり、アーク溶接構造をとる場合はＣｕの添加量は低く制限されていた。
【０００４】
例えば特開昭６４−４７８３０号公報では、小量のＣｒ及びＺｒを含有するＡｌ−Ｍｇ−Ｓｉ系アルミニウム合金押出形材においてＣｕの添加量を０．１％以下に制限し、特開平６−１７９９３５号公報では、小量のＣｒ及びＺｒを含有するＡｌ−Ｍｇ−Ｓｉ系アルミニウム合金押出形材においてＣｕの添加量を０．２％以下に制限し、特開平９−４１０６３号公報では、小量のＺｒを含有するＡｌ−Ｍｇ−Ｓｉ系アルミニウム合金押出形材においてＣｕの添加量を０．０５％以下に制限し、さらに、特開平９−２５６０９６号公報でも、Ｃｕの添加量を０．４％以下に制限している。
【０００５】
本発明はこのような状況下で、Ａｌ−Ｍｇ−Ｓｉ系アルミニウム合金押出形材からなり、高強度、高延性でかつ曲げ加工性とアーク溶接性に優れた曲げ加工及びアーク溶接用自動車フレーム構造材を得ることを目的とする。
【０００６】
【課題を解決するための手段】
本発明に係る曲げ加工及びアーク溶接用自動車フレーム構造材は、Ｍｇ：０．４〜０．８％、Ｓｉ：０．７〜１．１％、Ｃｕ：０．４超〜０．６５％、Ｔｉ：０．００５〜０．２％、Ｚｒ：０．０５〜０．３％を含有し、必要に応じてさらにＣｒ：０．０５〜０．５％とＭｎ：０．０５〜０．８％のうちいずれか一種又は２種を含有し、残部Ａｌ及び不純物からなるＡｌ−Ｍｇ−Ｓｉ系合金押出形材であって、結晶組織の全部又は大部分が繊維組織（以下、ファイバー組織という）であることを特徴とする。ここでファイバー組織とは押出材にみられる熱間加工組織で、押出方向に長く伸ばされた結晶粒組織のことであり、本発明においては、押出形材の断面の全てがファイバー組織で構成されるか、断面の肉厚の大部分、つまり肉厚の５０％以上の部分がファイバー組織で占められている（この場合、表面には再結晶組織が形成される）必要がある。
【０００７】
【０００８】
【発明の実施の形態】
以下、上記Ａｌ−Ｍｇ−Ｓｉ系合金押出形材の成分組成の限定理由について説明する。
Ｃｕ
Ａｌ−Ｍｇ−Ｓｉ系合金押出形材において、Ｃｕは析出強化により合金の強度を向上させるとともに、材料の延性を向上させるため、高強度、延性及び曲げ加工性を必要とされる構造材にはＣｕを添加したものも多い。しかし、Ｃｕの添加はＡｌ−Ｍｇ−Ｓｉ系合金の溶接性を阻害する（溶接割れを発生させる）という共通認識のもとで、溶接構造材として用いる押出形材では、Ｃｕの添加量は最大でも０．４％以下に抑えられていた。
【０００９】
ところが、本発明者らは、Ａｌ−Ｍｇ−Ｓｉ系合金押出形材の合金組織をファイバー組織とした場合、０．４％を超えるＣｕが含有されていてもアーク溶接性の低下がないことを見いだした。溶接性が低下しない理由は明らかではないが、ファイバー組織になることで粒界面積が増加するため、粒界に偏析しやすいＣｕが粒界に広く分散して析出し、単位粒界面積あたりのＣｕの濃度が低くなることが影響しているのではないかと推測される。
なお、０．４％を超えるＣｕが含有されたＡｌ−Ｍｇ−Ｓｉ系合金押出形材において、表面に形成される再結晶層が厚くなるとアーク溶接性の低下が著しくなる。従って、先に述べたように、押出形材の断面の全てがファイバー組織で構成されるか、断面の肉厚の５０％以上の部分がファイバー組織で占められている必要がある。望ましくは、表面再結晶層の全厚さ（両面の厚さの合計）が形材の肉厚の３０％以下（ファイバー組織が肉厚の７０％以上）になるように制御する。
【００１０】
本発明のＡｌ−Ｍｇ−Ｓｉ系合金押出形材において、０．４％超のＣｕは強度を向上させ、延性及び曲げ加工性を向上させる。しかし、過剰のＣｕはプレス焼入れ（押出直後に押出材の保有熱を利用して行う溶体化焼入れ）性を低下させ、また、アーク溶接性を阻害するようになるので、Ｃｕの添加量は０．６５％以下に制限する必要がある。特にＣｕ：０．４５〜０．５５％の範囲が望ましい。
【００１１】
Ｃｒ、Ｍｎ、Ｚｒ
Ｃｒ、Ｍｎ、Ｚｒは均質化熱処理時に鋳塊中に金属間化合物として析出し、熱間加工等における再結晶を抑制し、金属組織を微細にする。押出加工においては組織をファイバー組織とし、等軸再結晶組織に比べアーク溶接性を著しく改善する。それぞれ０．０５％以上の添加で再結晶抑制効果があるが、特に効果の高いのはＺｒであるので、Ｚｒをまず添加し、さらに必要があればＣｒ、Ｍｎを添加するようにすればよい。一方、過剰の添加は鋳造時に粗大な不溶性金属間化合物を生成しやすく、強度、延性の低下の原因となる。
従って、それぞれの添加量はＣｒ：０．０５〜０．５％以下、Ｍｎ：０．０５〜０．８％以下、Ｚｒ：０．０５〜０．３％以下とする。Ｍｎ、Ｚｒの特に望ましい範囲は、Ｍｎ：０．１５〜０．５％、Ｚｒ：０．１〜０．１５％である。
【００１２】
Ｍｇ、Ｓｉ
Ｍｇ及びＳｉは、合金に強度を付与する元素である。Ｍｇ含有量が０．３％未満又はＳｉ含有量が０．２％未満の場合、時効処理による強度向上の効果が得られない。逆に、Ｍｇ含有量が１．６％を越え又はＳｉ含有量が１．６％を越えると延性が阻害され曲げ加工性が低下し、また押出性も低下する。そのなかでも、強度、延性、曲げ加工性、押出性のバランスの面から、特にＭｇ：０．４〜０．８％、Ｓｉ：０．７〜１．１％の範囲が望ましい。
【００１３】
Ｔｉ
Ｔｉは溶解鋳造時に核生成し鋳造組織を微細にする働きがあり、０．００５％以上添加される。しかし、多すぎると粗大な化合物を生成しＡｌ−Ｍｇ−Ｓｉ系合金を脆弱にするので０．２％を上限とする。
不純物
不純物のうちＦｅはアルミニウム地金に最も多く含まれる不純物であり、０．３５％を超えて合金中に存在すると鋳造時に粗大な金属間化合物を晶出し、合金の機械的性質を損なう。従って、Ｆｅの含有量は０．３５％以下に規制する。
また、アルミニウム合金を鋳造する際には地金、添加元素の中間合金等様々な経路より不純物が混入する。混入する元素は様々であるが、Ｆｅ以外の通常の不純物は単体で０．０５％以下、総量で０．１５％以下であれば合金の特性にほとんど影響を及ぼさない。従って、これらの不純物は単体で０．０５％以下、総量で０．１５％以下とする。
【００１４】
上記Ａｌ−Ｍｇ−Ｓｉ系合金の押出加工にあたっては、その保有熱を利用して溶体化するのが工業上有利である。このため押出直後の形材温度が極力溶体化温度になるようにし、直ちに急冷（プレス焼入れ）する。この急冷により、同時に形材の再結晶を防止してファイバー組織をもつ押出形材を得る。一方、押出温度を高くしすぎると結晶組織の再結晶化が促進され、ファイバー組織から粗大な等軸再結晶粒へと変化する。再結晶を抑制し、ファイバー組織とするためには、押出工程では押出直後の形材温度を溶体化温度以上、固相線温度以下、すなわち５００℃以上５８０℃以下、望ましくは５１５℃以上、５５０℃以下に制御することが好適である。
【００１５】
【実施例】
以下、本発明の実施例を説明する。
表１に示す組成のアルミニウム合金をＤＣ鋳造にて直径１５５ｍｍの鋳塊に造塊し、５４０℃×４ｈｒの均質化処理後、ビレット温度５００℃に加熱し、押出速度５ｍ／ｍｉｎにて図１に示す板の形状（厚さ２ｍｍ、幅１１０ｍｍ）に押し出し、押出時水冷にてプレス焼入れを行った。その後、１８０℃×６ｈｒの人工時効処理を施し供試材とし、下記要領で各特性を調べた。その結果を表１にあわせて示す。
【００１６】
【表１】

【００１７】
機械的性質：押出方向と平行にＪＩＳ５号引張試験片を採取し、引張試験を行った。
結晶組織：押出方向に平行な断面を光学顕微鏡で観察した。（なお、溶着部を有する押出形材の場合は、当該部位近傍では再結晶層厚さがばらつくので、それ以外の部位で測定する。）
溶接性：図２に示すように２枚の供試材の押出方向に垂直なサイドを突き合わせ、表２に示す溶接条件にてＭＩＧ溶接を行ない、溶接部断面（図２（ｂ）に観察部位を示す）を目視又は光学顕微鏡で観察し、５段階で溶接性を評価した。５段階は、１：溶接割れなし、２：割れが１つの結晶粒界面におさまっており、その発生部の数もごく少ないもの、３：割れが１つの結晶粒界面におさまっているが、その発生部が断面に多数存在するもの、４：割れが複数の結晶粒界面にまたがっているもの、５：目視レベルで割れが観察できるもの（割れが少なくとも十数粒界に及ぶ）、とした。
【００１８】
【表２】

【００１９】
表１に示すように、成分組成が規定範囲内にあり、結晶組織がファイバー組織のＮｏ．１、２、５は、高強度、高延性を示し、溶接割れ性もＺｒを含まないＮｏ．３、４に比べて一段と優れていた。また、結晶組織がファイバー組織で、表面再結晶層が比較的厚く形成されたＮｏ．６〜８（プレス焼入れの冷却速度を調整）を、同じ組成のＮｏ．１と比べると溶接性がやや低く、かつ表面再結晶層が厚いほど溶接性が低くなっている。
一方、Ｃｒ、Ｍｎ、Ｚｒを含まないＮｏ．９は機械的特性はまあまあだが、結晶組織が等軸晶でＣｕの含有量が多いため溶接割れ性が劣っている。また、Ｎｏ．１０、１１は等軸晶であるが、Ｃｕの含有量が少ないため溶接割れ性はよい。しかし、強度及び伸びがＮｏ．１〜８に比べて劣る。
【００２０】
【発明の効果】
本発明によれば、Ａｌ−Ｍｇ−Ｓｉ系アルミニウム合金押出形材により、高強度、高延性でかつ曲げ加工性及びアーク溶接性に優れた曲げ加工及びアーク溶接用自動車フレーム構造材を得ることができる。この押出形材は、高強度を必要とし、曲げ加工による成形を施され、かつアーク溶接により組み付けられる自動車フレーム構造材として好適である。
【図面の簡単な説明】
【図１】実施例の押出形材の断面形状を示す図である。
【図２】実施例の溶接後の供試材（ａ）及びその断面図（ｂ）である。[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to an automobile frame structural material made of an extruded Al-Mg-Si alloy, subjected to forming by bending, and particularly suitable for being assembled by arc welding.
[0002]
[Prior art]
Al-Mg-Si-based aluminum alloys are relatively excellent in corrosion resistance among alloys that can obtain high strength, are widely available in the market as sash materials, etc., and are also superior to other types of aluminum alloys in terms of recycling. For this reason, application to various structural members as an extruded member has been attracting attention.
[0003]
[Problems to be solved by the invention]
Automobile frame structural materials may be required to be bent to a desired shape or to be joined by welding, etc., as required, and have high strength (tensile strength, proof stress), excellent ductility, and arc welding. It is desirable that the property is excellent. On the other hand, in order to improve strength and ductility in an Al-Mg-Si alloy, it is known to add Cu as an alloy element, but Cu impairs the arc weldability of the Al-Mg-Si alloy. In the case of an arc welding structure, the amount of Cu added was limited to a low level.
[0004]
For example, in Japanese Patent Application Laid-Open No. 64-47830, the addition amount of Cu is limited to 0.1% or less in an Al-Mg-Si-based aluminum alloy extruded material containing small amounts of Cr and Zr. In Japanese Patent Application Laid-Open No. Hei 9-41063, the amount of Cu added is limited to 0.2% or less in an Al-Mg-Si based aluminum alloy extruded material containing small amounts of Cr and Zr. In the extruded Al-Mg-Si-based aluminum alloy material containing a small amount of Zr, the addition amount of Cu is limited to 0.05% or less, and in Japanese Patent Application Laid-Open No. 9-256096, the addition amount of Cu is also limited to 0. It is limited to 4% or less.
[0005]
Under such circumstances, the present invention provides an automobile frame structure for bending and arc welding comprising an extruded Al-Mg-Si-based aluminum alloy material, having high strength, high ductility, and excellent bending property and arc weldability. The purpose is to obtain the material .
[0006]
[Means for Solving the Problems]
The automobile frame structural material for bending and arc welding according to the present invention has Mg of 0.4 to 0.8%, Si of 0.7 to 1.1%, Cu of more than 0.4 to 0.65%, Ti: 0.005 to 0.2%, Zr: 0.05 to 0.3%, Cr: 0.05 to 0.5% and Mn: 0.05 to 0.8 as required. % is any one or of containing two, a Al-Mg-Si series alloy extruded shape the balance being Al and impurities, all or most of the crystal structure fibrous tissue (hereinafter referred to as fiber tissue ). Here, the fiber structure is a hot worked structure found in the extruded material, which is a crystal grain structure elongated in the extrusion direction, and in the present invention, the entire cross section of the extruded shape is constituted by the fiber structure. Alternatively, most of the thickness of the cross section, that is, 50% or more of the thickness, must be occupied by the fiber structure (in this case, a recrystallized structure is formed on the surface).
[0007]
[0008]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, the reasons for limiting the component composition of the extruded Al-Mg-Si alloy material will be described.
Cu
In the extruded Al-Mg-Si alloy material, Cu improves the strength of the alloy by precipitation strengthening, and also improves the ductility of the material. Therefore, structural materials that require high strength, ductility, and bending workability are required. In many cases, Cu is added. However, based on the common understanding that the addition of Cu impairs the weldability of Al-Mg-Si alloys (causing weld cracking), the maximum amount of Cu added in extruded sections used as welded structural members is However, it was suppressed to 0.4% or less.
[0009]
However, the present inventors have found that, when the alloy structure of the extruded Al-Mg-Si alloy material is a fiber structure, there is no decrease in arc weldability even when Cu exceeds 0.4% is contained. I found it. It is not clear why the weldability does not decrease, but since the grain boundary area increases due to the fiber structure, Cu, which is easily segregated at the grain boundary, is widely dispersed and precipitated at the grain boundary, and per unit grain boundary area. It is presumed that the lowering of the Cu concentration may have an effect.
In an Al-Mg-Si based alloy extruded material containing more than 0.4% of Cu, if the recrystallized layer formed on the surface becomes thick, the arc weldability significantly decreases. Therefore, as described above, it is necessary that the entire cross section of the extruded profile is composed of the fiber structure, or that the fiber structure accounts for 50% or more of the thickness of the cross section. Desirably, the total thickness of the recrystallized surface layer (the sum of the thicknesses on both sides) is controlled to be 30% or less of the thickness of the profile (the fiber structure is 70% or more of the thickness).
[0010]
In the extruded Al-Mg-Si alloy material of the present invention, Cu exceeding 0.4% improves the strength, and improves the ductility and bending workability. However, an excessive amount of Cu reduces press hardening (solution quenching performed immediately after extrusion using the retained heat of the extruded material) and impairs arc weldability. .65% or less. In particular, Cu is preferably in the range of 0.45 to 0.55%.
[0011]
Cr, Mn, Zr
Cr, Mn, and Zr precipitate as intermetallic compounds in the ingot during the homogenization heat treatment, suppress recrystallization during hot working and the like, and refine the metal structure. In the extrusion process, the structure is a fiber structure, and the arc weldability is significantly improved as compared with the equiaxed recrystallized structure. Addition of 0.05% or more has an effect of suppressing recrystallization, but Zr is particularly effective. Therefore, Zr is added first, and if necessary, Cr and Mn may be added. . On the other hand, an excessive addition tends to generate a coarse insoluble intermetallic compound at the time of casting, which causes a decrease in strength and ductility.
Therefore, the respective addition amounts are Cr: 0.05 to 0.5% or less, Mn: 0.05 to 0.8% or less, and Zr: 0.05 to 0.3% or less. Particularly desirable ranges of Mn and Zr are Mn: 0.15 to 0.5% and Zr: 0.1 to 0.15%.
[0012]
Mg, Si
Mg and Si are elements that impart strength to the alloy. When the Mg content is less than 0.3% or the Si content is less than 0.2%, the effect of improving the strength by the aging treatment cannot be obtained. Conversely, if the Mg content exceeds 1.6% or the Si content exceeds 1.6%, ductility is impaired, bending workability decreases, and extrudability also decreases. Among them, the ranges of Mg: 0.4 to 0.8% and Si: 0.7 to 1.1% are particularly desirable from the viewpoint of balance of strength, ductility, bending workability, and extrudability.
[0013]
Ti
Ti has the function of forming nuclei at the time of melting casting and making the cast structure fine, and is added in an amount of 0.005% or more . However, if the content is too large, a coarse compound is formed and the Al-Mg-Si alloy is made brittle, so the upper limit is 0.2%.
Among the impurity impurities, Fe is the most frequently contained impurity in the aluminum ingot, and if present in the alloy exceeding 0.35%, a coarse intermetallic compound is crystallized during casting, which impairs the mechanical properties of the alloy. Therefore, the content of Fe is restricted to 0.35% or less.
Further, when casting an aluminum alloy, impurities are mixed from various routes such as a base metal, an intermediate alloy of an additive element, and the like. The elements to be mixed are various, but ordinary impurities other than Fe alone have a 0.05% or less, and if the total amount is 0.15% or less, it hardly affects the properties of the alloy. Therefore, these impurities are set to 0.05% or less in a simple substance and 0.15% or less in total.
[0014]
When extruding the Al-Mg-Si alloy, it is industrially advantageous to use the retained heat to form a solution. For this reason, the temperature of the shaped material immediately after extrusion is set to the solution temperature as much as possible, and the material is immediately quenched (press quenching). By this quenching, the extruded section having a fiber structure is obtained by simultaneously preventing recrystallization of the section. On the other hand, if the extrusion temperature is too high, recrystallization of the crystal structure is promoted, and the fiber structure changes to coarse equiaxed recrystallized grains. In order to suppress recrystallization and obtain a fiber structure, in the extrusion step, the temperature of the shaped material immediately after extrusion is equal to or higher than the solutioning temperature and equal to or lower than the solidus temperature, that is, 500 to 580 ° C, preferably 515 to 550 ° C. It is preferable to control the temperature to not more than ° C.
[0015]
【Example】
Hereinafter, examples of the present invention will be described.
An aluminum alloy having the composition shown in Table 1 was cast into an ingot having a diameter of 155 mm by DC casting, homogenized at 540 ° C. × 4 hr, heated to a billet temperature of 500 ° C., and extruded at an extrusion speed of 5 m / min. Was extruded into a plate shape (thickness: 2 mm, width: 110 mm) as shown in Table 1 and press-quenched by water cooling during extrusion. Thereafter, an artificial aging treatment at 180 ° C. for 6 hours was performed to obtain a test material, and each characteristic was examined in the following manner. The results are shown in Table 1.
[0016]
[Table 1]

[0017]
Mechanical properties: A JIS No. 5 tensile test piece was sampled in parallel with the extrusion direction, and a tensile test was performed.
Crystal structure: A cross section parallel to the extrusion direction was observed with an optical microscope. (In the case of an extruded profile having a welded portion, the thickness of the recrystallized layer varies in the vicinity of the site, so measurement is performed at other sites.)
Weldability: As shown in FIG. 2, two test pieces were joined with their sides perpendicular to the extrusion direction, MIG welding was performed under the welding conditions shown in Table 2, and the welded section (Fig. Was visually observed or observed with an optical microscope, and the weldability was evaluated in five stages. The five stages are: 1: no weld cracks, 2: cracks are contained in one crystal grain interface, and the number of occurrences is very small. 3: Cracks are contained in one crystal grain interface. The number of occurrences was large in the cross section, 4: the cracks straddled a plurality of crystal grain interfaces, and 5: the cracks could be observed at a visual level (the cracks spread over at least a dozen grain boundaries).
[0018]
[Table 2]

[0019]
As shown in Table 1, the component composition was within the specified range, and the crystal structure was No. 1 of the fiber structure . Nos. 1 , 2 , and 5 show high strength and high ductility, and have no weld cracking property . It was much better than 3 and 4 . In addition, in the case of No. 1 in which the crystal structure was a fiber structure and the surface recrystallized layer was formed relatively thick. Nos. 6 to 8 (adjusting the cooling rate of press quenching) were used for No. 6 having the same composition. The weldability is slightly lower than that of No. 1, and the weldability is lower as the surface recrystallized layer is thicker.
On the other hand, No. 3 containing no Cr, Mn and Zr. 9 has moderate mechanical properties, but is poor in weld cracking property due to its equiaxed crystal structure and high Cu content. No. Although 10 and 11 are equiaxed, they have good weld cracking properties due to the low Cu content. However, the strength and elongation were no. Inferior to 1-8.
[0020]
【The invention's effect】
According to the present invention, it is possible to obtain an automobile frame structural material for bending and arc welding having high strength, high ductility, and excellent bending property and arc weldability by using an extruded aluminum-Mg-Si-based aluminum alloy material. it can. This extruded profile requires high strength, is formed by bending , and is suitable as an automobile frame structural material assembled by arc welding.
[Brief description of the drawings]
FIG. 1 is a view showing a cross-sectional shape of an extruded profile of an example.
FIG. 2 is a specimen (a) after welding and a cross-sectional view (b) thereof of an example.

Claims

Mg: 0.4 to 0.8% (mass%, the same applies hereinafter), Si: 0.7 to 1.1%, Cu: more than 0.4 to 0.65%, Ti: 0.005 to 0.2 %, Zr: an Al-Mg-Si based alloy extruded material containing 0.05 to 0.3%, the balance being Al and impurities, wherein all or most of the crystal structure is a fiber structure (hereinafter referred to as fiber). The structure of the automobile frame for bending and arc welding, wherein the total thickness of the surface recrystallized layer is 12% or less of the thickness of the profile.

Mg: 0.4 to 0.8%, Si: 0.7 to 1.1%, Cu: more than 0.4 to 0.65%, Ti: 0.005 to 0.2%, Zr: 0.05 Al-Mg- containing any one or two of Cr: 0.05 to 0.5% and Mn: 0.05 to 0.8%, with the balance being Al and impurities. An extruded Si-based alloy material, wherein all or most of the crystal structure is a fiber structure, and the total thickness of the surface recrystallized layer is 12% or less of the thickness of the shape material. And automotive frame structural materials for arc welding.